US3117311A - Fire detectors - Google Patents

Description

Jan. 7, 1964 F. c. LEMAIRE 3,117,311

FIRE DETECTORS Filed June. 19, 195i 4 Sheets-Sheet 1 Fig.2

- o 20 6o t(c) Jan. 7, 1964 F. c. LEMAIRE 3,117,311

FIRE DETECTORS Filed June 19, 1959 4.Sheets-Sheet s F. c. LEMAIRE 3,117,311

Jan. 7, 1964 FIRE DETECTORS Filed June 19, 1959 4 Sheets-Sheet 4 j i .D 7'71 70 A11. ARM PEA/ Y INVEN TOR.

ATTORNEYS Fem/s c. A swam/s United States Patent 3,117,311 FIRE DETECTORS Frans C. Lemaire, 115 Rue du Cuirasse, Ghent, Belgium Filed June 19, 1959, Ser. No. 821,442 Claims priority, application Belgium June 21, 1958 5 Claims. (El. 340233) The present invention relates in general to electronic fire detectors and in particular to an alarm circuit for detecting a predetermined rate of temperature rise or a predetermined temperature rise.

Fire detectors can be based'on one of two criteria the thermometric criterion, or the exceeding of a critical temperature considered dangerous, or the velocimetric criterioni.e., an excessively rapid rising in the temperature of the area to be supervised. Apparatus based on the first criterion usually make use of the expansion of a temperature-sensitive member which is used similarly to the expanding substance of a thermometer. A temperature change therefore manifests itself mainly by a deformation or an increase in volume leading to the physical movement of a mechanical member or to the opening or closure of a contact. Such an apparatus must not trip below a minimum temperature, otherwise it will set off alarms accidentially.

Whatever the physical construction of apparatus of the kind specified, the thermometric criterion is of doubtful value, because the time taken to reach the required critical temperature varies considerablywith the initial temperaturei.e., the normal temperature. Such detectors therefore usually provide secondary supervision.

Velocimetric apparatusi.e., apparatus sensible to the rate of temperature rise-are more reliable for they respond to a rate of increase and are therefore basically independent of normal temperature conditions. As a rule, velocimetric detectors use the same elements as the absolute detectors but the temperature-sensitive elements are doubled and, although mounted close together, are differentiated by their thermocoupling with the surroundings to be supervised. One sensitive element is directly associated with the surroundings, while the other sensitive element is suitably isolated therefrom. Hence, if the ambient temperature rises sufficiently rapidly, the temperature of each sensitive element follows a different pattern. The delay in the temperature of the isolated element becoming adapted to the new ambient temperature is greater in proportion as the latter element is better protected and as the temperature rises more rapidly. If this difference in thermal adaptation can be made obvious, .a velocimetric detector is provided. Slow temperature variations will reduce the difiere'nces between the temperature of the elements, the difference between the physical conditions of the elements will therefore be negligible, and no warning signal will be given.

Sensitive elements such as solids, springs, filaments, strips, etc., or liquids such as mercury, alcohol, etc., or gases such as air, nitrogen, etc. may be utilized.

Temperature differences between solids are indicated by the relative movement of members connected to such solids. When liquids are employed, vessels are used which communicate with one another and which are insulated differently from the external surrounding medium. The vessels contain a liquid which vaporizes at ordinary temperatures, and due to the slight temperature difference when the; branches are externally heated, the difference in insulation leads to a difference in vapour pressure. The difference in vapour pressure in turn leads to a difference in liquid level, with the result that an electric contact is operated.

Similarly, with gases, two seal-tight enclosures filled with gas, such as air, nitrogen or the like, are also difierently insulated from the surroundings. A rapid temperature variation leads to a pressure difference operating, for instance, a differential manometer which, in turn, operates a mechanical member or an electric contact.

Electrical apparatus operating along the same lines consists of a circuit which comprises one or more temperature-sensitive elements connected into one arm of a Wheatstone bridge, a similar number of identical electrical elements being placed in' the opposite arm. 'However, the last-mentioned elements are thermally insulated from the surroundings or have an elevated heat inertia, for instance, by virtue of their mass. When the temperature varies fairly slowly, both systems vary similarly for the same temperature rise, but their resistance differ from one another when the temperature rises rapidly. In the first case, the bridge remains substantially balanced, while in the second case an out-of-balance voltage appears which can be used as a warning signal.

In other apparatus, temperature variations are applied to a chamber exposed to the surrounding medium. The

temperature variations lead to an excess pressure being applied to a controlled aperture valve, so adjusted that the excess gas associated with reduced rates of temperature variation can escape to atmosphere or to a compensating chamber (apparatus comprising a compensated tubular aerothermal chamber), with the result that there is scarcely any rise in the pressure in the main chamber.

However, rapid temperature variations lead to a considerable excess pressure which cannot be neutralized by the valvenotwithstanding the increased rate of gas fiow therethrough. The excess pressure operates a suitable pressure gauge which in turn operates a control member or electrical contact.

Apparatus of the kind specified can be highly sensitive, but high sensitivity is achieved at the cost of stability, for in very sensitive apparatus it is difficult to provide a stable boundary between the operative and the inoperative states.

Also, high mechanical precision must be embodied in expansion apparatus, since the force applied by the expanded elements are reduced a significant amount. An inevitable result of mechanical precision is high sensitivity to shocks and atmospheric agents such as salt air, dusty or corrosive atmosphere, dampness, etc. Similar considerations apply to apparatus having progressive escape valves, in which accidental blockage occur which are often invisible and, for that reason, all the more difficult to locate. When intended for use on' ships, such apparatus is usually provided with an anti shock mechanical suspension and made completely seal-tight. As a result, sensitivity is usually reduced, dimensions are incerased and the appearance is not very aesthetic.

