CA1176345A - Temperature control means for a humidity detector - Google Patents
Temperature control means for a humidity detectorInfo
- Publication number
- CA1176345A CA1176345A CA000348669A CA348669A CA1176345A CA 1176345 A CA1176345 A CA 1176345A CA 000348669 A CA000348669 A CA 000348669A CA 348669 A CA348669 A CA 348669A CA 1176345 A CA1176345 A CA 1176345A
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- CA
- Canada
- Prior art keywords
- detector
- temperature
- resistance
- control arrangement
- humidity
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K7/00—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements
- G01K7/16—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using resistive elements
- G01K7/22—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using resistive elements the element being a non-linear resistance, e.g. thermistor
- G01K7/24—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using resistive elements the element being a non-linear resistance, e.g. thermistor in a specially-adapted circuit, e.g. bridge circuit
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
- G01N27/22—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance
- G01N27/223—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance for determining moisture content, e.g. humidity
- G01N27/225—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance for determining moisture content, e.g. humidity by using hygroscopic materials
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Chemical & Material Sciences (AREA)
- General Health & Medical Sciences (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- Electrochemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Nonlinear Science (AREA)
- Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
- Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
The invention provides a control arrangement for a humidity detector, the operation of which is based on a change of impedance of the detector, the detector being electrically heated under the control of said arrangement by means of a heating resistance to a temperature (Ts) which is higher than the ambient temperature (Ta) of the detector with a view to improving the accuracy of measurement of the detector or to increasing its life, said control arrangement comprising a bridge circuit or equiva-lent containing temperature-dependent resistance elements responsive to said ambient temperature (Ta) and said detector temperature (Ts), an error voltage of said bridge circuit being used as a feedback signal for controlling the electric heating of the detector,said heating resistance for the detector comprising a resistance element with a positive temperature coefficient which at the same time serves as a sensor of the detector's temperature, and said temperature-dependent resistance element responsive to said ambient temperature(Ta) comprising a resistor/
thermistor assembly enabling a function Ts = f(Ta) peculiar to each detector type to be realized with the aid of the control arrangement.
The invention provides a control arrangement for a humidity detector, the operation of which is based on a change of impedance of the detector, the detector being electrically heated under the control of said arrangement by means of a heating resistance to a temperature (Ts) which is higher than the ambient temperature (Ta) of the detector with a view to improving the accuracy of measurement of the detector or to increasing its life, said control arrangement comprising a bridge circuit or equiva-lent containing temperature-dependent resistance elements responsive to said ambient temperature (Ta) and said detector temperature (Ts), an error voltage of said bridge circuit being used as a feedback signal for controlling the electric heating of the detector,said heating resistance for the detector comprising a resistance element with a positive temperature coefficient which at the same time serves as a sensor of the detector's temperature, and said temperature-dependent resistance element responsive to said ambient temperature(Ta) comprising a resistor/
thermistor assembly enabling a function Ts = f(Ta) peculiar to each detector type to be realized with the aid of the control arrangement.
Description
~3L7~3~5 This invention relates to a control arrangement for a humidity detector, the operation of which is based on the changing impedance of the de~ect~r, and in particular a capacitive humidity aetector having as its insulator a polymer substance, the di-electric constant varies as a function of humidity. The control arrangement controls the heating of the detector with electricity either directly or indirectly to obtain a temperature higher than the ambient temperature in order to improve the accuracy of measurement and/or increase the life of the detector. The control arrangement comprises a bridge circuit or equivalent containing temperature-dependent resistor elements with the aid of which said ambient te~perature and the temperature of the detector are monitored~ The error voltage of the bridge circuit is employed as a feedback slgnal by which the electric power heating of the detector is controlied. ~
The applicant's Finnish Patent No. 48229 discloses a capacitive humidity detector having as its dielectric material a polymer film, the dielectric constant of which is a function of the quantity of water which the polymer film has absorbed. In the above-presented, and even other humidity detectors working on the basis of a change in impedance, undesirable phenomena occur, in particular when high humidities are being measured.
These phenomena include for instance, slow creep of -the detector which may be due to a number of factors. As a rule, however, these are reversible phenomena. It is an object of the invention to control them better than be~ore. An object of the invention, therefore, is to increase the response time of the detector and to improve the accuracy of measurement, in particular at high relative humidities, such as those above 75 to 90~.
In the accompanying drawings:
Fig. 1 shows a prior art humidity control arrangement;
Fig. 2 shows schematically, a humidity detector and a ~ .
63~S
control arrangement in accordance with the invention attached thereto;
Fig. 3 shows a section along the line II-II in Fig. 2;
Fig. 4 shows a bridge circuit employed in various control arrangements according to the invention and current regulating means associated therewith;
Fig. 5 shows schematically an embodiment of a measuring head for a humidity detector provided with a control arrangement according to the invention;
Fig. 6 shows in greater detail an embodiment of a bridge circuit employed in a control arrangement according to the invention;
Fig. 7 displays the differential temperature ATS
established with the aid of a control arrangement according to the invention, as a function of the ambient temperature Ta; and Fig. 8 shows the detailed circuit diagram of a measuring instrument provided with a control arrangement according to the invention.
~h c~ 3/7,~
~ The applicant's Finni~h-patent application No. ~3~
discloses a method for reducing the undesirable properties of an electrical humidity detector. This method is mainly characterized in that the humidit~-sensitive material of the humidity pick-up is heated, at least when operating in a higher relative humidity range, to a temperature higher than the ambient temperature.
In association to the preceding, reference is further-more made to the paper presented at the Transducer ~78 Conference.
"A Stable Thin Film Humidity Sensor, Philip H. Chawner and Cecil A Cove", wherein a procedure is described according to which the temperature of the detector is kept at a level consistent with the equation Ts ~ K . Ta ~ C, where Ts is the detector temperature and Ta is the ambient temperature. The paper gives for the constants K and C the values 1.041856 and 4.02497, and 1.0352 ~1~7~5 and 4.1000 for various ranges of the temperature Ta, whereby the humidity detect~r "sees" 0.75 times the prevailing ambient humidity.
