US3082540A

US3082540A - Hair drier

Info

Publication number: US3082540A
Application number: US49006A
Authority: US
Inventors: Hiltenbrand Charles
Original assignee: Societe de Ventilation et dElectricite Appliquees Velecta
Current assignee: Societe de Ventilation et dElectricite Appliquees Velecta
Priority date: 1956-07-26
Filing date: 1960-08-11
Publication date: 1963-03-26
Anticipated expiration: 1980-03-26

Description

March 26, 1963 c. HILTENBRAND 3,082,540

HAIR DRIER Filed Aug. 11, 1960 5 Sheets-Sheet l March 26, 1963 c. HILTENBRAND 3,

HAIR DRIER Filed Aug. 11, 1960 s Sheets-Sheet 2 March 26, 1963 Filed Aug. 11, 1960 C. HILTENBRAND HAIR DRIER 5 Sheets-Sheet 3 @MM f Jah- March 26, 1963 c. HILTENBRAND 3, ,540

HAIR DRIER Filed Aug. 11, 1960 5 Sheets-Sheet 4 C. HILTENBRAND March 26, 1963 HAIR DRIER 5 Sheets-Sheet 5 Filed Aug. 11, 1960 WWW tates This application is a continuation-in-part of my copending applications Serial No. 673,799, filed July 24, 1957 and now abandoned and Serial No. 813,746 filed May 18, 1959 and now abandoned.

It is known that to speed up the drying of hair use is made of special driers based on a common principle consisting in pulsating heated air by means of a motor-driven turbine or fan and blowing this air onto the hair.

This drying step is accomplished at variable adjustable temperatures and its duration is determined empirically by the hairdresser as a function of the volume of hair, its physical condition and more particularly by guess-work. However, this empirical estimation is attended by very numerous and serious inconveniences.

Therefore, it is the essential object of the present invention to permit a reliable and efficient control of the hair drying operation by constantly checking the degree of dryness of the whole hair. Therefore, this invention provides a hair drier comprising means for controlling the evolution and completion of the hair drying operation.

To this end, the invention provides a hair drier comprising first conduit means for picking up used air having already passed through the hair and second conduit means for picking up external air, means for equalizing the temperatures of these two air streams, and means for detecting the hygrometric degree of one and the other air streams subsequent to the equalization of their temperatures, said detector means consisting for example either of two detectors disposed respectively in contact with one and the other air streams and of which the indications can be easily compared with each other, or a single detector adapted to be connected by turns with one and the other air stream.

In the first case, the end of the drying operation is indicated by the concordance of the data supplied by the detector member contacting the used air with those supplied by the detector member contacting the external atmosphere.

In the second case, the end of the drying operation is indicated when the single detector member shows identical data whether it is connected with the used air stream or with the external air stream.

In both cases, the hair-drying control action is based on the checking of the humidity excess absorbed by the drying air during its passage through the hair. In fact, the air blown through the hair is heated and forced by the drying machine, and has well-defined temperature and humidity characteristics in relation to the initial air, these characteristics being those of the room atmosphere or ambient air but modified by the heating within the drying machine, for example 55 C. with a 20% humidity content, representing a rate of 20.5 gr. per cubic meter. On contacting the hair, this air will absorb therefrom a certain quantity of moisture constituting a fresh charge or overload which, according to psychometric measurements, is estimated to lie in the range of from 8 to 10 gr. per cubic meter. Of course. this overload varies during the drying operation; it will f rstly increase as the hair temperature increases, and become stabilized during some time, until it begins to decrease and finally attains a zero value when the hair is dried. In this last case the air having flown through the hair has resumed the characteristics of the initial air used for the drying operation.

Thus, the device controlling simultaneously the hygrometric degree of the used air and that of the external air 3,082,540 Patented Mar. 26, 1963 fed to the drier affords a very accurate and reliable control of the end of the drying period.

It may be pointed out that the supervision ensured by the control device cannot be disturbed by variations in the hygrometric degree of the external air since the control device will constantly check the hygrometric degree of this air.

In a preferred form of embodiment, the humidity detecting member or members consist of a probe or like sensitive member comprising electrical characteristics of which at least one varies in proportion to the variations of the hygrometric degree of the air flowing thereon, said probe or probes being connected in an electrical circuit comprising indicating apparatus whereby the operator or the patient herself may read the variations occurring in the characteristics of the probe, or probes, and therefore check the evolution and the completion of the drying operation.

However, substantial differences in the hygrometric state of the ambient air may constitute a source of difficulties. In fact, in order to maintain an adequate sensitivity in the electrical control circuit without necessitating the use of different probes each consistent with predetermined hygrometric values, some means must be provided for varying the impedance of the feed circuits and voltages. These adjustments, although relatively simple in themselves, require a certain skill from the operator.

Under these conditions, it is also within the scope of this invention to provide means for permitting the operation of the hygrometric control probe or probes within selected limits so that their sensitivity remains maximum as a function of the circuit characteristics without necessitating complicated or delicate adjustments for the operator.

To this end, the invention provides a preferred form of embodiment of the hair drier as broadly set forth hereinabove wherein the suction chamber for taking external air by means of the fan equipping the apparatus comprises a heating resistor and a thermostat adapted to maintain the stream of air flowing through this chamber at the temperature at which the hygrometric state of the reference air controlled by the probe or probes remains substantially at the value corresponding to the basic sensitivity of said probe or probes. Of course, this value should correspond to the minimum possible moisture in winter, that is, for example 35% 25%. As will be seen presently the ad justment of the heating temperature of the induction chamber may be effected by the operator after checking the conditions of operation of the check probe. This adjustment may also be effected automatically by means of an additional device associated with the control device of this invention.

Preferably, in specific form of embodiment the check probe or probes are disposed within said external-air induction chamber.

A few typical embodiments of a hair-drying machine constructed in accordance with the teachings of this invention are described hereafter with reference to the accompanying drawings forming part of the specification and illustrating diagrammatically by way of example the manner in which the invention may be carried out in the practice. In the drawings:

FIGURE 1 is a diagrammatic axial section showing a first embodiment of a hair-drying machine;

FIGURES 2 and 3 are two illustrative examples of electrical circuits associated with the hygrometric probes;

FIGURES 4 and 5 are other diagrammatic axial sections showing two other embodiments of the hair-drying machine;

FIGURE 6 is a diagram illustrating the operation of the machine shown in FIGURE 5;

FIGURE 7 is a perspective view showing another form of embodiment of the hair drier;

FIGURE 8 is a fragmentary elevational view showing the possible arrangement, on the casing of the drying apparatus of the dial of a control galvanometer with its adjustment knobs;

FIGURE 9 is a diagrammatic vertical section of the same apparatus;

FIGURE 10 is a fragmentary vertical section taken upon a plane at right angles and on a larger scale to show a detail of the apparatus;

FIGURE 11 is a typical example of a wiring diagram for the electrical check probes of the same apparatus;

FIGURES 12 and 13 illustrate two forms of embodiment of the wiring diagram of an additional device adapted to be associated with the control device, if desired.