Finally, apparatus using bimetallic strips, trimetallic strips, filaments, springs, diaphragms, etc., often goes out of order due to a slow change in the nature of the metal (aging, recrystallization, creep, slow deformation due to the variation of internal strains) or to a lack of seal-tightness.

Furthermore, if there is a considerable and rapid drop in temperature the very nature of vel'ocimetric detectors prevents them from immediately adapting themselves to the new heat conditions. Hence, while they are adapting themselves their velocimetric sensitivity is bound to be reduced. This case occurs, for instance, when the place to be protected is suddenly subjected to an outside winter temperature (opening of the doors of hangers, sheds, etc.).

Detectors in which the temperature gives rise to purely electrical effects in the sensitive element form a class apart. Amongst such devices are thermo-couple devices; the pot and cold junctions are differently insulated from the surroundings so that fairly rapid temperature 3 variations produce a potential difference in the circuit due to the behaviour of the junctions, and the potential difference is used as a warning signal.

Another device uses a thermistor supplying a relay and is based on the fact that under certain circumstances the thermistor may have a negative resistance. When cold the thermistor operates in the linear part of its voltagecurrent characteristics; its resistance is high and the current is very low. When the temperature rises above a certain level, the resistance of the thermistor decreases and the current rises, and this state of affairs is amplified due to dissipation by Joules effect. The thermistor finally operates in the negative part of its characteristic and the current rapidly rises to the value required to trip the warning relay.

The novel electronic detectors to be described hereinafter have all the possibilities of conventional detectors but, due to the inherent flexibility of their circuits, they can be readily adapted to special detection requirements. This applies not only to giving a warning but also to the immediate indication of faults in the detectors themselves.

To show the novelty of the operating principles and properties of the apparatus according to this invention, a survey will be made of the operational role of various elements of conventional detectors.

Some known velocimetric devices give a warning on the basis of a comparison between an inoperative state, when the temperature of the sensing element or elements is stationary, and a new thermal condition associated with the start of a fire and distinguished by a temperature rising at an excessive rate. This comparison implies that a semi-permanent memory element is always available which at any give moment records the thermal conditions during a specified and immediately previous period of about 30 to 60 seconds or several minutes, while gradually becoming adapted to the new thermal conditions which are slowly being established. In most apparatus this memory element takes the form of a second temperaturesensitive sensing element which doubles the actual supervisory sensing element. However, the thermal change of the secondary sensing element is delayed as compared with the thermal change of the main element because the secondary element is not subjected to heating in the same way as the main element. The delay is produced either by additional thermal insulation or by appropriately increasing the thermal mass of the sensitive element. The arrangement of sensitive elements is so contrived that when the difference between the thermal changes of the elements reaches a value considered excessive, such difference produces an electrical or mechanical effect which can be indicated and fixed. On the other hand, when the heat change of the supervised area is slow, the thermal delay of the secondary element is reduced and cannot cause any noticeable effect.

Other detectors comprise only one sensitive element so arranged that the physical state and sensitive characteristics of the element (as a rule, the pressure of a gas or of air) are continuously adapted to slow temperature variations, whereas with rapid temperature variations there is an increasing delay in adaptation leading to a physical change which can be demonstrated.

In the electronic devices to be described hereinafter the compensation for a slow temperature increase is provided by the slow discharge of a condenser, the electric circuit not delivering any signal. However, when the rate of temperature increase exceeds a value considered excessive, the condenser discharge rate becomes inadequate and a warning signal is given. Hence the condenser and an associated discharge circuit form the semi-permanent memory element hereinbefore described. As will be described hereinafter, the condenser also acts as an element for decoupling and concentrating a number of detector circuits in the same indicating circuit.

The electronic detectors to be described hereinafter are mixed detectors having both thermometric and velocimetric sensitivity. If required, however, they may have only one kind of sensitivity. If they have velocimetric sensitivity, thermal compensation for slow heat changes is progressive and is provided by the slow discharge of the condenser.

The detectors according to this invention each comprise only one sensing element which can, however, be subdivided into a number of units distributed in the area to be supervised. The sensing element comprises a temperature-sensitive element associated with a heat collector. Since the mass of the sensitive element is grealy reduced, an assembly comprising a sensing element and a collector is no bigger than the diameter of a large coin.

The present novel detector is distinguished by the follOWing exclusive features:

(1) The sensitive element, hereinafter referred to as the sensing element, is completely static. It is of very small dimensions, has low thermal inertia and consists of a substance insensitive to air composition and to dust. The heat collector to which it is secured can be of reduced dimensions. Detector elements can therefore be of discrete appearance yet have the required sensitivity; also, the sensing element is completely unaffected by shock, vibration, dust and the ambient atmosphere (detectors are subjected to dust in industrial installations and to salt air on board ship).

(2) The sensing element associated with the electronic indicating circuit can have either a thermometric or a velocimetric sensitivity or both, simultaneously, yet independent control thereof is possible.

(3) The semi-permanent memory element providing gradual compensation of a slow temperature rise is a condenser, the charge of which continuously adapts to the temperature of the sensing element, providing that the temperature rise thereof is slow. However, the critical value of the variation rate can be adjusted as required. it rapid drop in temperature is compensated without de- (4) As normally provided, the sensing element is of the point kind, although a number of point elements can be connected in series or parallel in the same circuit. When distributed over the area to be supervised, the sensing elements are the equivalent of a compensated tubular aerothermal detector.

(5) Substantially any number of circuits of the kind just described can be connected to a single centralized warning installation.

(6) The aforesaid compensating condensers not only provide compensation but also act as decoupling elements by blocking the DC. component of the voltage delivered by each of the circuits with which the condensers are associated.

(7) Although a number of detector circuits are concentrated on one central warning device, the velocimetric and thermometric sensitivity of each detector circuit can be individually adjusted, as required, within wide limits.