The equation stated, Ts = K . Ta + C, is an approximate equation, by which one achieves for instance within the temperature range between -20 and +100C, about -~0.2~C accuracy in the control of the detector temperature Ts The electronic circuit design of this apparatus of prior art is as shown in the attached Fig. 1, where -the resistors R31,R32,Ra and Rs constitute, together with the resistance elements connected across them, a bridge circuit, to which has furthermore been connected in the right branch of the bridge, additional components to serve purposes which will become apparent later on. By suitably selecting the values of said resistors and additional components, and when Ra is a PTC resistor with nearly linear dependence of the ambient temperature Ta and Rs is a PTC resistor with equally nearly linear dependence of the pick-up's temperature Ts, in theexemplary case both being platinum resistors PtlOO, the bridge will be in equilibrium as soon as the said equation Ts =
~0 K . Ta + C is satisfied.
The humidity detector (not visible in Fig. 1), the re-sistor Rs and a separate heating resistance RH are mechanically joined by cement, whereby there is a therm~l coupling TC to the resistor Rs and to the humidity detector attached thereto.
Assuming the gain of the operational amplifier ICl to be very high, which is satisfied in practice, the mode of operation of the circuit is physically easy to unders~and. Power is supplied continuously to the resistance RH which maintains the bridge in eguilibrium by controlling the temperature, and thereby the resistance, of the temperature-sensitive resistor Rs. If for instance the ambient temperature Ta increases, the resistance of resistor Ra will increase. The bridge becomes unbalanced, causing ^ 3L~';'~i,;~5 an increase of the current passing through the transistor TRO
connected after ICl, increaslng the power inpùt into the heating resistance RH, and as a result the temperature of the resistor Rs will rise until equilibrium of the brid~e has been attained.
A substantially constant current through the temperature-sensitiue resistors is accomplished by stabilizing the volta~e supplying the bridge and by selecting the resistors R31 and R32 to have high resistivity, for example, 10 kohms.
The con~tant K is accomplished by so adjusting the resistance RVl that the current ratto Ia/I will have a numerical value equalling the desired K value. The constant C is adjusted as desired by alterin~ the resistance RV2, thereby affecting the error voltage between the points X,Y.
It has been possible, in the manner just described, to carry out almost satisfactorily the controlled adjustment of the humidity detectors temperature. However, certain drawbacks encumber this procedure of prtor art, and these shall be considered in detail in the following.
One requires In the apparatus, adjoined to the humidity ~0 detector, two separate additonal elements: the temperature-sensitivè resistor Rs and the heating resistance RH, and this makes the detector/resistor combination hard to manufacture.
The resistors Rs and Ra with temperature-dependent resistance are, out of practical considerations, industrially made resistive metallic temperature detectors having a relatively low impedance, J ~00 -oh~ ~ .t ,'n ~r~?
and they are~for instance.PtlO0 resistors. Ra in particular, which has to present low thermal inertia, musk be prevented from heating itself, whlch implies that care must be taken to apply only a tn ~
low voltage across Ra, in the order of 150 ~ It follows that exceedingly high requirements have to be imposed on the electron-ic components, for instance, on the operational amplifier ICl, and the components are also difficult to calibrate and their operation requires special care, ~or instance, in avoiding contact potentials at the connectors. Moreover it is a fact that the temperature control in the apparatus of prior art, when occurring in accordance with the equation Ts = K . Ta f C, causes significant errors in the results of measurement. It is also one of the drawbacks that the apparatus requires a considerable number of high precision components and a stabilized voltage source.
The object of the invention i5 to provide a control means in which the drawbacks mentioned are eliminated. It is also an object: to provide a control means by the aid of which it is possible if necessary to correct also the temperature depèndence of the humidity detector.
In order to attain the objects mentioned and others which will become apparent later on, the invention is mainly characterized in that as a heating resistance for the detector is used such a resistance element having a positive temperature coefficient, for instance, a platinum resistor, which at the same time serves as temperature sensor of the pick-up, and that as ambient temperature sensor such a resistor/thermistor arrangement is used that a given function Ts = f(Ta) character-istic of each individual pick-up type is realized with the aid of the control means.
In accordance with the invention, therefore, there is provided a control arrangement for a humidity detector, the operation ofwhich isbased ona change of impedance of the detector, the detector being electrically heated under the control of said arrangement by means of a heating resistance to a temperature (Ts) which is higher than the ambient temperature (Ta) of the detector with a view to improving the accuracy of measurement of the detector or to increasing its li~e, said control arrangement comprising a bridge circuit or equivalent containin~ te~perature-i34~i dependent resistance element~ r~sive to said ambient tempera-ture (Ta) and said detector temperature ~Ts)~ an error ~oltage of said bridge circuit being used as a feedback signal or con-trolling the electric heating of the detector, said heating resis-tance for the detector comprising a resistance element with a posi-tive temperature coefficient which at the same time serves as a sensor of the detector's temperature, and said temperature-depen-dent resistance element responsive to said ambient temperature (Ta)ccm~rising an assembly of a resistor and thermistor enabling a function Ts = f(Ta) peculiar to each detector type to be realized with the aid of the control arrangement.
As an additional advantage, the invention affords a simple possibility to provide a humidity meter indicating not only the relative humidity (R.H.) but also the absolute moisture content and/or the dewpoint. Additional advantages of the control means of the invention are: its simple implementation, and low energy comsumption.
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings introduced above and which are based on a capacitive humidity pick-up having an organic polymer as its humidity-sensitive mater-ial.