FIGURE 14 is a view similar to FIGURE ll, but corresponding to another form of embodiment of the drier, wherein only one detector probe is provided.

FIGURES 15 and 16 illustrate two forms of embodiment of the wiring diagram of this single detector probe.

The hair-drying machine illustrated in FIGURE 1 comprises a conventional-type case 1 forming at its lower portion a helmet 2 and containing in the known fashion a fan 3 driven from an electromotor 4. A partition 5 separates the helmet portion 2 from the inner space of the case 1. However, the outer periphery of this partition 5 forms a free annular gap through which the air pulsated by the fan may flow onto the patients hair, this air being heated for example by means of electric resistors R.

On the other hand, the partition 5 comprises in its central portion an aperture 6 for recovering the exhaust or used air from the upper portion of the helmet, which is the air having partially dried the hair and having therefore become laden with humidity. This exhaust airis actually sucked by the central portion of the fan 3 so as to be blown again onto the hair after being re-heated.

The partition 5 carries in its central portion two chambers 7a, 7b enclosing two identical

hygrometric probes

8, 8b. The chamber 7a is open at either ends and registers with the aperture 6 of the partition 5 so that the exhaust air sucked by the fan 3 is forced through this chamber. The other chamber 71) is closed on the side opposite to the fan and communicates through the medium of a pipe 9 with the air outside the case 1. Thus, the air flowing through this chamber 7b is taken from the outside but heated to the temperature of the used air. In fact, this ambient air flowing the chamber 7b is heated on account not only of the very position of this chamber in the zone where the used air is recovered but also of the path followed by the pipe 9 inside the helmet.

Each

hygrometric probe

8a or 8b is highly sensitive and constitutes an electrical resistor the resistance of which decreases in proportion to the hygrome-tric degree of the air flowing theret-hrough. Each probe may consist of a support of insulating materialcarrying two separate conducting circuits it), 11 which are insulated on their outer surface and provided with very-closely-. spaced portions so that the very moisture of the airflowing' through the probe may provide an electrical connection between these 'circiuts, the electrical resistance of the probes varying with the hygrometric degree of the air. These probes may be constructed either in the form of printed electric circuits, or in the form of imbricated comb-like metal structures. On the other hand, the sensitivity of the probes of this type may be varied to a substantial extent by altering the gaps separating the elements of the two poles and by coating them with a film layer 'of a hygroscopic cement (having for example a sodium-silicate or magnesia-silicate base).

These probes may'also be provided with means for varying their capacity and resistance or their impedance as a function of the hygrornetric condition of the air. The two

probes

8a and 8b are connected with a differential type electrical circuitas shown in FIGURE 2, which comprises a suitably adapted source E of AC. or D.C.,

and two resistors R1 and R2. From this circuit two

conductors

12 and 13 lead to a voltmeter or a galvanometer G constituting the instrument for indicating the actual degree of hair drying. In fact, the voltage value shown by this voltmeter decreases as the hair dry up and becomes zero when the hair is dried completely.

According to a further embodiment illustrated in FIG- URE 3, the ditierential circuit comprises an adjustment potentiometer P and the two

conductors

12 and 13 lead to an amplifier A feeding through a circuit 37 the winding 33 of an electromagnet maintaining a switch 34 closed against the elastic force of a return spring 35, this switch being inserted in the feed circuit 36 of the heating resistors R and driving motor M of the apparatus. Thus, when the energization of winding 33 is discontinued, the circuit 36 is opened automatically, thus stopping the apparatus.

As both

probes

8a, 8b are mounted in chambers 7a, 7b respectively, they will detect the one the humidity content of the air which has become laden with humidity by flowing through the air, and the other the humidity content of the external air heated by the internal heat of the helmet. In other words, the diiference between the resistances of the probes will be only a function of the amount of humidity absorbed by the used air and this difference will decrease with this amount, that is, when the hygrometric degree of the air within the helmet and that of the external air will become equal. At this time, which occurs as the hair has dried completely, the differentialaction circuit of both

probes

8a, 8b actuates the relay and the machine is thus stopped.

In fact, the circuit 1213 will become de-energized since the two probes have the same resistance and balance each other, the electromagnet 33 releasing the switch 34 so that the latter i opened by the return spring 35.

This stoppage of the drier is obtained automatically when the hair is dry.

In the case of the circuit illustrated in FIGURE 2, the operator is simply warned that the drying operation is completed by observing the voltmeter V or the galvanometer G, the end of the drying operation corresponding to the complete de-energization of circuit 12-43. Then the operator may him or herself stop the apparatus.

It may be pointed out that the control of the drying operation is quite reliable since it is constantly related to the hygrometric degree of the ambient air as a function of its temperature. If desired, the potentiometer P may be used for pro-adjusting the diiferential system to properly balance the differential circuit and therefore stop the drier at the desired time, that is, when it is desired to eifect a more or less pronounced drying. However, any risk of detrimentally overheating the hair is efinitely precluded.

FIGURE 4 illustrates a modified embodiment of the hair-drying machine shown in FIGURE 1. In this modified embodiment the drying action is controlled in a substantially different way, although the same basic principle is still adhered to. Thus, the fan 3a of this drying machine forces air at a certain pressure through the per forations of a helmet 2a. Besides, this machine com prises a double-

walled case

14, 15 surrounding the helmet 2a and having a lower aperture 16 for re-sucking the used air from the bottom, the resulting annular chamber 17 communicating at 18 with the central portion of the fan 3a.

The probe 8a for controlling the used or exhaust air is mounted in this annular chamber 17, and the other probe 8b for controlling the heated ambient air is mounted in its chamber 712 but the latter is adjacent to the first probe 312 in the annular chamber 17. The probe chamber 7b is open on the fan side and its opposite side communicates with the outside through a pipe 9a forming a loop in the chamber 17. Thus, the ambient air flowing through the probe 8b is also heated to the temperature of the used air.

The two probes of this modified embodiment are connected, as in the case of the first embodiment shown in FIGURE 1, to a differential circuit controlling a relay adapted to stop the drying machine automatically when the air has dried to the proper degree.