(8) Any circuit giving a warning is identified through the agency of the central warning device. The identified circuit appears on a central panel as an individual light signal; also a permanent general acoustic warning is operated. Finally, the useless detector circuit is eliminated.

(9) When a detector circuit or one or more sensing elements thereof ceases or cease to operate, even for a very brief time, a warning other than the fire warning is given, the faulty circuit being identified, marked and eliminated as in the previous case. These operations take a short fraction of a second.

The central warning circuit is immediately made available again to the unaffected detector circuits.

(10) The current consumption of the detector circuits and warning circuit is greatly reduced, hence the size of the supply batteries, and the capacity of the charger therefor reduced a substantial amount.

The detectors to be described hereinafter comprise a number of semi-conductor sensing elements, installed in groups or individually, sensitive to the temperature of their surroundings. The elements are electrically connected to an indicating or warning device which interprets the signal from the sensing elements and considers, as required, the rate of variation or the absolute amplitude of such signals or both such rate and such amplitude. The Warning device provides a permanent warning'signal and identifies the detector circuit which has tripped. Also, the warning circuit provides constant supervision of the state of operation of each circuit, and for any technical defect, and identifies the faulty circuit.

Any sensing eleemnt can be adapted to respond either to an excessive rate of temperature rise or to an absolute temperature or to both. The values which are considered to be critical for such rates and absolute temperatures can be set individually in each sensing element or sensing element group associated with a given detector circuit.

The invention will be fully understood from the following description and the drawing in which:

FIG. la is a top view of a temperature sensing element;

FIG. 1b is a sectional view taken along the line X-X, YY of FIG. 1a.;

FIGS. 2, 44, and 5 are graphs of the characteristics of the temperature sensing device;

FIGS. 3, 6 and 17 are diagrams of the temperature sensing circuit;

FIGS. 7 to 9 and 11 are graphs explanatory of the operation of the circuit of FIG. 6;

FIG. is a diagram of the amplifier and relay portion of the circuit according to the invention;

FIGS. 12 to 16 are circuit diagrams of various embodiments of the invention.

A description will hereinafter be given of the various operationally separable elements of the electronic detector of this invention, as follows:

(1) The sensing elements and the corresponding sensing circuits;

(2) The differentiating circuits associated therewith to form detector circuits;

(3) The simple indicating circuit associated with the preceding circuits;

(4) The indicating circuit associated with several grouped detector circuits jointly;

(5) The identification and marking circuit.

I. Sensing Elements-Sensing Circuit These mainly comprise a temperature-sensitive electrical conductor or semiconductor element secured to a suitable heat collector. An example of a. sensing element having a semi-conductor body Th and a collector C of thin sheet copper or aluminium is illustrated in FIGS. 1a and 1b. Of course, other physical construction bringing together the same operating elements can be considered and therefore fall under the invention.

More particularly, the sensing element for supervising temperature variations of a solid such as, for instance, a machine bearing or an electric cable, come within the same classification, for in such a case the solid to be supervised is thermally connected to the sensitive element, for instance, by a plate or flange secured to or around the solid.

It will be hereinafter assumed that the sensitive element is a negative temperature coefiicient resistance, for instance, a thermistor. Of course, sensitive elements having, for instance, a positive coefiicient, .can be used without departing from the scope of the invention. The circuits can be adapted accordingly.

The relationship between the absolute temperature T of the thermistor and its resistance Th can be stated by the formula:

Th-=Ae X%= (in per degrees Kelvin) (2) The thermistor isconnected in series with a resistance and battery to provide a sensing circuit (FIG. 3).

'It will be assumed that the current flowing through Th is never sufiicient to raise the temperature thereof much above ambient temperature.

If the temperature is varied by a value dT (or dt) from a value at which Th equals R, it can readily be shown that the elementary variation of the potential of the point M (FIG. 3) is:

ail ODOZSaEdt volts/ degree =Xdt volts/ degree where X represents the elementary signal corresponding to aone degree temperature variation.

If the same circuit is now considered to be at a temperature at which Th no longer equals R and if K: Th/R, the elementary signal obtained is:

KX t M d The function (K) has the same values for a given value of the variable and its reciprocal. Hence the curve f(K) shown as following a logarithmic scale for K is symmetrical about the abscissa K=I (curve 1 of FIG. 4).

A value of R will be assumed such that K is unity for the mean temperature of the temperature range under consideration. It will be noted in particular that, if K lies between 0.38 and 2.6, the function f(K) varies by not more, than 10% around its mean value in this interval. This means that, with an appropriate choice of temperature coefiicient, detector sensitivity can be constant to within in this interval. For instance, if X=3.3%, the interval extends over 60, for instance, from -15 to +45 In fact, the coefiicient or is not .constant and its variation is less negligible in proportion as the temperature is lower; It can be shown that the sensitivity curve will therefore be the curve 2 shown in FIG. 4 which has substantially the same temperature range between its end values hereinbefore describedi.e., between K and I The value of R will be so chosen that at the mean temperature of the range to be considered, the value of K will be equal to /KK. in the value of R.

The resistance Th can be subdivided into a number of parts forming as many separate sensing elements each having its own collector and distributed over the area to be supervised and connected either in parallel or in se ries in the same circuit. The rest temperature of all the thermistors is therefore substantially identical. If a fire starts, one or more sensing elements will be affected and their resistance will decrease, While the resistance of the unaffected elements does not change. The presence of the inactive elements weakens the signal delivered by the affected elements. All the sensingelements must, of course, be installed in the same thermal surroundings to ensure that the rest temperature (and therefore the inoperative resistance) of all n sensing elements is substantially the same. The sensitivity of the indicating or warning circuit can therefore be increased so that a signal delivered by in affected sensing elements is enough to initiate a Warning signal although m-n elements remain inactive.

volts/degree=XE(K)dt (4) This leads to a slight change These sensing elements, being at the same rest temperature in common thermal surroundings, will hereinafter he referred to as thermally dependent.