The humidity detector shown in Figs. 2 and 3 is described in applicant's Finnish Patent No. 48229. The base of the detector 10 is a substrate 11 which is passive as regards absorption of water, such as a glass plate for instance. In a way known in the art from thin film technology, bottom contacts 12 have been formed by metallizing on the substrate 11 and by soldered joints 16 there-to have been attached contact leads, from which the capacitance is measured and indicated with the apparatus schematically represented in Fig. 2 ~'7t~ 5 by the blocks 20 and 22. The detector comprises, as its active substance, a thin (for instance, thickness about lO ~m order of magnitude) polymer film. Upon the polymer film 13 there has been formed by vacuum evaporation, by sputtering or chemically, a thin surface contact 14 permeable to water vapour and which is not in galvanic contact with either one of the bottom contacts 12. Hereby the capacitance to be measured CM is established through the series connection of the capacitances formed between the bottom contacts 12 and the surface contact 14 in the areas d and e (Fig. 3).
In Fig. 2, the block 17 has been included to represent those means by which the ambient temperature Ta is measured, its signal being conducted through the connector l9 to the measuring and control means 20. Fig. 3 reveals the resistance element RST
produced e.g. by evaporation in conjunction with the substrate ll, this resistance serving to keep the pick up lO, using the control means of the invention, at a temperature Ts which is higher than the ambient temperature Ta. The connector 18 in Fig.
The applicant's Finnish Patent No. 48229 discloses a capacitive humidity detector having as its dielectric material a polymer film, the dielectric constant of which is a function of the quantity of water which the polymer film has absorbed. In the above-presented, and even other humidity detectors working on the basis of a change in impedance, undesirable phenomena occur, in particular when high humidities are being measured.
These phenomena include for instance, slow creep of -the detector which may be due to a number of factors. As a rule, however, these are reversible phenomena. It is an object of the invention to control them better than be~ore. An object of the invention, therefore, is to increase the response time of the detector and to improve the accuracy of measurement, in particular at high relative humidities, such as those above 75 to 90~.
In the accompanying drawings:
Fig. 1 shows a prior art humidity control arrangement;
Fig. 2 shows schematically, a humidity detector and a ~ .
63~S
control arrangement in accordance with the invention attached thereto;
Fig. 3 shows a section along the line II-II in Fig. 2;
Fig. 4 shows a bridge circuit employed in various control arrangements according to the invention and current regulating means associated therewith;
Fig. 5 shows schematically an embodiment of a measuring head for a humidity detector provided with a control arrangement according to the invention;
Fig. 6 shows in greater detail an embodiment of a bridge circuit employed in a control arrangement according to the invention;
Fig. 7 displays the differential temperature ATS
established with the aid of a control arrangement according to the invention, as a function of the ambient temperature Ta; and Fig. 8 shows the detailed circuit diagram of a measuring instrument provided with a control arrangement according to the invention.
~h c~ 3/7,~
~ The applicant's Finni~h-patent application No. ~3~
discloses a method for reducing the undesirable properties of an electrical humidity detector. This method is mainly characterized in that the humidit~-sensitive material of the humidity pick-up is heated, at least when operating in a higher relative humidity range, to a temperature higher than the ambient temperature.
In association to the preceding, reference is further-more made to the paper presented at the Transducer ~78 Conference.
"A Stable Thin Film Humidity Sensor, Philip H. Chawner and Cecil A Cove", wherein a procedure is described according to which the temperature of the detector is kept at a level consistent with the equation Ts ~ K . Ta ~ C, where Ts is the detector temperature and Ta is the ambient temperature. The paper gives for the constants K and C the values 1.041856 and 4.02497, and 1.0352 ~1~7~5 and 4.1000 for various ranges of the temperature Ta, whereby the humidity detect~r "sees" 0.75 times the prevailing ambient humidity.
The equation stated, Ts = K . Ta + C, is an approximate equation, by which one achieves for instance within the temperature range between -20 and +100C, about -~0.2~C accuracy in the control of the detector temperature Ts The electronic circuit design of this apparatus of prior art is as shown in the attached Fig. 1, where -the resistors R31,R32,Ra and Rs constitute, together with the resistance elements connected across them, a bridge circuit, to which has furthermore been connected in the right branch of the bridge, additional components to serve purposes which will become apparent later on. By suitably selecting the values of said resistors and additional components, and when Ra is a PTC resistor with nearly linear dependence of the ambient temperature Ta and Rs is a PTC resistor with equally nearly linear dependence of the pick-up's temperature Ts, in theexemplary case both being platinum resistors PtlOO, the bridge will be in equilibrium as soon as the said equation Ts =
~0 K . Ta + C is satisfied.
The humidity detector (not visible in Fig. 1), the re-sistor Rs and a separate heating resistance RH are mechanically joined by cement, whereby there is a therm~l coupling TC to the resistor Rs and to the humidity detector attached thereto.
Assuming the gain of the operational amplifier ICl to be very high, which is satisfied in practice, the mode of operation of the circuit is physically easy to unders~and. Power is supplied continuously to the resistance RH which maintains the bridge in eguilibrium by controlling the temperature, and thereby the resistance, of the temperature-sensitive resistor Rs. If for instance the ambient temperature Ta increases, the resistance of resistor Ra will increase. The bridge becomes unbalanced, causing ^ 3L~';'~i,;~5 an increase of the current passing through the transistor TRO
connected after ICl, increaslng the power inpùt into the heating resistance RH, and as a result the temperature of the resistor Rs will rise until equilibrium of the brid~e has been attained.
A substantially constant current through the temperature-sensitiue resistors is accomplished by stabilizing the volta~e supplying the bridge and by selecting the resistors R31 and R32 to have high resistivity, for example, 10 kohms.
The con~tant K is accomplished by so adjusting the resistance RVl that the current ratto Ia/I will have a numerical value equalling the desired K value. The constant C is adjusted as desired by alterin~ the resistance RV2, thereby affecting the error voltage between the points X,Y.
It has been possible, in the manner just described, to carry out almost satisfactorily the controlled adjustment of the humidity detectors temperature. However, certain drawbacks encumber this procedure of prtor art, and these shall be considered in detail in the following.
One requires In the apparatus, adjoined to the humidity ~0 detector, two separate additonal elements: the temperature-sensitivè resistor Rs and the heating resistance RH, and this makes the detector/resistor combination hard to manufacture.