FIGURE 5 illustrates another modified embodiment of the hair-drying machine according to this invention. In this case the machine is of the same general type as that illustrated in FIGURE 1, but the mounting of the two 'hygrometric probes differs. In fact, in the embodi ment illustrated in FIGURE 5 the probe 817 controlling the ambient air is mounted in a chamber 18b communicating directly with the surrounding atmosphere. This chamber is open at both ends and has one end coincident with an aperture 19 formed in the case 1b of the machine. Thus, the air is drawn directly from the surrounding atmosphere and sucked by the fan 3a; if desired, this same air may be caused to flow around the electromotor 4 to cool same, for example by providing a suitable cowling 29.

The other probe 8a controlling the used air is mounted in a chamber 18a communicating with the centre of the fan 3 through a passage 21 leading to the inside of the cowling 20. A pipe 22 connects the centre of the aperture 6 in the partition 5 to the opposite end of the chamber 1811 to supply the latter with air sucked *by the fan 3. This pipe 22 forms turn or loops 22a -(see diagram of FIGURE 6) registering with a relatively large aperture 23 formed in the case 1b through which the external air is drawn by the fan 3. Thus, the used air circulating in this pipe is cooled to the temperature of the ambient air before flowing round the probe 8a.

The two

probes

8a, 8b receive the one used air and the other ambient or external air both of which have substantially the same temperature. However, in the present case, the ambient air is at its normal temperature and it is the temperature of the used air that is altered by being cooled down to the same value as that of the ambient air. The

probes

8a, 8b are connected as in the preceding embodiment in the differential circuit controlling the operation of the drying machine so that the latter is stopped automatically when the hair is dry.

FIGURES 7 to of the drawings illustrate another modified embodiment of the hair drier of this invention.

In this alternate embodiment, there is provided a chamber 195 through which the external air is sucked by the turbine rotor or impeller 168, this chamber communicating through a conduit 107 with a chamber formed inside the helmet by means of two spaced

parallel partitions

103 and 104. Chamber 105 has mounted therein a filter 120 for removing any dust from the stream of air circulating therethrough, and an electrical resistor 121 for heating this stream. A thermostat 122 controlling the operation of this resistor is also mounted in the duct 105 and comprises an adjustment knob 123 disposed on the outer wall of the duct 1115 (see FIG- URE 9). If desired, this adjustment knob may be replaced with a knob 123a mounted on the front of the casing 161 (see FIGURE 7) and connected with the thermostat through flexible driving means.

The upper portion of this casing encloses the motor 166 of which the shaft carries the turbine impeller or rotor 103 in the upper portion of the helmet 1G2 and in the vicinity of the perforations 104a formed in the central portion of the partition 104 of the inner suction chamber 1tl31il4. i

This impeller or rotor carries a disk 108a extending at right angles to its axis and at a level intermediate the vertical dimension of the rotor blades. Thus, this disk will divide the rotor into two portions, that is, an upper portion drawing fresh air through the duct 1115 and perforations 104a and delivering it to the hair,

and a lower portion redrawing used air from the hair and delivering it again towardthe hair. The intermediate partition 19852 will thus prevent the re-drawn used air from being mixed with the fresh air in the rotor. A circulation of used air is thus established in closedcircuit conditions in the direction of the arrows f Resistors R are provided for heating the air stream directly toward the hair.

Registering with the lower portion of the rotor is the bent end portion 111a of a duct 111 adapted to receive the used air re-drawn by the rotor. Also registering with the upper portion of this rotor 197 is the bent end portion of a similar duct i112 receiving the fresh air delivered from this rotor.

Both

pipes

111 and 112 have the same cross-sectional area and extend side by side firstly between the two partitions 10 3, 104 then within the main duct 1115 down to the elbow thereof. At this location both pipes 11-1 and 112 extend on either side of a case .113 containing the

hygrometric probes

8c and 8d for checking the humidity content of the used air and of the fresh air, this case 1 13 having an outlet pipe extension 115 emerging from the main duct 105. Thus, this case 113 is dis posed within the duct 105 constituting the suction chamber of the turbine blower of the drier.

The two

ducts

111 and 1 12 communicate with the inside of the enclosure through a pair of small pipes 111k and 11% of same cross-sectional area which lead to a pair of compartments 113a and 113d of enclosure 1.13 which contain the two

probes

8c and 8d adapted to be connected in a differential-action circuit of the type i1- lustrated in FIGURE 2 or FIGURE 3.

However, in this hair drier (as shown in FIGURES 7 to 10) the air circulating in the feed chamber 105 may be brought to the temperature at which its hygrometric state corresponds substantially to the desired value corresponding to the maximum basic sensitivity of the detector probes 8c and 8d.

Thus, the

probes

8c and 8d may have a maximum sensitivity when the air flowing therearound has a 35% relative humidity. Under these conditions, if the surrounding atmosphere has a higher humidity content the thermostat 122 will maintain within the duct 105 a temperature higher than the room temperature I", this difference corresponding to the temperature increment necessary to bring the humidity content of the air taken from the surrounding medium to the aforesaid value of 35% :L5%. Thus, for example, the humidity percentage of the surrounding air may be 45% at 68 F., and in this case the thermostat will keep the inner volume of duct 5 at a temperature of 68 F.+7 F. in order to reduce the humidity content of the reference air to 35% Preferably, the thermostat 122 is operatively connected to a temperature detector disposed on the outside, so that this thermostat will maintain a constant temperature discrepancy between the suction chamber 105 and the surrounding space, in case the temperature of the incoming air were subject to variations.

After the adjustment step, the drying operation proper may be performed as in the preceding example and checked by means of the two probes and their differentialaction control circuit.

The control means provided by this invention is all the more reliable and accurate as the two air streams flowing successively around both

probes

8c and 8d have the same temperature conditions and as their outputs are the same. As a mater of fact, the respective crosssectional areas of both

pipes

111 and 1 12 and of the small branch pipes 11112 and 1121) are the same. More over, due to the fact that the two

pipes

111 and 112 extend on a certain length side by side in the space pro vided between the two partitions 10 3 and 164 and then in the upper portion of the main air duct 1115, the air circulating in these pipes is brought substantially to the same temperature. Finally, it may be noted that the 7 probe containing case 113 is similarly located Within the main duct 105.

Moreover, this device is characterized by the essential advantage that the used air, from the point where it is taken inside the helmet and during its travel along the pipe leading same to the atmosphere surrounding the probe-containing case, is maintained at a temperature higher than that corresponding to the dew point, with due account for the excess of humidity with which it has become loaded during its passage through the hair, which is a condition necessary for ensuring a proper control action.

