GB2047431A

GB2047431A - Temperature control means in a humidity pick-up

Info

Publication number: GB2047431A
Application number: GB8010526A
Authority: GB
Original assignee: Vaisala Oy
Current assignee: Vaisala Oy
Priority date: 1979-03-29
Filing date: 1980-03-28
Publication date: 1980-11-26
Also published as: FI58403B; DE3011525A1; DE3011525C2; CA1176345A; JPS5629151A; FI58403C; GB2047431B; FR2452709A1

Abstract

A capacitive humidity pick-up (10), Fig. 4 the dielectric constant of which varies as a function of humidity, has a temperature control arrangement whereby it is heated electrically to a temperature (Ts) which is a function of and is higher than the ambient temperature (Ta), such that (Ts) = f(Ta). The control arrangement comprises a bridge circuit Fig. 3 including an NTC thermistor RAT responsive to ambient temperature and a PTC resistor RST which is responsive to the pick-up temperature and also serves as the heating element for the pick-up. <IMAGE>

Description

SPECIFICATION Control means in a humidity pick-up The present invention concerns a control means in a humidity pick-up of which the operation is based on changing impedance of the pick-up, in particular in a capacitive humidity pick-up having as its insulator a polymer substance of which the dielectric constant varies as a function of humidity, and said control means being applied to control the heating of the pick-up with electricity either directly or indirectly to obtain a temperature higher than the temperature ambient to the pick-up, to the purpose of improving the pick-up's accuracy of measurement and/or to increase its life span, and said control means comprising a bridge circuit or equivalent containing temperature-dependent resistor elements by the aid of which said ambient temperature and the temperature of the pick-up are monitored, and the error voltage of the bridge circuit being employed as feedback signal by which the electric power heating the pick-up is controlled.

The same applicant's Finnish Patent No. 48229 discloses a capacitive humidity pick-up having as its dielectric material a polymer film, of which the dielectric constant is a function of the water quantity which the polymer film has absorbed. In the above-presented, and even other humidity pick-ups working on the basis of a change in impedance, undesirable phenomena occur, in particular when high humidities are being measured. These phenomema include for instance, slow creep of the pick-up, which may be due to a number of factors. As a rule however reversible phenomena are then concerned, and their controlling better than heretofore is the aim of the invention, the object being to achieve higher than before speed of the pick-up and to improve the accuracy of measurement, in particular at high relative humidities, such as those above 75 to 90% for instance.

The same applicant's Finnish patent application No. 773680 discloses a procedure for reducing the undesirable properties of an electrical humidity pick-up, such procedure being mainly characterized in that the humidity-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 furthermore 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 pick-up is kept at a height consistent with the equation T5 = K Ta + C, where T5 is the pick-up 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 and 4.1000 for various ranges of the temperature Ta, whereby the humidity pick-up "sees" 0.75 times the prevailing ambient humidity.

The equation stated, T5 = K Ta + C, is an approximate equation, by which one achieves for instance within the temperature range between - 20 and + 1 00 C, about + 0.2"C accuracy in the control of the pick-up temperature Ta. The electronic circuit design of this apparatus of prior art is as shown in the attached Fig. A, where the resistors R31, R32, Ra and Ra 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 on the ambient temperature Ta and R5 is a PTC resistor with equally nearly linear dependence of the pick-up's temperature T5, in the exemplary case both being platinum resistors Pt100, the bridge will be in equilibrium as soon as the said equation T5 = K Ta + C is satisfied.

The humidity pick-up (not visible in Fig. A), the resistor Rs and a separate heating resistance RH are mechanically joined by cementing, whereby there is a thermal coupling TC to the resistor R5 and to the humidity pick-up attached to these. Assuming the gain of the operation amplifier IC1 to be very high, which is satisfied in practice, the mode of operation of the circuit is physically easy to understand. To the resistance RH continuously a power is supplied which maintains the bridge in equilibrium 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 is hereby unbalanced, causing increase of the current passing through the transistor TRO connected after IC1, increasing the power input into the heating resistance RH, and as a result the temperature of the resistor Rs will rise until equilibrium of the bridge has been attained. A substantially constant current through the temperature-sensitive resistors is accomplished by stabilizing the voltage supplying the bridge and by selecting the resistors R31 and R32 to have high resistivity, for instance 10 kohms.