The resistors Rs and Ra with temperature-dependent resistance are, out of practical considerations, industrially made resistive metallic temperature detectors having a relatively low impedance, J ~00 -oh~ ~ .t ,'n ~r~?
and they are~for instance.PtlO0 resistors. Ra in particular, which has to present low thermal inertia, musk be prevented from heating itself, whlch implies that care must be taken to apply only a tn ~
low voltage across Ra, in the order of 150 ~ It follows that exceedingly high requirements have to be imposed on the electron-ic components, for instance, on the operational amplifier ICl, and the components are also difficult to calibrate and their operation requires special care, ~or instance, in avoiding contact potentials at the connectors. Moreover it is a fact that the temperature control in the apparatus of prior art, when occurring in accordance with the equation Ts = K . Ta f C, causes significant errors in the results of measurement. It is also one of the drawbacks that the apparatus requires a considerable number of high precision components and a stabilized voltage source.
The object of the invention i5 to provide a control means in which the drawbacks mentioned are eliminated. It is also an object: to provide a control means by the aid of which it is possible if necessary to correct also the temperature depèndence of the humidity detector.
In order to attain the objects mentioned and others which will become apparent later on, the invention is mainly characterized in that as a heating resistance for the detector is used such a resistance element having a positive temperature coefficient, for instance, a platinum resistor, which at the same time serves as temperature sensor of the pick-up, and that as ambient temperature sensor such a resistor/thermistor arrangement is used that a given function Ts = f(Ta) character-istic of each individual pick-up type is realized with the aid of the control means.
In accordance with the invention, therefore, there is provided a control arrangement for a humidity detector, the operation ofwhich isbased ona change of impedance of the detector, the detector being electrically heated under the control of said arrangement by means of a heating resistance to a temperature (Ts) which is higher than the ambient temperature (Ta) of the detector with a view to improving the accuracy of measurement of the detector or to increasing its li~e, said control arrangement comprising a bridge circuit or equivalent containin~ te~perature-i34~i dependent resistance element~ r~sive to said ambient tempera-ture (Ta) and said detector temperature ~Ts)~ an error ~oltage of said bridge circuit being used as a feedback signal or con-trolling the electric heating of the detector, said heating resis-tance for the detector comprising a resistance element with a posi-tive temperature coefficient which at the same time serves as a sensor of the detector's temperature, and said temperature-depen-dent resistance element responsive to said ambient temperature (Ta)ccm~rising an assembly of a resistor and thermistor enabling a function Ts = f(Ta) peculiar to each detector type to be realized with the aid of the control arrangement.
As an additional advantage, the invention affords a simple possibility to provide a humidity meter indicating not only the relative humidity (R.H.) but also the absolute moisture content and/or the dewpoint. Additional advantages of the control means of the invention are: its simple implementation, and low energy comsumption.
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings introduced above and which are based on a capacitive humidity pick-up having an organic polymer as its humidity-sensitive mater-ial.
The humidity detector shown in Figs. 2 and 3 is described in applicant's Finnish Patent No. 48229. The base of the detector 10 is a substrate 11 which is passive as regards absorption of water, such as a glass plate for instance. In a way known in the art from thin film technology, bottom contacts 12 have been formed by metallizing on the substrate 11 and by soldered joints 16 there-to have been attached contact leads, from which the capacitance is measured and indicated with the apparatus schematically represented in Fig. 2 ~'7t~ 5 by the blocks 20 and 22. The detector comprises, as its active substance, a thin (for instance, thickness about lO ~m order of magnitude) polymer film. Upon the polymer film 13 there has been formed by vacuum evaporation, by sputtering or chemically, a thin surface contact 14 permeable to water vapour and which is not in galvanic contact with either one of the bottom contacts 12. Hereby the capacitance to be measured CM is established through the series connection of the capacitances formed between the bottom contacts 12 and the surface contact 14 in the areas d and e (Fig. 3).
In Fig. 2, the block 17 has been included to represent those means by which the ambient temperature Ta is measured, its signal being conducted through the connector l9 to the measuring and control means 20. Fig. 3 reveals the resistance element RST
produced e.g. by evaporation in conjunction with the substrate ll, this resistance serving to keep the pick up lO, using the control means of the invention, at a temperature Ts which is higher than the ambient temperature Ta. The connector 18 in Fig.
2 illustrates that functional connection by which the heating ~0 current I2 is carried to the resistance RST and through which the temperature Ts of the detector lO is measured.
In the bridge circuit of Fig. ~, RST is a low-impedance, temperature-sensitive resistance element, for instance, a PtlOO
platinum resistance, which measures and through the self-heating caused by the measuring current increases the temperature of the humidity detector lO or of the fluid surrounding it, which usually is air. The resistance element RST has been affixed to the humidity detector 10 or been formed on the surface of the detector 10, or it is a separate element heating the fluid with which the pick-up 10 comes into contact. In the bridge circuit of Fig. 4, RAT is a high-impedance NTC element dependent on the ambient temperature, for instance, a t~ermistor/resistor ~ ~'7~
circuit assembly which has been made such that the impedance of RAT is a function of the detector and ambient temperature Ta in a certain pre-selected way. The resistances Rl and R2 in the bridge circuit have a constant resistance value and mlnimal temperature dependence, whereby they may as well be placed elsewhere, not in the immediate vicinity of the detector 10, in a temperature differing from that of the fluid which is the subject of humidity measurement.
As shown in Fig. 4, the equilibrium of the bridge RAT, Rl,R2,RST is measured by observing the error voltage ~U, by which the operation ampli~ier ICl is governed, this amplifier in turn controlling the transistor TR3, through which the current I =
Il+I2 is conducted to the bridge.
As shown in Fig. 5, the resistance elements RAT and RsT
and the humidity detector 10 have been accommodated in one measuring head, close to each other.