, According to a first modified form of embodiment of the drying apparatus illustrated in FIGURES 7 to 10, the two

probes

8c and 8d are mounted in a differentialaction circuit designed to permit an easy initial adjustment of the desired temperature in the chamber 105 through which the apparatus is fed with air. This circult, of which a typical example is illustrated in FIG- URE 14, differs from those of FIGURES 2 and 3 in that it comprises an additional resistor R3 connected in series with the probe 80 controlling the used air and in the same branch asthe latter probe.

There is also provided in this arrangement a conductor 124 adapted to shortcircuit this additional resistor, this conductor comprising a normally open switch 125 controlled by an external press-button 126.

This press-button is mounted on the top of the helmet of the drier (see FIGURES 7 and 8) near the dial 110 of the galvanometer G inserted in the control circuit, and close to another knob 127 controlling the potentiometer P of this circuit. The button 123a, for adjusting the thermostat controlling the heating of chamber 105 is also located in the vicinity of these members.

The scale of the galvanometer dial 11% comprises two reference zones E and S. The first zone indicates the position of the galvanometer pointer when the circuit is balanced; The other zone S corresponds to the position of this pointer for a condition of maximum unbalance of the differential circuit when the probe 8c contacts used air having a relatively high hygrometric degree.

' The value of this additional resistor R3 of the control circuit corresponds to the resistance difference of probe So when the later contacts air having a relative humidity content corresponding to its basic or pre-adjusted sensitivity, and when it contacts air having a humidity content exceeding a predetermined value. Thus, the subsequent suppression of this resistor by short-circuiting same causes a reduction in the resistance of the corresponding branch of the Wheatstone bridge which is expected to cause a predetermined movement of the galvanometer pointer to the region S of the dial 110 of galvanometer G if this pointer was set beforehand on the reference line of region E which was the initial cir cuit gauging position.

' Before carrying out a first drying operation, the operator runs the drier during a few minutes in order to enable same to attain its proper operating temperature, the thermostat 122 being set to ensure a minimum heating. of the feed air. Then the operator sets the circuit by adjusting the control knob 127 of potentiometer P so as to bring the galvanometer pointer approximately in the center of region B.

Then, the operator checks whether the conditions of operation correspond to those consistent with the basic sensitivity of the probe. To this end he simply depresses the push-button 126 in order to short-circuit the additional resistor R3. As already explained hereinabove, the short-circniting of this resistor, while reducing the resistance of the corresponding circuit branch, should cause the galvanometer pointer to move to the dial region S. If the pointer oversteps this position, or if on the contrary it cannot attain it, this means that the conditions of operation do not correspond to the basic conditions consistent with the pre-set sensitivity of the probe, or in other words, that the hygrometric degree of the feed air is not the proper one. If the pointer oversteps this region S, it indicates that the feed air (that is, the reference air) is overladen with humidity and therefore it is necessary to increase the heat supplied by the resistor 121 by properly adjusting the thermostat control knob 123, until the pointer is again positioned within the region S of the galvanometer dial. Conversely, if the pointer does not attain this region S, the heating must be reduced accordingly to bring the pointer in the dial region S.

When this adjustment is completed, the operator has the certainty that the hygrometric state of the reference air corresponds to that for which the sensitivity of the probe is adequate. is controlled in a very accurate manner since at this time the used air flowing on and around the probe will have attained a hygrometric state approximating that for which the sensitivity of this probe is maximum as a function of the impedance characteristics of the circuits.

After this checking and adjustment operation, the drying operation proper may perfectly and safely be accomplished and controlled. In fact, at the beginning of the drying operation, when the used air is very moist, the galvanometer pointer lies in the zone S. Then, as the hair become dry, it moves toward the zone E and stops in this zone upon completion of the drying operation, the electrical circuit stopping automatically the apparatus at the same time.

According to another modified embodiment of the hair drying apparatus which is illustrated in FIGURES 7 to 10 of the drawings, there is associated with this apparatus an additional device adapted automatically to adjust the temperature of the feed air supplied to the drier as a function of its hygrometric degree relative to the ambient temperature in order to stabilize the relative hygrometry to this air which acts somewhat as reference air.

This complementary device, FIGURE 12, comprises a hygrometric probe 123 of which the electrical characteristicsresistance, capacity or impedancevary as a function of the humidity content of the air contacting same. This probe is located in the suction chamber 105, in close proximity of the case 1 13 and is connected to one of the branches of an electrical circuit forming a Wheatstone bridge the other branches of which incorporate respectively, in case the probe 128 is a variable resistor, a variable adjustment resistor R and a pair of balancing resistors R4 and R5.

This circuit is fed through a winding 0 of a transformer T this winding being provided in the junction line 130 of the Wheatstone bridge circuit. Connected in the other junction line 131 is an electrical device adapted to control the supply of current to resistor 121 by which the chamber is heated, this resistor 121 being fed through a winding o of transformer T In the example illustrated this apparatus consists of a triode gas-filled or thyratron tube the perforated grid of which is connected to one end of the junction line 131, the anode and cathode of this tube being connected, the one to the ground and the other to the circuit supplying current to resistor 121, the winding 2 and the other end of the junction line 131 being grounded.

Thus, the humidity content of the supply air introduped into the chamber 105 increases as the value of the resistance of probe 128 decreases.

The bridge is unbalanced and the unbalance voltage is fed to the grid of the thyratron tube 129 which becomes conducting and supplies current to the corresponding heating resistor 121 until the condition of equilibrium is restored in the bridge circuit, that is, until the hygrometric degree of the supply air has resumed the desired value.

Thus, this additional device will automatically and efiiciently stabilize the hygrometric value of the air fed to the drier as reference air at exactly the desired value corresponding to that at which the

probes

8c and 8d have their maximum sensitivity. This value may be set before- Thus, the end of the drying periodhand by adequately adjusting the variable resistor R If the control device is equipped with this complementary stabilizing device, the electrical circuit of the

detectors

8c and 8d does not require an additional resistance R and its short-circuiting means 24 for the manual adjustment. The wiring diagram of this circuit in this case will be that shown in FIGURE 3 or 2.

If desired an amplifier A may be provided between the thyratron tube 129 and the Wheatstone bridge. If the heat to be dissipated by resistor 121 is very considerable, the thyratron tube 129 may be adapted to control a relay 132 controlling in turn the supply of current to the heating resistor 121 (see the wiring diagram of FIGURE 13).