In practice, an absolute temperature is bound to be exceeded at a low rate of temperature rise, otherwise the detector circuit would already have responded. It can therefore be assumed that when the critical temperature is reached, all the sensing elements are at substantially the same temperature. Hence, since all the thermistors behave in unison as a single thermistor, the critical level will be exceeded nearly at the same temperature, however many sensing elements are connected to the warning circuit.

Summing up, the differential sensitivity of a detector circuit comprising one or more thermally dependent sensing elements is usually less than the differential sensitivity of a similar circuit comprising only one sensing element subjected to the same heat source, although the decrease in sensitivity is independent of the rest temperature of the arrangement. On the other hand, the sensitivity of an absolute temperature detector circuit is little affected by the number of elements it comprises, provided that they all are mounted in the same thermal surroundingsi.e., that they are thermally dependent.

Conventionally, the sensitivity of a fire detector is defined as the minimum rate (assumed to be constant) of temperature rise required for a warning signal to be given within a specified time, usually 30 to 60 seconds. Of course, this assumption of linear temperature variation is not found in practice but is a convenient bases for comparing the sensitivity of various detectors and makes it possible to state the exact technical requirements of a particular detector circuit. As a rule, practical values of sensitivity lie between 2 and l5/minute. It will be assumed hereinafter that the ambient temperature Ta varies linearly in time. If the collector mass is low, if its thermal conductivity is high and when the thermal coupling with the sensitive element is efficient, the temperature T thereof varies in substantially the same Way after an initial period (FIG. 5). It therefore follows that the electric signal delivered at M by the thermistor in the circuit shown in FIG. 3 is also substantially linear. This signal is applied to the differentiating circuit.

H. Simple Detector Circuit (FIG. 6)

This term denotes an assembly comprising the sensing circuit herein before described and now associated with a specially-designed difierentiating circuit.

The same comprises a condenser C connected in series with a resistance 1' through which flows a current i supplied through a diode D.

The diode and the constant current it delivers will temporarily be neglected. The inoperative potential of the point X is therefore that of the positive pole of the battery. If a voltage which increases linearly at a rate of k volts/ second is applied to the condenser C, the current flowing through the resistance r is a function which is a solution of the following differential equation of equilibrium:

ri+l/Cfidt=kt There are two possible cases:

(1) The voltage k increases during a period which is short or at the most of the same order of magnitude as the time constant ;-C of the circuit and therefore remains constant. The current i therefore varies in accordance with two segments of exponentials which increase and decrease consecutively with the time constant of the circuit.

(2) The voltage k increases during a period which is long relatively to the time constant 1C, and then remains constant.

In this case, the current curve takes the form of a pulse, the flanks of which are segments of exponentials, while the final amplitude is kC amperes (FIG. 8).

8 A complete differentiating circuit is shown in FIG. 6 and will now be considered. It will be assumed that the diode is connected to the potential E". Therefore, and in the absence of any signal at M, the potential of the point X is nearly E", while an inoperative current flows through the resistance r. The value of E must be less than the lowest potential which can be assumed by the point Mi.e., at the highest temperature to which the thermistors may raise; for the condenser C, being of a relatively high capacity, is of the electrolytic kind and should therefore always have a voltage applied across it in the same direction. As will become apparent hereinafter, the best arrangement is that in which the negative pole is connected to X. This value of 15 is therefore fairly low in practice, for example, 10-20% of E.

If a linearly decreasing potential is applied to M due to linear increasing of the temperature of Th, Formula 6 shows that the condenser delivers to the resistance r a current of the kind shown in BT68. 7 or 8.

Provided that the latter current remains less than (EE)/r, no differential signal appears at the point X which remains at the rest potential. On the other hand, if the condenser discharge current rises, the point X will tend to become more negative and the potential of the point X will vary along an exponential path of the kind shown in FIG. 9. It will he noted that when the potential of X has dropped a little, the diode D is biased to cut off and a supplementary reverse current i which is constant and, if the diode is correctly chosen, very low, is taken from the condenser. Such current accelerates the discharge of C in the proportion of i /C volts/ second. However, the effect of the diode D can be eliminated if a signal of i C volts/ second is subtracted from the total signal delivered by the thermistor at M. Similar considerations apply if a diode d is connected, for a purpose to be described hereinafter, across the condenser C in the reverse direction. If the currents of these diodes are i and i respectively, the effect of the diodes can be eliminated by subtracting the current (i +i )/C from the total signal, and the net available signal will hereinafter be considered as k(i +i )/C:k' volts/ second. The minimum variations of the potential of M required to produce a differentiated signal of a specific level is therefore greater when the presence of the diodes is taken into account. Since the reverse current of the diodes is temperature-dependent, the diodes will be such that their maximum reverse current is low relative to the mean current absorbed by r.

In some extreme cases Where the temperature is very high, silicon diodes may be used, in which case It substantially=k.

Summing up, upon the receipt of any incident signal of sufficient variation the normally energized differentiating circuit hereinbefore described delivers an exponential differentiated signal if k'C is greater than i, the latter signal having an asymptotic level of kCr volts.

The dilferentiating circuit therefore acts after the fashion of a variable signal limiter and detector, the detector having a fixed and arbitrarily adjustable threshold (adjustable by the level of i). The differentiating circuit therefore rejects any incident signal having a variation below a specified minimum. In some special cases it may be convenient to connect the resistance r, not to the positive pole of the battery, but to a potential E between E and E (FIG. 12).

The marginal sensitivity of a detector circuit can therefore be defined by a careful choice of i and by acting on E (or E 13 and r.

The time constant of the differentiating circuit is determined by appropriate dimensions of C and r. A suitable combination of these values will provide different warning criteria which can be adapted to particular protection problems.

g. The signal available at X is finally supplied to an indicating amplifier.