The constant K is accomplished by so adjusting the resistance RV, that the current ratio la/15 will have a numerical value equally the desired K value. The constant C is adjusted as desired by altering the resistance RV2, thereby affecting the error voltage between the points X, Y.

It has been possible, in the way just described, to carry out almost satisfactorily the controlled adjustment of the humidity pick-up's temperature. However, certain drawbacks encumber this procedure of prior art, and these shall be considered in detail in the following.

One requires in the apparatus, adjoined to the humidity pick-up, two separate additional elements: the temperature-sensitive resistor Ra and the heating resistance RH, and this makes the pick-up/resistor combination hard to manufacture. The resistors Ra and Ra with temperaturedependent resistance are, out of practical considerations, industrially made resistive metallic temperature pick-ups having a relatively low impedance, and they are for instance Pt100 resistors. Ra in particular, which has to present low thermal inertia, must be prevented from heating itself, which implies that care must be taken to apply only a low voltage across Ra, on the order of 1 50 m.It follows that exceedingly high requirements have to be imposed on the electronics, for instance on the operation amplifier IC1, and the electronics will also be difficult to calibrate and their operation will require special care for instance in avoiding contact potentials at connectors. Moreover it is a fact that the temperature control in the apparatus of prior art, when occurring in accordance with the equation Tea = K Ta + 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 is 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 dependence of the humidity pick-up.

In order to attain the objects mentioned and others which will become apparent later on, the invention is mainly characterized in that as heating resistance for the pick-up 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 Tea = f(Ta) characteristic of each individual pick-up type is realized with the aid of the control means.

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 consumption.

In the following the invention is described in detail with reference being made to certain embodiment examples of the invention presented schematically in the figures of the attached drawing and which are based on a capacitive humidity pick-up having an organic polymer as its humidity-sensitive material.

Figure A presents the control means of prior art which was discussed above.

Figure 1 presents, schematically, the humidity pick-up and the control means thereto attached.

Figure 2 shows the section along the line ll-ll in Fig. 1.

Figure 3 presents the bridge circuit employed in the various control means of the invention and the current regulating means associated therewith.

Figure 4 presents schematically the embodiment of the measuring head for a humidity pick-up provided with a control means according to the invention.

Figure 5 shows in greater detail the embodiment of the bridge circuit employed in the control means of the invention.

Figure 6 displays the differential temperature Aa established by the aid of the control means of the invention, as a function of the ambient temperature Ta Figure 7 presents the detailed circuit diagram of a measuring instrument provided with the control means of the invention.

The humidity pick-up depicted in Figs. 1 and 2 is in itself known in the art through the same applicant's Finnish Patent No. 48229. The base of the pick-up 10 is a supporting 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 the thin film technology, bottom contacts 1 2 have been produced by metallizing on the substrate 11 and by soldered joints 1 6 thereto have been attached contact leads, from which the capacitance is measured and indicated with the apparatus schematically represented in Fig. 1 by the blocks 20 and 22. The pick-up comprises, as active substance, a thin (for instance thickness about 10 itm order of magnitude) polymer film.Upon the polymer film 1 3 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 1 2. Hereby the capacitance to be measured 0m is established through the series connection of the capacitances formed between the bottom contacts 1 2 and the surface contact 14 in the areas d and e (Fig. 2).