In another form of embodiment, the drier may comprise only one hygrometric probe 8 similar to the

probes

8a and 812, or 8c and 8d of the preceding form of embodiment. In this case the hair drying apparatus may he of the type illustrated in FIGURES 7 to 9, but the enclosure 113 provided with two compartments for twin probes is replaced with an enclosure 113a without compartment wherein a single probe 8 is mounted.

The two

ducts

111 and 112 communicate with the enclosure 113a through two small bent pipes 1110 and 112a. A valve 114 registers with the orifices by which these branch pipes communicate with the case 113. This valve 114 is adapted to close one or the other of these orifices, adequate control means being provided for operating this valve. If desired, a push-button (not shown) may be provided to this end and the valve in its normal position closes the orifice leading from pipe 112 while allowing the flow of used air to circulate from pipe 111 to case 113.

The probe 8 may be connected with the control electrical circuit for example of the type illustrated in FIG- URE 15. This circuit consists of a Wheatstone bridge comprising'in its other branches balance resistors R6, R7 and R8. This circuit comprises a potentiometer P, and its lead out wires 12 and 13' are connected either to an electromagnet controlling the drying apparatus, as in the case of the circuit of FIGURE 3, or to a galvanometer G of which the dial 116 is mounted on the top of the drying helmet. The characteristics of this electrical circuit are such that it is balanced when the resistance value of probe 8 is that obtaining when this probe contacts air having a humidity content corresponding to the hygrometric degree at which its sensitivity is maxi mum.

Having firstly adjusted the potentiometer P in its intermediate position, the drying apparatus is started empty in order to attain its operating temperature, whereafter, by properly adjusting the thermostat 122 (see FIGURE 9) the feed air heating system is regulated to set the galvanometer pointer close to its zero position, that is, in front of a reference mark provided in the region E of the dial (see FIGURE 7). Thus, the chamber 105 is heated to the degree necessary to bring the air fed to the drier and taken from the surrounding atmosphere, to the hygrometric value at which the probe 8 has its maximum sensitivity.

After this adjustment step, the drying operation proper may take place. The valve 114 being set in the position shown in thick lines in FIGURE 14, that is, the position in which the used air is caused to impinge against the probe 8, immediately as the helmet 102 has been placed on the head of the person, Whose hair is to be dried, the galvanometer pointer starting from the dial region corresponding to the maximum condition of unbalance in the Wheatstone bridge circuit, due to the reduction of the value of the resistance offered by the probe 8 as the latter is now placed within a stream of air having a high hygrometric value.

As the drying operation proceeds the galvanometer pointer moves back to region E. The operator may check whether the drying operation takes places correctly, with due consideration for the hygrometric value of the feed air, by actuating from time to time the manual control member associated with the valve 114 so as to reverse the position of this valve and deliver air taken from the surrounding atmosphere through pipe 111 to the case 113 in which the probe 8 is mounted. Each time air from the surrounding medium is delivered to the probe, the galvanometer pointer moves back to the dial region E.

When the drying operation is about to be completed, the pointer becomes stabilized in this region B and the probe is then in its maximum sensitivity condition. However, if during the drying operation a change occurred in the hygrometric state of the surrounding atmosphere, the pointer will become stabilized in the region B, but not on the reference line previously used for gauging or pre-setting purposes.

The fact that the galvanometer pointer remains in the same position whether air taken from the surrounding atmosphere or used air from the recycle circuit flows the probe 8, means that the drying operation is completed. As a matter of fact this equality in the indica tions resulting from difierent air sources means that the used or re-cycle air does not contain any more an excess of humidity with respect to the room atmosphere, and that consequently the hair has attained the proper degree of dryness beyond which it would become overdried it the drying operation were continued.

According to a modified form of the embodiment of the drier described hereinabove the manual control device associated with the valve 114, for example a pushbutton or the like, is replaced by an automatic device whereby the valve will be positioned now in one position, now in the other position. This device may be mechanical, electromagnetic or other character, or consist of a bimetal heated by an electric resistor having a certain thermal inertia. The frequency of operation of this automatic reversing device may be such that the valve 114 remains during a time period of the order of 5 to 10 seconds in each of its successive or alternate positions.

During the operation of the drier according to this specific form of embodiment of the invention, the galvanometer pointer will oscillate at the same frequency as the valve 114 is reversed. During the initial period of the drying operation, when used or re-cycle air is caused to flow around the probe 8, the pointer will move completely to the right-hand side of the dial, and is returned to the region B thereof each time the probe receives air from the surrounding atmosphere. However, the amplitude of oscillation of this pointer decreases as the hair humidity content decreases. When the hair has been dried completely, the pointer stops oscillating and becomes stabilized in the dial region E.

Thus, when using a drier constructed according to this modified form of embodiment, the operator has simply to check now and then the oscillation of the galvanometer pointer, and when the oscillation stops and the pointer becomes stabilized the operator knows that the air is properly dried.

The drying operation controlled by this device is very accurate for although it comprises only one detector probe, the control action is subordinate, however, to the exact hygrometric state of the ambient airfed to the apparatus, as this hygrometric degree may differ considerably from one case to another and may vary even during the drying operation proper.

In a first form of embodiment of this control device provided with a single detector probe, there is provided an auxiliary device for stabilizing the hygrometric degree of the feed air supplied to the chamber of the apparatus, that is, a device of the type illustrated in FIG-

URE

12 or 13.

In a second modification of the control device provided with a single detector probe 8, this probe is connected in an electrical control circuit similar to that shown in FIG- 1 1 URE 11, that is, comprising an additional resistor R3 adapted to be short-circuited for checking the adjustment of the device. FIGURE 16 shows this circuit, in the form of a Wheatstone bridge, comprising in one branch the probe 8 and the resistor R3 as well as the short-circuiting conductor 124, and in the other branches balancing resistors R6, R7 and R8. The

outlet conductors

12 and 13 lead either to electromagnets controlling the stoppage of the drying operation in the circuit of FIGURE 11, or to a galvanometer G of which the dial 110 is mounted on the helmet. The short-circuit control switch 125 is controlled by a press-button 126 mounted on the helmet.