III. Simple Indicating or Warning Circuit This circuit serves to show that'a warning signal has been initiated.

It comprises a transistorised current emplifier supplying a polarized relay P as shown in FIG. The relationship between the current delivered by the amplifier and the voltage applied to the input-i.e., across the base of Ta and the emitter of Tb-is shown in FIG. 11 which also shows the operating current limits of the polarized relay. The relay is energized at a clearly-defined input voltage. The relay has an armature which can take up two operating positions and one neutral position. The amplifier provides a gain of from 2000 to 4090. If the relay P is sensitive and can operate, for instance, on 300' to 400 microamperes, the relay will be energized upon receipt of an input signal of little more than a few hundred millivolts and with an input current of a smaller fraction of a milliampere. If the circuit shown in FIG. 10 is associated with the circuit shown in FIG. 6, the damage alarm indication circuit shown in FIG. 12. is produced. Since the current i=:(E E)/r is much greater than the current consumed by the amplifier even at the instant when the relay operates, the amplifier current and the presence of the amplifier can be neglected. The potential at X therefore varies as shown by the curves previously explained, and the appearance of a signal at X depends solely upon the factor k' i/Ci.e., upon whether the rate of temperature rise is high as compared with the maximum normal rise corresponding to i".

The circuit constants are so chosen that the net signal reaches the critical value krC of 200 mv. within a given time for a given excess of rate of temperature rise maintained for such given time. For instance, no appreciable net signal is produced by a rise of 3/rninute, whereas a warning is initiated within 40 seconds by an increase of 40/minute; i.e., by an excess rate of rise of only 1/minute maintained for 1 minute.

If the linear increase of the signal at M is maintained indefinitely, the potential at X decreases along an elongated exponential path (according to the time constant rC) tending towards the value krC. The time constant is adapted to the detection conditions required by the user.

It will be apparent from the foregoing that in the present case It is when DT/dt=l C./minute. The factors 1', C, K, E" can always be combined to provide the required sensitivity. When the thermistor cools, the increase of potential at M causes the condenser C to be recharged through the diode D and resistance r (FIG. 12). Since'the value of this resistance is much lower than that of r, being, for instance, one-thousandth of r, the time constant r C is very low (for instance, less than one second), so that the potential at Y is scarcely affected by the charging current. Hence, when the sensing element is cooled, the circuit is adapted without delay to the new temperature conditions.

The circuit always provides the condenser C with a charge corresponding to sensing element temperature, whether the same is constant or slowly rising or rapidly dropping. It will be readily apparent that the detector circuit is therefore fully sensitive at any time and operates without any delay to show that there has been an abrupt and unexpected temperature rise during cooling of the area under supervision.

The indicating or warning circuit is also required to provide a separate technical incident indication of any complete or partial failure of a detector circuit. These faults are either the short-circuiting, earthing or accidental opening of one or more sensing elements in any way associated with the circuit. Whatever the fault, it is seen by the point M as an abrupt potential variation in one or the other direction (FIG. 13). Since the variation is abrupt, it is transmitted without attenuation to the point X to which two technical incident indication transisors T and T are connected through condensers c and c (FIG. 13). As can be seen in FIG. 13, the latter transistors must be of complementary polarity, since technical warning signals may be of either polarity. The transis'tors are operated by the differentiating circuits formed by r 0 and r 6 respectively. The time constants r 0 and r 0 of these circuits are very low so that the current produced even by maximum potential variation at X due to heating of the thermistors is insufficient to open the transistors. These amplifiers therefore remain inoperative, in particular forany fire alarm indication. On the other hand, a technical incident signal, even if of low amplitude, is transmitted with its complete initial amplitude and applies to the transistors a large exponential signal. Nevertheless, such signal lasts long enough to operate one of the corresponding technical incidents alarm relays AT or AT shown in FIG. 13.

The low time constants of the circuits associated with the transistors T and T are produced by reducing c and c for instance, to one-tenth or one-twentieth of C. 1- and r are adjusted to give a time constant of, for instance, to 200 milliseconds.

Of course, the condenser 0 reduces the net fire alarm signal available at X due to the current which it draws because of the potential reduction at the point X. It can readily be shown that, if the time constant r c is small relative to rC, as is the case in practice, the presence of 0 leads to an imaginary reduction of C in the ratio C/(C-t-c which means in practice that the capacity of C is reduced by 10%.

It will be noted that theamplifier receives a fire alarm signal if a sensing element is accidentally short-circuited. Such. an unreal alarm signal must be neutralised.

To this end, the winding I of the polarized relay is shunted by an element representing a voltage threshold Z of a level such that the maximum current normally required to energize the relay produces a voltage drop slightly below said threshold (FIG. 13). On the other hand, the technical incident signal just considered produces in the same Winding a current limited to a value slightly above the required energizing current, the excess current being absorbed by the element Z The relay would therefore tend to give a fire alarm, were it not for the fact thatall the current delivered by the technical incident amplifier flows through a second and opposing winding II. The winding II is such that the opposing ampere-turns predominate so that the polarized relay is immediately operated in the sense of a technical alarm. A diagram illustrating these principles is given in FIG. 14.

In addition to the elements already described, there can be seen a rectifier bridge B for enabling either of the amplified technical incident. signals delivered by T or T to be applied to the winding II of the polarized relay P. The fire alarm or technical incident signals are kept on permanently by relays RAS and RAT respectively which latch to operate the suitable warning signal permanently.

Any accidental shorting of the condenser C is indicated as a technical alarm due to the abrupt and considerable rise in the potential at X. This is one of the main reasons why the potential of E is set below the lowest possible potential of the point M. y

In the foregoing a description has been given of the manner in which the alarm circuit is operated by an excessixe rate of temperature rise. If required, an alarm can also be operated for any excess of an absolute temperature considered dangerous. To this end a diode D is connected across the condenser C with its cathode connected to the positive terminal. of C.