As taught by the invention, when RAT is in appropriate manner a function of the ambient temperature Ta and the operational amplifier ICl depicted in Fig. 4 has a high gain, it ~ecomes possible for the arrangement to control the temperature Ts =
f(Ta) of the detector 10 so that f(Ta) is a function of the kind desired. As taught by the invention, Ts = f(Ta) is fixed so that in every instance ~he humidity pick-up 'sees' a value which is suitably lower than the prevailing relative humidity, for instance, 0.75 x RH (rel. humidity), similarly as in means known in prior art, but it is in addition possible to apply a correction for the pick-up's temperature dependence by using a given, other P dence Ts fl(Ta) proper for the case in hand In the invention, the currents flowing in the arms of the bridge of Fig. 4 have been so adjusted that the current I2 heats the resistor RST, which at the same time operates as resistive temperature detector, measuring its own tempexature, ~7~3~S
whereby I2 will always be adjusted so that the e~uilibrium equation of the bridge Rl.~2 RAT RST
condition I1~<I2 ~s catered for by ensuring that the impedance (RAT+Rl) is >> (~2+RST) Also within the scope of the present invention is the way in which the desired temperature dependence of RAT is achieved.
It may first be observed that f(Ta) is most favourably a conductance-linear function of the temperature, in contrast with designs previously disclosed (Fig. 1), where Ta is an almost resistance-linear function of temperature.
To carry out the invention, the element RAT may be assembled of commercially available, so-called thermilinear thermistor components and of suitably chosen resistors.
Operative within a wide temperature range is a circuit as shown in Fig. 6, where Tl and T2 are a commercially available thermistor combination. The resistors R3 and R5 are resistors supplied by a manufacturer of the thermilinear thermistor, or other equivalent.
R6 and R4 are selected to have a resistance such that the requisite Ts = f(Ta) is accomplished, if desired also taking into account the temperature coefficient of the hu~idity detector 10, in which case Ts = fl(Ta).
Also within the scope of the invention is the replacement o~ the temperature-dependent RAT with a resistor independent of temperature. It is obvious that if RAT has constant resistance, then the bridge of Fig. 4 will be in equilibrium at a certain constant value TSl of the detector temperature Ts, and Ts will remain constant at TSl if TSl ~ Ta, whereby the temperature of the respective humiditv detector 10 is then also constant. The detector 10 indicating the RH value may then be calibrated to indicate either the dewpoint or the absolute humidity, with certain restrictions.
As shown in Fig. 5, ~he measuring head comprises a cylindrical, elongated housing 100, within which all the electronic components of the arrangement have been mounted on a clrcuit board 102. To the insulator body 101 has been attached a radiation shield 103, for instance a piece of sheet metal with mirror surface. On one side of the shield 103 has been placed the humia~ty detector 10, provided with a heating resistance RST as described above, and on the other side a thermistor assembly To monitoring the ambient temperature Ta, comprising the thermist~rs Tl and T2 of the resistor/thermistor assembly RAT. The lead 104 departs from the measuring head 100 to the indicator instrument.
Fig 8 shows, in addition to the electronic components of the control means of the inventlon, also the electronic circuit measuring the capacitance CM of the humidity detector 10.
In the following are given the resistance and capacitance values of the resistors and capacitors used in the . .
circuit of Fig. 8 and the type designations of its other components.
Rl = 350 ohms Cl = 10 nF
R2 = 10 kohms C2 = 10 nF
R3 = 10 kohms C3 = 10 nF
R4 = 390 ohms C4 = 10 nF
R5 = 390 ohms C5 = 39 pF
R6 = 390 ohms C6 = 0.8 - 10 pF
R7 = 2 kohms, trimmer C7 = 22 nF
R8 = 5700 ohms, 0.1% C8 = 10 ~F, tantalum Rg = 1200 ohms, 0.1% Cg = 1 ~F, tantalum Rlo = 2 ohms Rll = 100 ohms TRl = 2N 3227 - R12 = 4698 ohms TR2 = 2N 3227 R14 = 4110 ohms TR3 = 2N 3904 R15 = 47 kohms Rs = P~100, Heraeus ICl = LM 35 ~ YSl thermistor 44202 T2 = ) The following resistors are equivalent in Figs. 6 and
In the bridge circuit of Fig. ~, RST is a low-impedance, temperature-sensitive resistance element, for instance, a PtlOO
platinum resistance, which measures and through the self-heating caused by the measuring current increases the temperature of the humidity detector lO or of the fluid surrounding it, which usually is air. The resistance element RST has been affixed to the humidity detector 10 or been formed on the surface of the detector 10, or it is a separate element heating the fluid with which the pick-up 10 comes into contact. In the bridge circuit of Fig. 4, RAT is a high-impedance NTC element dependent on the ambient temperature, for instance, a t~ermistor/resistor ~ ~'7~
circuit assembly which has been made such that the impedance of RAT is a function of the detector and ambient temperature Ta in a certain pre-selected way. The resistances Rl and R2 in the bridge circuit have a constant resistance value and mlnimal temperature dependence, whereby they may as well be placed elsewhere, not in the immediate vicinity of the detector 10, in a temperature differing from that of the fluid which is the subject of humidity measurement.
As shown in Fig. 4, the equilibrium of the bridge RAT, Rl,R2,RST is measured by observing the error voltage ~U, by which the operation ampli~ier ICl is governed, this amplifier in turn controlling the transistor TR3, through which the current I =
Il+I2 is conducted to the bridge.
As shown in Fig. 5, the resistance elements RAT and RsT
and the humidity detector 10 have been accommodated in one measuring head, close to each other.