As in the specific case of the device provided with the control circuit of FIGURE 11, the operator may check the adjustment at will by depressing the press-button 126 to short-circuit the additional resistor R3. As already explained hereinabove, the short-eircuiting of this resistor, while reducing the resistance of the corresponding circuit branch, should cause the galvanometer pointer to move to the dial region S. If the pointer oversteps this position, or if on the. contrary it cannot attain it, this means that the conditions of operation do not correspond to the basic conditions consistent with the preset sensi tivity of the probe, or in other words, that the hygrometric degree. of the feed air is not the proper one. 7 Therefore, the operator must increase or decrease the heating of the feed chamber 165 of the drawing apparatus through the resistor 121 by depressing the thermostatcontrolling button 123. In the difierent forms of embodi ment of the hair drying apparatus of this invention, the control device with twin detector probes or one detector probe may, instead of automatically stopping the apparatus at the end of the drying operation, may actuate an alarm device of the sound or luminous type, to warn the operator that the drying operation is completed so that he may stop the drier at the proper time. This alarm device may be controlled by a photoelectric cell actuating the pointer of the galvanometer connected in the control circuit of the detector or detectors.

This apparatus may also consist of a light-ray syntony indicator lamp of which the light ray becomes continuous when the circuit is stabilized, so that it constitutes a visual-type warning device, this lamp being connected in this casein the circuit as a substitute for the galvanometer. Of course, this warning apparatus may be combined with a system stopping automatically the operation of the drying apparatus, or if desired, it may be combined with this apparatus.

If desired, if the control device. comprises two moisture detectors, these detectors may consist of thermometers, thermo-couples or thermistances, as the dry temperatures of the two airs are equal. These two detectors may be combined to permit a direct reading (for example two bulb thermometers controlling two concentric pointers displaceable on a common dial may be used), or for actuating a sound or luminous alarm device, or for controlling .the stoppage of the drier at the end of the hair drying operation. The two pointers indicating the moist temperatures (ambient air and used air) may be replaced with two discs each formed with a slit at the pointer location; if a lamp is placed within the casing behind the dial, a light ray will appear when the slits are superposed to each other, that is, when the temperatures become equal, thus indicating the end of the, drying operation. A photoelectric cell or a photo-transistor disposed at a proper location may control the stoppage of the drier by means ofa relay.

What I claim is:

1. A hair drying apparatus comprising, in combination, a hair drying hood; first passage means for directing a stream of ambient air along a first path into said hood; means for directing air in said hood onto the. hair to be dried while simultaneously heating said air; second passage means for directing a stream of used air which has passed through the hair to be dried along a second separate path; means for changing the temperature of at least one of said streams until both are at substantially the same temperature at predetermined portions of said paths while transferring heat from said used air to at least part of said stream of ambient air entering said hood; humidity detecting means cooperating with the air passing through said predetermined portions of said paths for detecting the humidity of the stream of used air and the humidity of the stream of ambient air, while both of said streams of air are of substantially the same temperature so that the diflierence between the humidity of the used air and that of the ambient air may be determined; and additional passage means communicating with said second passage means and with said means for directing air in said hood onto the hair to be dried for recirculating at least a portion of said used air onto the hair to be dried.

2. A hair drying apparatus comprising, in combination, a hair drying hood; first passage means for directing a stream of ambient air along a first path into said hood; means for directing air in said hood onto the hair to be dried while simultaneously heating said air; second passage means for directing a stream of used air which has passed through the hair to be dried along a second separate path, said first and second passage means being in direct heat exchanging relation with each other so that heat from said used air stream is transferred to said stream of ambient air until both air streams are of substantially the same temperature at predetermined portions of said paths; humidity detecting means cooperatingwith the air passing through said predetermined portions of said paths for detecting the humidity of the stream of used air and the humidity of the stream of ambient air, while both of said streams of air are of substantiially the same temperature, so that the diiference between the humidity of the used air and that of the ambient air may be determined; and additional passage means communicating with said second passage means and with said means for directing air in said hood onto the hair to be dried for recirculating at least a portion of said used air onto the hair to be dried.

3. A hair drying apparatus comprising, in com-bination, a hair drying hood; first passage means for directing a stream of ambient air along a first path into said hood; means for directing air in said hood onto the hair to be dried while simultaneously heating said air; second passage means for directing a stream of used air which has passed through the hair to be dried along a second separate path, at least part of said first passage means extending through at least part of said second passage means so that heat from said stream of used air is transferred to said stream of ambient air until both air streams are of substantially the same temperature at predetermined portions of said paths; humidity detecting means cooperating with the air passing through said predetenmined portions of said paths for detecting the l1't1- midi ty of the stream of used air and the humidity of the stream of ambient air, while both of said streams of air are of substantially the same temperature, so that the difference between the humidity of the used air and that of the ambient air may be determined; and additional passage means communicating with said second passage meansand with said means for directing air in said hood onto the hair to be dried for recirculating at least a portion of said used air onto the hair to be dried.

4. A hair drying appanatus comprising, in combination, a hair drying hood; first passage means for directing a stream of ambient air along a first path into said hood; means for directing air in said hood onto the hair to be dried while simultaneously heating said air; second passage means for directing a stream of used air which has passed through the hair' to be dried along a second separate path; means for changing the temperature of at least one of said streams until both are at substantially the same temperature at predetermined portions of said paths while transferring heat from said used air to 13 at least part of said stream of ambient air entering said hood; a pair of humidity detecting means respectively located at said portions of said paths for respectively detecting the humidity of the stream of used air and the humidity of the stream of ambient air while both of said streams of air are of substantially the same temperature; indicator means associated with said pair of humidity detecting means for indicating the difference between the humidity of the used air and that of the ambient air; and additional passage means communicating with said second passage means and with said means for directing air in said hood onto the hair to be dried for recirculating at least a portion of said used air onto the hair to be dried.

5. A hair drying apparatus comprising, in combination, a hair drying hood; first passage means for directing a stream of ambient air along a first path into said hood;

means for directing air in said hood onto the hair to be dried While simultaneously heating said air, said means including air moving means and heater means for heating the air moved by the air moving means; second passage means for directing a stream of used air which has passed through the hair to be dried along a second separate path, said first and second passage means being in direct heat exchanging relations with each other so that heat from said used air stream is transferred to said stream of ambient air until both air streams are of substantially the same temperature at predetermined portions of said paths; humidity detecting means cooperating with the air passing through said predetermined portions of said paths for detecting the humidity of the stream of used air and the humidity of the stream of ambient air While both of said streams of air are of substantially the same temperature; means cooperating with said humidity detecting means and operatively connected to said air moving means for shutting off the latter when the humidity of the stream of used air is substantially equal to that of the ambient air; and additional passage means communicating with said second passage means and with said means for directing air in said hood onto the hair to be dried for recirculating at least a portion of said used air onto the hair to be dried.