The value of Th (or of R) should be such that, when the critical temperature is reached, the additional reduction in the resistance of Th decreases the potential at M to below E", so that the diode D becomes conductive and the potentials at M and X act in unison and vary together upon any further temperature rise. Hence the slightest excess of the critical temperature produces at X a signal of sufficient amplitude to open the transistorized amplifier and operate the relay P to give a fire alarm warning. It will be noted that the diode D is biased in the opposite direction for any temperature of Th below the critical figure, for the potential of M is greater than that of E. If the reverse saturation current of the diode D is negligible, the operation of the circuit as a velocirnetric detector is not affected by the presence of the latter diode. If required, D can be a silicon diode, in which case it requires a minimum voltage of about 0.4 v. to become conductive, so that a suitable value of Th (or R) must be chosen.

There is no disadvantage in the condenser C becoming slightly polarized in the opposite direction when D becomes conductive.

Voltage-step elements Z, Z and Z are provided to stabilize the voltage E and E +e despite variations of the battery voltage, for the least such variation would produce a fire alarm or technical incident signal at X.

A protective resistance r limits the base current of Th to a safe value. r is low enough not to cause a harmful voltage drop in the fire alarm signals applied to the transistor.

The resistances 2' and r stabilize the inoperative current of the bases of T and T in the absence of any signal. The shunt diodes discharge the condensers c and c after a technical incident alarm has been given.

The device hereinbefore described forms a complete alarm circuit which can be operated by one or more sensing elements installed in the same thermal surroundings and connected to a single sensing circuit. Of course, the sensitivity of each sensing element operated individually is less than the sensitivity which the same sensing element Would have alone in the circuit (with the value K=I at the means temperature). This reduction in individual sensitivity becomes greater in proportion as more elements are associated in a single circuit. As has been shown, this reduction in individual sensitivity is not a disadvantage, provided that the associated sensing elements are disposed in the same thermal surroundings, for an outbreak of fire would affect a number of adjacent sensing elements simultaneously and their individual signals would be superimposed upon one another to provide a high-amplitude alarm signal.

However, in some cases a sensing element may be required to retain its individual sensitivity although installed with other elements in the same room. To this end, each sensing element can be provided with an individual differentiating circuit and the complete arrangement can be connected to a common indicating or warning circuit. The connection thereto is such that the behaviour of any detector circuit is not affected by the thermal condition of the others, just as if the detector was alone in the circuit. Sensing elements thus installed and the associated detector circuits will hereinafter be referred to as ther mally independent. It will be apparent that, even if a considerable number of thermally independent sensing elements are provided in one room, there is no need to identify the element which responded to an outbreak of fire, for the same can be immediately located by identifying the common site of the sensing elements associated with the same circuit.

Substantially any number of identical sensing circuits are connected in parallel with the voltage E (FIG. 15). Each of these circuits of order k has its own differentiating circuit r C The detector circuits thus formed are connected to a complete indicating and warning circuit similar to the kind hereinbefore described by means of two groups of diodes S and G each forming an or-gate. Two examples, k and k+I only, of such detector and differentiating circuits are shown.

The diodes Si are silicon diodes. The resistance r,. and

12 the diodes D, G G etc. at each connected across a resistance r maintain the diodes Si when the circuit is at rest, at a potential slightly below the potential on their voltage-current characteristics where they start to become conductive (FIG. 15).

The complete circuit operates as follows (FIG. 15): if a sensing element, for instance Th is heated at a rate greater than the critical rate, the potential drops at M and X as hereinbefore described, so that the diode G is cut off and the diode Si is rendered conductive. The transistors T and T are, therefore, in turn opened and the alarm relay P is quickly operated to give a fire alarm. Similiarly, when a sensing element Th is ruptured or short-circuited, the abrupt potential variation at M and X opens the diode G or the diode Si respectively, so that the potential variation of M is applied either to the transistor T or the transistor T and the relay P operates to give a technical alarm.

When the diode Si is operated, it biases the 12-1 passive diodes Si of the gate and the diode G to cut-off. Hence the reverse current of these nl diodes, since it is applied in toto to the differentiating circuit r C must be greatly reduced, whereas the reverse current of the diode D can be greater, provided that it is a small fraction of the current absorbed by r The potential at X or Y, therefore, depends only upon the rate of change of the potential at the places M of the detector circuit operated; i.e., upon the rate of ambient temperature change, just as assumed in the theory of the simple detector circuit.

IV. Identifying and Marking Circuits These circuits identify and locate detector circuits in which one or more sensing elements have set off an alarm or have gone out of order.

Detector devices with individual indications of the kind shown in FIG. 14 provide, by their very nature, an individual alarm, the reason for which can be traced. This is not the case, however, with the detectors having central indicating circuits of the kind shown in FIG. 15, Where alarm signals are essentially anonymous. As previously stated, sensing elements connected to the centralized indicating circuits must be placed in the same room.

The centralized indicating circuit can, however, be adapted to cases where detector circuits are to be fitted in different places and are thermally independent such as, for instance, in hotel rooms, enclosed places at public performances, etc. In this case the central indicating circuit is supplemented by as many individual identifying and marking circuits as there are centralized detector circuits to be identified and by an automatic connection circuit.

The circuit to be described hereinafter and illustrated in FIG. 16 utilizes the centralization possibilities of the indicator described in section III hereof yet provides immediate identification of the detector circuit concerned and of the nature of the alarm (fire or technical).

FIG. 16 is given by way of example and is not in itself a feature limiting the scope of the invention.