As taught by the invention, when RAT is in appropriate manner a function of the ambient temperature Ta and the operational amplifier ICl depicted in Fig. 4 has a high gain, it ~ecomes possible for the arrangement to control the temperature Ts =
f(Ta) of the detector 10 so that f(Ta) is a function of the kind desired. As taught by the invention, Ts = f(Ta) is fixed so that in every instance ~he humidity pick-up 'sees' a value which is suitably lower than the prevailing relative humidity, for instance, 0.75 x RH (rel. humidity), similarly as in means known in prior art, but it is in addition possible to apply a correction for the pick-up's temperature dependence by using a given, other P dence Ts fl(Ta) proper for the case in hand In the invention, the currents flowing in the arms of the bridge of Fig. 4 have been so adjusted that the current I2 heats the resistor RST, which at the same time operates as resistive temperature detector, measuring its own tempexature, ~7~3~S
whereby I2 will always be adjusted so that the e~uilibrium equation of the bridge Rl.~2 RAT RST
condition I1~<I2 ~s catered for by ensuring that the impedance (RAT+Rl) is >> (~2+RST) Also within the scope of the present invention is the way in which the desired temperature dependence of RAT is achieved.
It may first be observed that f(Ta) is most favourably a conductance-linear function of the temperature, in contrast with designs previously disclosed (Fig. 1), where Ta is an almost resistance-linear function of temperature.
To carry out the invention, the element RAT may be assembled of commercially available, so-called thermilinear thermistor components and of suitably chosen resistors.
Operative within a wide temperature range is a circuit as shown in Fig. 6, where Tl and T2 are a commercially available thermistor combination. The resistors R3 and R5 are resistors supplied by a manufacturer of the thermilinear thermistor, or other equivalent.
R6 and R4 are selected to have a resistance such that the requisite Ts = f(Ta) is accomplished, if desired also taking into account the temperature coefficient of the hu~idity detector 10, in which case Ts = fl(Ta).
Also within the scope of the invention is the replacement o~ the temperature-dependent RAT with a resistor independent of temperature. It is obvious that if RAT has constant resistance, then the bridge of Fig. 4 will be in equilibrium at a certain constant value TSl of the detector temperature Ts, and Ts will remain constant at TSl if TSl ~ Ta, whereby the temperature of the respective humiditv detector 10 is then also constant. The detector 10 indicating the RH value may then be calibrated to indicate either the dewpoint or the absolute humidity, with certain restrictions.
As shown in Fig. 5, ~he measuring head comprises a cylindrical, elongated housing 100, within which all the electronic components of the arrangement have been mounted on a clrcuit board 102. To the insulator body 101 has been attached a radiation shield 103, for instance a piece of sheet metal with mirror surface. On one side of the shield 103 has been placed the humia~ty detector 10, provided with a heating resistance RST as described above, and on the other side a thermistor assembly To monitoring the ambient temperature Ta, comprising the thermist~rs Tl and T2 of the resistor/thermistor assembly RAT. The lead 104 departs from the measuring head 100 to the indicator instrument.
Fig 8 shows, in addition to the electronic components of the control means of the inventlon, also the electronic circuit measuring the capacitance CM of the humidity detector 10.
In the following are given the resistance and capacitance values of the resistors and capacitors used in the . .
circuit of Fig. 8 and the type designations of its other components.
Rl = 350 ohms Cl = 10 nF
R2 = 10 kohms C2 = 10 nF
R3 = 10 kohms C3 = 10 nF
R4 = 390 ohms C4 = 10 nF
R5 = 390 ohms C5 = 39 pF
R6 = 390 ohms C6 = 0.8 - 10 pF
R7 = 2 kohms, trimmer C7 = 22 nF
R8 = 5700 ohms, 0.1% C8 = 10 ~F, tantalum Rg = 1200 ohms, 0.1% Cg = 1 ~F, tantalum Rlo = 2 ohms Rll = 100 ohms TRl = 2N 3227 - R12 = 4698 ohms TR2 = 2N 3227 R14 = 4110 ohms TR3 = 2N 3904 R15 = 47 kohms Rs = P~100, Heraeus ICl = LM 35 ~ YSl thermistor 44202 T2 = ) The following resistors are equivalent in Figs. 6 and
3 19~ R4 R22' Rs = R18, and R6 = R20 In the embodiment of Fig. 8, the operating temperature range has been chosen to be -5C < Ta < 45C. The detector temperature Ts is so controlled that the humidity 'seen' by the detector is 0.75 times the humidity of the fluid under measurement.
Furthermore, correction is applied for the temperature dependence of the humidity detector, which is assumed to be +0.07% RH/1C.
For the temperature-dependent part of RAT there has been chosen the thermi-linear thermistor package 44202 of YSI (Yellow Springs Instrument Co., Ohio), comprising the thermistors Tl and T2 and the resistors R3 = 5700 ohms and R5 = 12000 ohms. RST is a PtlOO (DIN 43760~ platinum -temperature detector.
The desired temperature dependence Ts = f(Ta) was to begin with tabulated, using tables published in the Smithsonian Meteorological Tables. A new, corrected table was then prepared for Ts = fl(Ta) which accounts for the temperature correction of the humidity detect~r 10. With the aid of the equilibrium equations of the brid~e and the resistance values of thermistors Tl and T2, values are calculated for R4 and R6 which meet the required temperature dependence Ts = fl(Ta).
When for the resistors R4 and R6 the values of, respectively, 4698 ohms and 2 ohms are chosen and when the product of ~1 and R2 is 411037 ohms, the theoretical deviation s from the desired temperature Ts = fl(Ta) shown in the graph of Fig. 6 is obtained at different values of ambient temperature Ta in the range from ~5 to 45DC.
Furthermore, correction is applied for the temperature dependence of the humidity detector, which is assumed to be +0.07% RH/1C.
For the temperature-dependent part of RAT there has been chosen the thermi-linear thermistor package 44202 of YSI (Yellow Springs Instrument Co., Ohio), comprising the thermistors Tl and T2 and the resistors R3 = 5700 ohms and R5 = 12000 ohms. RST is a PtlOO (DIN 43760~ platinum -temperature detector.
The desired temperature dependence Ts = f(Ta) was to begin with tabulated, using tables published in the Smithsonian Meteorological Tables. A new, corrected table was then prepared for Ts = fl(Ta) which accounts for the temperature correction of the humidity detect~r 10. With the aid of the equilibrium equations of the brid~e and the resistance values of thermistors Tl and T2, values are calculated for R4 and R6 which meet the required temperature dependence Ts = fl(Ta).