6. A hair drying apparatus comprising, in combination, a hair drying hood; a main duct communicating with the ambient air and the interior of said hood; air moving and directing means for directing the air entering the hood through said main duct onto the hair to be dried; heating means coordinated with said air moving and directing means for heating the air before it is passed onto the hair .to be dried; first conduit means communicating with said air moving and directing means upstream of said heating means for directing part of the air along a first path; second conduit means for directing a stream of used air which has passed through the hair to be dried along a second separate path, said first and second conduit means extending closely adjacent to each other in direct heat exchanging relation through part of said main duct so that both air streams passing through said conduit means will reach at a predetermined portion of said paths substantially the temperature of the air passing through said main duct while heat is transferred from said stream of used air to said stream of air passing through said first conduit means and to the air passing through said main duct; humidity detecting means cooperating with the air passing through said predetermined portions of said paths for detecting the humidity of the used air passing through said second conduit means and the humidity of the air passing through said first conduit means at said predetermined portions of said paths; and additional heating means in said main duct upstream of said portions of said paths for controlling the temperature and therewith the humidity of the air entering through said main duct.

7. A hair drying apparatus comprising, in combination, a hair drying hood; a main duct communicating with the ambient air and the interior of said hood; air moving 14 and directing means for directing the air entering the hood through said main duct onto the hair to be dried; heating means coordinated with said air moving and directing means for heating the air before it i passed onto the hair to be dried; first conduit means communicating With said air moving and directing means upstream of said heating means for directing part of the air along a first path; second conduit means for directing a stream of used air which has passed through the hair to be dried along a second separate path, said first and second conduit means extending closely adjacent to each other in direct heat exchanging relation through part of said main duct so that both air streams passing through said conduit means will reach at a predetermined portion of said paths substantially the temperature of the air passing through said main duct while heat is transferred from said stream of used air to said stream of air passing through said first conduit means and to the air passing through said main duct; humidity detecting means cooperating With the air passing through said predetermined portions of said paths for detecting the humidity of the used air passing through said second conduit means and the humidity ofthe air passing through said first conduit means at said predetermined portions of said paths; additional heating means in said main duct upstream of said portions of said paths for controlling the temperature and therewith the humidity of the air entering through said main duct; and means cooperating with said air moving and directing means for recirculating at least part of said used air past said first mentioned heating means onto the hair to be dried.

8. A hair drying apparatus comprising, in combination, a hair drying hood; a main duct communicating with the ambient air and the interior of said hood; air moving and directing means for directing the air entering the hood through said main duct onto the hair to be dried; heating means coordinated with .said air moving and directing means for heating the air before it is passed onto the hair to be dried; first conduit means communicating With said air moving and directing means upstream of said heating means for directing part of the air along a first path; second conduit means for directing a stream of used air which has passed through the hair to he dried along a second separate path, said first and second conduit means extending closely adjacent to each other in direct heat exchanging relation through part of said main duct so that both air streams passing through said conduit means will reach at a predetermined portion of said paths substantially the temperature of the air passing through said main duct while heat is transferred from said stream of used air to said stream of air passing through said first conduit means and to the air passing through said main duct; an electrical heating resistor in said main duct upstream of said portions of said paths for controlling the temperature and therewith the humidity of the air entering through said main duct; an electrical circuit for energizing said electrical heating resistor in said main duct; and humidity detector means in said main duct and operatively connected to said electrical circuit for controlling said electrical heating resistor as a function of the humidity of the entering the main duct so as to maintain a preselected value of humidity in said main duct.

9. A hair drying apparatus comprising, in combination, a hair drying hood; a main duct communicating with the ambient air and the interior of said hood; air moving and directing means for directing the air entering the hood through said main duct onto the hair to be dried; heating means coordinated with said air moving and directing means for heating the air before it is passed onto the hair to be dried; first conduit means communieating with said air moving and directing means upstream heat exchanging relation through part of said main duct so that both air streams passing through said conduit means will reach at apredetermined portion of said paths substantially the temperature of the air passing through said main duct While heat is transferred from said stream of used air to said stream of air passing through said first conduit means and to the air passing through said main duct; an enclosure located in said main duct in the region of said predetermined portions of said paths and having a dividing wall dividing said enclosure into two compartments, said first and second conduit means communicating respectively with said compartments; a pair of humidity detecting means respectively located in said compartments for detecting respectively the humidity of the stream of used air passing through said second conduit means and the stream of air passing through said first conduit means while both of said streams of air are of substantially the same temperature; indicator means operatively connected to said pair of humidity detecting means for indicating the difference between said humidities; and additional heating means in said main duct upstream of said portions of said paths for controlling the temperature and therewith the humidity of the air entering through said main duct.

10. A hair drying apparatus comprising, in combination, a hair drying hood; a main duct communicating with the ambient air and the interior said hood; air moving and directing means for directing the air entering the hood through said main duct onto the hair to be dried; heating means coordinated with said air moving and directing means for heating the air before it is passed ontothe hair to be dried; first conduit means communicating with said air moving and directing means upstream of said heating means for directing part of the air along a first path; second conduit means for directing a stream of used air which has passed through the hair to be dried along a second separate path, said first and second conduit means extending closely adjacent to each other in direct heat exchanging relation through part of said main duct so that both air streams passing through said conduit means will reach at a predetermined portion of said paths substantially the temperature of the air passing through said main duct While heat is transferred from said stream of used air to said stream of air passing through said first conduit means and to the air passing through said main duct; an enclosure located in said main duct in the region of said predetermined portions of said paths; 2. single humidity detecting means in said enclosure; means for alternatively placing said enclosure in communication with said predetermined portion of the other path, for successively ascertaining by means of said single humidity detecting means the humidity of the used air passing through said second conduit means and the humidity of the stream of air passing through said first conduit means while both of said streams of air are of substantially the same temperature; indicator means operatively connected to said single humidity detecting means so that the difference between said humidities may be determined; and additional heating means in said main duct upstream of said portions of said paths for controlling the temperature and therewith the humidity of the air entering through said main duct.

11. A hair drying apparatus comprising, in combination, a hair drying hood; first passage means for directing a stream of ambient air along a first path onto said hood; means for directing air in said hood onto the hair to be dried While simultaneously heating said air; second passage means for directing a stream of used air which has passedthrough the air to be dried along a second separate path; means for changing the temperature of at least one of said streams until both are at substantially the same temperature at predetermined portions of said paths while transferring heat from said used air to at least part of said stream of ambient air entering said hood; an enclosure adjacent said hood; a single humidity detecting means in said enclosure; a pair of pipe elements respectively communicating with said predetermined portions of said paths and the interior of said enclosure; valve means for alternatively closing said pipe elements; means for actuating said valve means so that said humidity detecting means will alternatively detect the humidity of the stream of used air and the humidity of the stream of ambient air while said streams of air are of substantially the same temperature; indicator means associated with said humidity detecting means so that the ditterence between said humidities can be determined; and additional passage means communicating with said second passage means and with said means for directing air in said hood onto the hair to be dried for recirculating at least a portion of said used air onto the hair to be dried.