It will be seen that in FIG. 16 the circuit shown in FIG. 15 is associated with a number of indicating and marking circuits individually associated with n detector circuits. In FIG. 16 the illustration of these individual circuits is limited to the circuit associated with the detector circuit k. It comprises fire alarm and technical incident relays RAS; and RAT, supplied through transistors T and T respectively. There are therefore as many pairs of relays and transistors as there are detector circuits to be individualized. These individual circuits are connected to the central indicating device by connecting relays Co. This relay comprises as many change-over contacts as there are circuits to be individualized. Two general failure and fire arm relays RAS and RAT associated jointly with all the individual detector circuits complete the circuit. The latter relays operate when any detector circuit has initiated an alarm and ensure that the alarm is given permanently 13 by latching. An acoustic warning (not shown) is then operated permanently.

The circuit operates as follows:

When the circuit is in the rest-state, all the relays are released and are in the position shown in FIG. 16. The detector circuits aretherefore connected to the central indicating circuit in the same way as in FIG. 15.

If a detector. circuit of identity k initiates a fire alarm, the relay P is operated as hereinbefore described and in turn operates an auxiliary relay P which in turn energizes the connecting relay CO. The same immediately interrupts the connection between the condensers C (k=1 n) to the gates formed by the diodes Si and G and connect each of them individually to the transistor pair T andT (k l 21) associated with each detector circuit. Sincethe transistor emitters are connectedto a potential close to E", the new connection made by C does not alter the base potentials of transistors associated with circuits not giving a fire alarm. On the other hand, the detector circuit which has operated the relay P opens the transistor T associated therewith so that the corresponding individual damage alarm relay M8,; is operated. The same latches and gives a light warning at position k of the individual alarm panel (not shown). Also, the relay RAS immediately connects the condenser C to the potential E". Finally, it operates the general fire alarm relay RAS to start a permanent damage audiblewarning.

However, once the relay Co has operated, the negative side of C is dissociated from the potential at X, and sothe relay P and therefore the relay -P falloff. Since the relay P is shunted by a diode, its-falling-off is fairly delayed, so that the delay C0 can remain on despite the disappearance of the signal from X. Thelatter relay is also provided with a diode-for delaying its falling-off so that the signal produced at X is kept applied to the bases of T and T longenough forthe relay RAS to operate and'latch on. Since the alarm signal at X is applied to the only conductive-transistor T -(instead of'the two series connected transistors T and T the current flowing through the base thereof is considerable and is sufficient for the transistor T to operate the individual alarm relay despite the accelerated discharge of the condenser C Finally, P, P and CO fall off due to the operation and latching of RAS and RAS. This means that the detector circuit giving a fire alarm has been identified, marked and cut out of operation, that its alarm condition has been remotely indicated by an audible warning and that the centralized general alarm circuit is reconnected to the remaining n-l operative detector circuits.

It will now be assumed that the identity detector lc has just been short-circuited. The relay P therefore gives a technical alarm as described with reference to FIG. 15. The auxiliary relay P is operated and in turn operates a connecting relay Co. The same operates as in the case just described so that the relay RAT is operated through the agency of the transistor T (to which it has been connected by a make-contact of the connecting relay P and the general technical incident alarm relay RAT operates and latches on to provide an appropriate permanent alarm, while the detector circuit of order k is eliminated and the central indicating circuit is reconnected to the remaining operative n1 detectors.

Finally, it will be assumed that the identity detector circuit k has just been opened. The process is exactly as in the case just described (except that the transistor T operates the relay RAT and leads to the same alarm relays being operated as were considered in the previous case.

In short, the detector circuits have sensing elements which are completely static, strong, unchanging, free from any electrical contact and of reduced dimensions. These static elements deliver a signal interpreted by an RC- circuit biased by an inoperative current. The latter circuit defines the maximum admissible rate of temperature rise or the velocimetric sensitivity of the detector circuit. Also, it may define an absolute temperature level which must not be exceeded; i.e., the thermometric'sensitivity; These sensitivities can readily be adjusted independently of one another. Any'desir'ed number of sensing elements can be associated with the same detector circuit. Simi larly, any desired number of detector circuits can be associated with the same central indicating or warning circuit, yet the individual sensitivities of such circuits remain independent and individually adjustable. Such centralization doesnot impair identification of the sensing circuit which has operated. The detector circuits are supervised continuously to see that they are doing their job. Any fault in anyof them is shown up,'an'd thefaulty circuit is. identified, marked and eliminated and'a permanent special alarm is given. The consumption of all the detector and indicating circuits is greatly reduced so that small batteries can be used. Semiconductors make the circuits very stable. Highly reliable operation is ensured by using hermetically sealed polarized relays, while the use of sealed batteries supplied at a constant voltage reduces maintenance costs to a very low figure;

The device differs from known devices in that the sensing elements are very'small, static, sturdy and free of electric contacts and in that a second and insulated thermal element'is not required, slow thermal changes being compensated for by electrical means. Also, and in contrast with known devices, the velocimetric sensitivity and the thermometric sensitivity are defined by a single indicating or warning circuit which interprets the state of a single sensing element, or of a group of associated sensing elements, on essentially electrical bases.

In contrast to known devices, a considerable number of detector circuits can be associated with one central indicating circuit, yet the detector circuits remain independent of one another and the circuit giving the alarm or having the defect can stillbe identified.

Finally, and in contrast to mostof the known circuits, electronic detectors consume very little current.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the following claims.

What I claim is:

1. A circuit for detecting a rate of temperature change, comprising a source of direct voltage, a first resistor, a sensing element having a temperature-dependent coefficient of resistance series connected at a first junction with said first resistor across said voltage source, a second resistor connected at one end to said voltage source, a normally forward biased unilateral conducting means having one side tied to a point of constant potential and another side connected at a second junction to the other end of said second resistor for normally maintaining a constant flow of current through said second resistor, capacitive means connected between said first junction and said second junction to block the passage of DC. therebetween and to produce a second current through said second resistor in response to a change of resistance of said sensing element, said second current flowing in a direction to produce a voltage drop across said second resistor tending to cut off said unilateral conducting means, and output means connected to said second junction for detecting signals produced by predetermined rates of temperature change, whereby upon a predetermined rate of temperature change in a given direction said unilateral conducting means is biased off to disconnect said constant potential point from said output means.