When for the resistors R4 and R6 the values of, respectively, 4698 ohms and 2 ohms are chosen and when the product of ~1 and R2 is 411037 ohms, the theoretical deviation s from the desired temperature Ts = fl(Ta) shown in the graph of Fig. 6 is obtained at different values of ambient temperature Ta in the range from ~5 to 45DC.
Claims (9)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A control arrangement for a humidity detector, the operation of which is based on a change of impedance of the detec-tor, the detector being electrically heated under the control of said arrangement by means of a heating resistance to a temperature (Ts) which is higher than the ambient temperature (Ta) of the detector with a view of improving the accuracy of measurement of the detector or to increasing its life, said control arrangement comprising a bridge circuit containing temperature-dependent resis-tance elements responsive to said ambient temperature (Ta) and said detector temperature (Ts), an error voltage of said bridge circuit being used as a feedback signal to control a current source for heating the detector, said heating resistance for the detector comprising a resistance element with a positive tempera-ture coefficient, which at the same time serves as a sensor of the detector's temperature, and said temperature-dependent resistance element responsive to said ambient temperature (Ta) comprising an assembly of at least one resistor and termistor enabling a function Ts = f(Ta) peculiar to each detector type to be realized with the aid of the control arrangement.
2. A control arrangement according to claim 1, wherein said assembly is dimensioned to realize a given said function Ts =
f1(Ta) by applying a suitable correction for the temperature depen-dence of the detector.
f1(Ta) by applying a suitable correction for the temperature depen-dence of the detector.
3. A control arrangement according to claim 1 or 2, wherein the conductance (GAT) of said assembly is a substantially linear function of the pick-up's ambient temperature (GAT = 1/RAT =
K1 x Ta + C1, where K1 and C1 are expediently selected constants).
K1 x Ta + C1, where K1 and C1 are expediently selected constants).
4. A control arrangement according to claim 1, where-in the combination (RST) of heating resistance for the detector and sensor for the detector temperature (Ts) and said assembly are incorporated in opposing different arms of said bridge circuit.
5. A control arrangement according to claim 4, where-in the impedances of the different bridge arms are so chosen that the current (I) in that arm of the bridge which includes the heat-ing resistance + sensor combination (RST) is of greater order of magnitude than the current (I1) in the opposite arm (I2>>I1).
6. A control arrangement according to claim 1, wherein said humidity detector is a capacitive humidity detector having as its insulator material a polymer, the dielectric constant of which varies as a function of humidity.
7. A control arrangement according to claim 1, wherein said heating resistance for the detector is a platinum resistance.
8. A control arrangement for an absolute humidity or dewpoint detector, the operation of which is based on a change of impedance of the detector, the detector being electrically heated under the control of said arrangement by means of a heating resistance to a temperature (Ts) which is higher than the ambient temperature (Ta) of the detector with a view of improving the accuracy of measurement of the detector or to increasing its life, said control arrangement comprising a bridge circuit containing a temperature-dependent resistance element responsive to said detector temperature (Ts) and a given fixed substantially tempera-ture-independent resistance, an error voltage of said bridge circuit being used as a feedback signal to control a current source for heating the detector, said heating resistance for the detector com-prising a resistance element with a positive temperature coefficient, which at the same time serves as a sensor of the detector's tempera-ture, and said fixed re-sistance being selected such that the detector is heated to a given constant temperature (Ts1) which is higher than the abso-lute humidity or dewpoint observation temperature.
9. A control arrangement for a humidity detector, the operation of which is based on a change of impedance of the detector, the detector being electrically heated under the con-trol of said arrangement by means of a heating resistance to a temperature (Ts) which is higher than the ambient temperature (Ta) of the detector with a view of improving the accuracy of measurement of the detector or to increasing its life, said control arrangement comprising a bridge circuit containing a temperature-dependent resistance element responsive to said detector temperature (Ts), and a further resistance element, an error voltage of said bridge circuit being used as a feed-back signal to control a current source for heating the detect-or, said heating resistance for the detector comprising a resis-tance element with a positive temperature coefficient, which at the same time serves as a sensor of the detectors tempera-ture, said further resistance element being selected in accor-dance with predetermined heating requirements of said detector.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FI791061A FI58403C (en) | 1979-03-29 | 1979-03-29 | ADJUSTMENT OF FUNCTIONS |
| FI791061 | 1979-03-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1176345A true CA1176345A (en) | 1984-10-16 |
Family
ID=8512537
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000348669A Expired CA1176345A (en) | 1979-03-29 | 1980-03-28 | Temperature control means for a humidity detector |
Country Status (6)
| Country | Link |
|---|---|