12. A hair drying apparatus comprising, in combination, a hair drying hood; first passage means for directing a stream of ambient air along a first path into said hood; means for directing air in said hood onto the hair to be dried while simultaneously heating said air; second passage means for directing a stream of used air which has passed through the hair to be dried along a second separate path; means for changing the temperature of at least one of said streams until both are at substantially the same temperature at predetermined portions of said paths while transferring heat from said used air to at least part of said stream of ambient air entering said hood; an enclosure adjacent said hood; humidity responsive electrical variable resistor means located in said enclosure for providing resistance which is indicative of the humidity in the air contacted by said resistor means; valve means for alternatively connecting the interior of said enclosure with said predetermined portion of one path and with said predetermined portion of the other path in order to detect through the same variable resistor means the humidity of the used air and the humidity of the am.- bient air While both of said streams of air are at substantially the same temperature; an electric bridge circuit having a branch in which said resistor means is located and having balancing resistors in the other branches of said circuit; electrical meter means electrically connected with said bridge circuit for indicating the dilference between said humidities; and additional passage means communi eating with said second passage means and with said means for directing air in said hood onto the hair to be dried for recirculating at least a portion of said used air onto the hair to be dried.

13. A hair drying apparatus comprising, in combination, a hair drying hood; first passage means for directing a stream of ambient air along a first path into said hood; means for directing air in said hood onto the hair to be dried while simultaneously heating said air, said means including a fan and an electric motor for driving said fan; second passage means for directing a stream of used air which has passed through the hair to be dried along a second separate path; means for changing the temperature of at least one of said streams until both are at substantially the same temperature at predetermined portions of said paths While transferring heat from said used air to at least part of said stream of ambient air entering said hood; a pair of humidity responsive electrical resistor means located respectively at said portions of said paths for providing resistance which is indicative of the humidity in the air contacted by said resistors; an electric bridge circuit having branches in which said resistor means are respectively located and balancing resistors in the other branches of said circuit; and relay switch means connected electrically with said motor for energizing the latter when said switch means is closed, said switch means being electrically connected with said bridge circuit to be automatically opened when said pair of resistor means have the same resistance, whereby said motor will be stopped when the humidity of the used air is equal to the humidity of the ambient air.

14. A hair drying apparatus comprising, in combinaaosaeso 17 tion, a hair drying hood; a main duct communicating with the ambient air and the interior of said hood; air moving and directing means for directing the air entering the hood through said main duct onto the hair to be dried; heating means coordinated with said air moving and directing means for heating the air before it is passed onto the hair to be dried; first conduit means communicating with said air moving and directing means upstream of said heating means for directing part of the air along a first path; second conduit means for directing a stream of used air which has passed through the hair to be dried along a second separate path; said first and second conduit means extending closely adjacent to each other in direct heat exchanging relation through part of said main duct so that both air streams passing through said conduit means will reach at a predetermined portion of said paths substantially the temperature of the air passing through said main duct While heat is transferred from said stream of used air to said stream of air passing through said first conduit means and to the air passing through said main duct; 21 pair of humidity responsive electrical variable resistor means, said pair of resistor means being respectively located at said portions of said paths for providing electrical resistances which are respectively indicative of the humidity of the used air passing through said second conduit means and the humidity of the air passing through said first conduit means at said predetermined portions of said paths at which said streams of air are of substantially the same temperature; a bridge circuit having a pair of branches in which said pair of resistor means are respectively located, said bridge circuit including an additional resistor in series with the resistor means located in the path of used air, and means for short circuiting said additional resistor, said additional resistor having a resistance substantially equal to the difference between the resistance of the resistor means in the used air stream when this used air stream is substantially dry and the resistance of said resistor means when the used air stream has a humidity content exceeding a predetermined value; and electrical meter means electrically connected to said bridge circuit for indicating the difference between the resistance of said resistor means to indicate thereby the difference between the humidities of the air contacting said resistor means, respectively, whereby during initial dry run of the apparatus said additional heating means in said main duct may be properly set by reading the meter with said additional resistor respectively in circuit and short circuited.

References Cited in the file of this patent UNITED STATES PATENTS 2,643,464 Hadady June 30, 1953

Claims

1. A HAIR DRYING APPARATUS COMPRISING, IN COMBINATION, A HAIR DRYING HOOD; FIRST PASSAGE MEANS FOR DIRECTING A STREAM OF AMBIENT AIR ALONG A FIRST PATH INTO SAID HOOD; MEANS FOR DIRECTING AIR IN SAID HOOD ONTO THE HAIR TO BE DRIED WHILE SIMULTANEOUSLY HEATING SAID AIR; SECOND PASSAGE MEANS FOR DIRECTING A STREAM OF USED AIR WHICH HAS PASSED THROUGH THE HAIR TO BE DRIED ALONG A SECOND SEPARATE PATH; MEANS FOR CHANGING THE TEMPERATURE OF AT LEAST ONE OF SAID STREAMS UNTIL BOTH ARE AT SUBSTANTIALLY THE SAME TEMPERATURE AT PREDETERMINED PORTIONS OF SAID PATHS WHILE TRANSFERRING HEAT FROM SAID USED AIR TO AT LEAST PART OF SAID STREAM OF AMBIENT AIR ENTERING SAID HOOD; HUMIDITY DETECTING MEANS COOPERATING WITH THE AIR PASSING THROUGH SAID PREDETERMINED PORTIONS OF SAID PATH FOR DETECTING THE HUMIDITY OF THE STREAM OF USED AIR AND THE HUMIDITY OF THE STREAM OF AMBIENT AIR, WHILE BOTH OF SAID STREAMS OF AIR ARE OF SUBSTANTIALLY THE SAME TEMPERATURE SO THAT THE DIFFERENCE BETWEEN THE HUMIDITY OF THE USED AIR AND THAT OF THE AMBIENT AIR MAY BE DETERMINED; AND ADDITIONAL PASSAGE MEANS COMMUNICATING WITH SAID SECOND PASSAGE MEANS AND WITH SAID MEANS FOR DIRECTING AIR IN SAID HOOD ONTO THE HAIR TO BE DRIED FOR RECIRCULATING AT LEAST A PORTION OF SAID USED AIR ONTO THE HAIR TO BE DRIED.