2. A circuit according to claim 1, including a semiconductor amplifier, a relay connected to the output of said amplifier, the input of said amplifier being connected to said second junction a difierentiating circuit having a low time constant relative to the temperature changes, and a fault detector, one end of said difierentiating circuit connected to said second junction and the other end of said differentiating circuit connected to said fault detector, said amplifier being responsive to predetermined voltages at said second junction.

3. A system for detecting excess rates of temperature rise comprising a source of direct voltage, a plurality of detector circuits connected in parallel, each detector circuit including a first resistor connected at one end to the positive side of said voltage source, a sensing thermistor connected at one end to the other end of said first resistor, at a first junction, the other end of said thermistor being connected to the negative side of said voltage source, a capacitor connected at one end to the junction of said first resistor and thermistor, a second resistor connected at one end to the positive side of said voltage source, a first diode, a second diode, the anode of said first diode being connected to the other side of said second resistor, the other end of said capacitor and the cathode of said second diode, a constant potential resistor connected at one end to the cathode of each of said first diodes in said plurality of detector circuits, the other end of said constant potential resistor being connected to a point of constant potential, a detector means connected to the anode of each of said second diodes in said plurality of detector circuits, whereby the sensing of a predetermined excessive rate of temperature rise by any of said thermistors actuates said detector means.

4. A system according to claim 3 including a pnp transistor and an npn transistor, a first predetermined small time constant RC circuit connected to the base of said pnp transistor the anodes of said second diodes being connected to said first RC circuit, a second predetermined small time constant RC circuit connected to the base of said npn transistor the cathodes of said first diodes being connected to said second RC circuit whereby a malfunction in the sensing circuit produces a signal which is sensed by one of said transistors.

5. A circuit for detecting an excess rate of temperature rise comprising a source of direct voltage, a first resistor connected at one end to the positive side of said voltage source, a sensing thermistor connected at one end to the other end of the first resistor at a first junction, the other end of said thermistor being connected to the negative side of said voltage source, a capacitor connected at one end to the junction of said first resistor and thermistor, a second resistor connected at one end to the positive side of said voltage source, a diode having its cathode connected to a point of constant potential and its anode connected to the other side of said second resistor and the other side of the capacitor at a second junction, the circuit parameters being dimensioned so that a stand-by current flows in said second resistor and said diode during decreasing temperatures, constant temperatures and temperature rate increases below a predetermined level, said diode being cut off when a rate of temperature increase in excess of said level is detected by said thermistor, detecting means connected to said second junction whereby a detection signal is produced at the anode of said diode, a third resistor con nected to the cathode of said diode, the other end of said third resistor being connected to the positive side of said voltage source, a pnp transistor, a first predetermined small time constant RC circuit connected at one end to the anode of the diode and the other end of said first RC circuit connected to the base of the pnp transistor, an npn transistor, a second predetermined small time constant RC circuit connected at one end to the cathode of the diode and the other end of said second RC circuit connected to the base of the npn transistor whereby a malfunction in the sensing circuit produces a signal which is sensed by one of said transistors.

References Cited in the file of this patent UNITED STATES PATENTS 2,742,634 Bergen Apr. 17, 1956 2,827,624 Klein Mar. 18, 1958 2,901,740 Cutsogeorge Aug. 25, 1959 3,038,106 Cutsogeorge et a1. June 5, 1962

Claims (1)

1. A CIRCUIT FOR DETECTING A RATE OF TEMPERATURE CHANGE, COMPRISING A SOURCE OF DIRECT VOLTAGE, A FIRST RESISTOR, A SENSING ELEMENT HAVING A TEMPERATURE-DEPENDENT COEFFICIENT OF RESISTANCE SERIES CONNECTED AT A FIRST JUNCTION WITH SAID FIRST RESISTOR ACROSS SAID VOLTAGE SOURCE, A SECOND RESISTOR CONNECTED AT ONE END TO SAID VOLTAGE SOURCE, A NORMALLY FORWARD BIASED UNILATERAL CONDUCTING MEANS HAVING ONE SIDE TIED TO A POINT OF CONSTANT POTENTIAL AND ANOTHER SIDE CONNECTED AT A SECOND JUNCTION TO THE OTHER END OF SAID SECOND RESISTOR FOR NORMALLY MAINTAINING A CONSTANT FLOW OF CURRENT THROUGH SAID SECOND RESISTOR, CAPACITIVE MEANS CONNECTED BETWEEN SAID FIRST JUNCTION AND SAID SECOND JUNCTION TO BLOCK THE PASSAGE OF D.C. THEREBETWEEN AND TO PRODUCE A SECOND CURRENT THROUGH SAID SECOND RESISTOR IN RESPONSE TO A CHANGE OF RESISTANCE OF SAID SENSING ELEMENT, SAID SECOND CURRENT FLOWING IN A DIRECTION TO PRODUCE A VOLTAGE DROP ACROSS SAID SECOND RESISTOR TENDING TO CUT OFF SAID UNILATERAL CONDUCTING MEANS, AND OUTPUT MEANS CONNECTED TO SAID SECOND JUNCTION FOR DETECTING SIGNALS PRODUCED BY PREDETERMINED RATES OF TEMPERATURE CHANGE, WHEREBY UPON A PREDETERMINED RATE OF TEMPERATURE CHANGE IN A GIVEN DIRECTION SAID UNILATERAL CONDUCTING MEANS IS BIASED OFF TO DISCONNECT SAID CONSTANT POTENTIAL POINT FROM SAID OUTPUT MEANS.

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