| JP (1) | JPS5629151A (en) |
| CA (1) | CA1176345A (en) |
| DE (1) | DE3011525C2 (en) |
| FI (1) | FI58403C (en) |
| FR (1) | FR2452709A1 (en) |
| GB (1) | GB2047431B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2018188636A1 (en) * | 2017-04-13 | 2018-10-18 | 深圳市科兰德实业发展有限公司 | Method and system for controlling dehumidifying device of bridge cable |
Families Citing this family (23)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5870152A (en) * | 1981-10-23 | 1983-04-26 | Fuji Xerox Co Ltd | Detector for temperature and humidity |
| JPS5870153A (en) * | 1981-10-23 | 1983-04-26 | Fuji Xerox Co Ltd | Detector for temperature and humidity |
| DE3525115A1 (en) * | 1984-08-25 | 1987-01-22 | Hoelter Heinz | Hazardous-material sensor for motor vehicles, working-protection cabs and other stressed spaces having analogous compensation for the influences of temperature and air humidity on semiconductor gas sensors |
| DE3500412C1 (en) * | 1985-01-11 | 1986-08-28 | Beutler Maschinenbau- und Vertriebsgesellschaft Inhaber Wolfgang Beutler, 1000 Berlin | Device for indicating an undesired gas |
| DE3704534C1 (en) * | 1987-02-13 | 1988-10-27 | Bert Dr-Ing Kueppers | Circuit arrangement for temperature-dependent heating elements |
| DE3708697A1 (en) * | 1987-03-18 | 1988-09-29 | Draegerwerk Ag | METHOD AND ARRANGEMENT FOR MEASURING THE DEW POINT |
| US4793182A (en) * | 1987-06-02 | 1988-12-27 | Djorup Robert Sonny | Constant temperature hygrometer |
| US4793181A (en) * | 1987-06-02 | 1988-12-27 | Djorup Robert Sonny | Constant temperature sorption hygrometer |
| DE3900184A1 (en) * | 1989-01-05 | 1989-08-03 | Jun Martha Resch | Precipitation-dependent control for covering systems |
| DE3919864A1 (en) * | 1989-06-19 | 1990-12-20 | Testoterm Mestechnik Gmbh & Co | CAPACITIVE HUMIDITY SENSOR |
| DE3924518A1 (en) * | 1989-07-25 | 1991-01-31 | Haefele Umweltverfahrenstechik | TEMPERATURE SENSOR AND METHOD FOR THE PRODUCTION THEREOF |
| DE4130513C2 (en) * | 1991-09-13 | 1996-10-24 | Pierburg Ag | Temperature controller |
| FI95626C (en) * | 1993-09-29 | 1996-02-26 | Vaisala Oy | Procedure and arrangement for measuring humidity, especially in radio probes |
| FI99164C (en) * | 1994-04-15 | 1997-10-10 | Vaisala Oy | Method for measuring the dew point or gas content and equipment for predicting freezing |
| DE4414736A1 (en) * | 1994-04-27 | 1995-11-09 | Roth Technik Gmbh | Controller |
| US5792938A (en) * | 1996-12-13 | 1998-08-11 | Panametrics, Inc. | Humidity sensor with differential thermal detection and method of sensing |
| DE19729697C1 (en) * | 1997-07-11 | 1999-02-11 | Mannesmann Vdo Ag | Arrangement for determining the relative humidity |
| GB2345597B (en) * | 1999-01-11 | 2003-07-02 | Otter Controls Ltd | Improvements relating to thermal controls |
| DE10033620A1 (en) * | 2000-07-11 | 2002-01-31 | Testo Gmbh & Co Kg | Device and method for determining the moisture in gases |
| DE10335553A1 (en) | 2003-08-02 | 2005-02-17 | E + E Elektronik Ges.M.B.H. | Method and arrangement for moisture measurement |
| US20160003975A1 (en) * | 2013-02-22 | 2016-01-07 | Vaisala Oyj | A radiosonde and a method for atmospheric measurements performed at an elevated temperature |
| DE102017210064A1 (en) * | 2017-06-14 | 2018-12-20 | E + E Elektronik Ges.M.B.H. | Method for operating a sensor arrangement and suitable sensor arrangement for this purpose |
| CN108535338B (en) * | 2018-01-25 | 2020-12-22 | 中国科学院东北地理与农业生态研究所 | Coarse spatial resolution satellite remote sensing soil moisture authenticity inspection method |
Family Cites Families (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2108287C3 (en) * | 1971-02-20 | 1975-05-15 | Varian Mat Gmbh, 2800 Bremen | Device for thermostatting a component, in particular a field probe for measuring the magnetic field strength in sector FeM mass spectrometers |
| US3743930A (en) * | 1971-08-13 | 1973-07-03 | Burrows Equipment Co | Moisture tester having temperature controlled variable time delay circuit |
| JPS4888997A (en) * | 1972-02-23 | 1973-11-21 | ||
| DE2247394A1 (en) * | 1972-09-27 | 1974-04-18 | Siemens Ag | THERMOSTAT WITH A TEMPERATURE SENSOR PROVIDED TO DETERMINE THE THERMOSTAT TEMPERATURE |
| DE2260959C3 (en) * | 1972-12-13 | 1978-03-30 | Siemens Ag, 1000 Berlin Und 8000 Muenchen | Continuously regulating thermostat |
| GB1451231A (en) * | 1973-03-30 | 1976-09-29 | Nat Res Dev | Gas sensitive devices |
| US3868846A (en) * | 1973-07-02 | 1975-03-04 | Ford Motor Co | Circuit for converting a temperature dependent input signal to a temperature independent output signal |
| US4080564A (en) * | 1975-10-02 | 1978-03-21 | Matsushita Electric Industrial Co., Ltd. | Humidity sensitive resistor device |
| FI58402C (en) * | 1977-12-02 | 1981-01-12 | Vaisala Oy | FARING EQUIPMENT WITHOUT ELECTRICAL EQUIPMENT HOS EN ELECTRICAL FUNCTIONAL EQUIPMENT |
-
1979
- 1979-03-29 FI FI791061A patent/FI58403C/en not_active IP Right Cessation
-
1980
- 1980-03-25 DE DE19803011525 patent/DE3011525C2/en not_active Expired
- 1980-03-28 GB GB8010526A patent/GB2047431B/en not_active Expired
- 1980-03-28 FR FR8007114A patent/FR2452709A1/en active Pending
- 1980-03-28 CA CA000348669A patent/CA1176345A/en not_active Expired
- 1980-03-29 JP JP4111780A patent/JPS5629151A/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2018188636A1 (en) * | 2017-04-13 | 2018-10-18 | 深圳市科兰德实业发展有限公司 | Method and system for controlling dehumidifying device of bridge cable |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2452709A1 (en) | 1980-10-24 |
| GB2047431B (en) | 1983-08-03 |
| DE3011525C2 (en) | 1985-03-28 |
| DE3011525A1 (en) | 1980-12-18 |
| FI58403C (en) | 1981-01-12 |
| FI58403B (en) | 1980-09-30 |
| GB2047431A (en) | 1980-11-26 |
| JPS5629151A (en) | 1981-03-23 |
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