In Fig. 1, the block 1 7 has been included to represent those means by which the ambient temperature Ta is measured, its signal being conducted through the connector 1 9 to the measuring and control means 20. Fig. 2 reveals the resistance element RST produced e.g. by evaporation in conjunction with the substrate 11, this resistance serving to keep the pick-up 10, using the control means of the invention, at a temperature Ta which is higher than the ambient temperature Ta. The connector 1 8 in Fig. 1 illustrates that functional connection by which the heating cuttent 12 is carried to the resistance RST and through which the temperature Ta of the pick-up 10 is measured.

In the bridge circuit of Fig. 3, RST is a low-impedance, temperature-sensitive resistance element, for instance a Pt100 platinum resistance, which measures and through the self-heating caused by the measuring current increases the temperature of the humidity pick-up 10 or of the fluid surrounding it, which usually is air. The resistance element RST has been affixed to the humidity pick-up 10 or been formed on the surface of the pick-up 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. 3, RAT is a high-impedance NTC element dependent on the ambient temperature, for instance a thermistor/resistor circuit assembly which has been made such that the impedance of RAT is a function of the pick-up and ambient temperature Ta in a certain pre-selected way, The resistances R1 and R2 in the bridge circuit have constant resistance value and minimal temperature dependence, whereby they may as well be placed elsewhere, not in the immediate vicinity of the pick-up 10, in a temperature differing from that of the fluid which is the subject of humidity measurement.

As shown in Fig. 3, the equilibrium of the bridge RAT, R1, R2, RST is measured by observing the error voltage AU, by which the operation amplifier IC1 is governed, this amplifier in turn controlling the transistor TR3, through which the current I = I, + 12 is conducted to the bridge.

As shown in Fig. 4, the resistance elements RAT and RST and the humidity pick-up 0 have been accomodated 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 operation amplifier IC1 depicted in Fig. 3 has a high gain, it becomes possible by the means to control the temperature Ta = f(Ta) of the pick-up 10 so that f(Ta) is a function of the kind desired. As taught by the invention, Ta = f(Ta) is fixed so that in every instance the 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 dependence Tea = f1(Ta) proper for the case in hand.

In the invention, the currents flowing in the legs of the bridge of Fig. 3 have been so adjusted that the current 12 heats the resistor Rust, which at the same time operates as resistive temperature pick-up, measuring its own temperature, whereby 12 will always be adjusted so that the equilibrium equation of the bridge R, R2 = RAT RUST is satisfied. The condition 12 < < 12 is catered for by taking care that the impedance (RAT + R1) is > > (R2 + Rust) 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.A), where Ta is an almost resistance-linear function of temperature.

For carrying out the invention 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. 5, where T1 and T2 are a commercially available thermistor combination. The resistors R3 and R5 are resistors supplied by the manufacturer of the thermilinear thermistor, or other equivalent.

R6 and R4 are selected to have such resistance that the requisite Ta = f(Ta) is accomplished, if desired also taking into account the temperature coefficient of the humidity pick-up 10, in which case Tea = f1(Ta).

Also within the scope of the invention is the replacement of the temperature-dependent RAT with a resistor independent of temperature. It is obvious that if RAT has constant resistance, then the bridge of Fig. 3 will be in equilibrium at a certain constant value Tai of the pick-up temperature Ta, and Ta will remain constant at Tai if Tai > Ta. whereby the temperature of the respective humidity pick-up 10 is then also constant. The pick-up 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. 4, the measuring head comprises a cylindrical, elongated housing 100, within which all the electronic components of the means have been mounted on a circuit 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 humidity pick-up 10, provided with a heating resistance RST as described above, and on the other side a thermistor assembly To monitoring the ambient temperature Tat comprising the thermistors Ta and T2 of the resistor/thermistor assembly RAT. The lead 104 departs from the measuring head 100 to the indicator instrument.

Fig. 7 shows, in addition to the electronics of the control means of the invention, also the electronic circuit measuring the capacitance CM of the humidity pick-up 1 0.

In the following are given the resistance and capacitance values of the resistors and capacitors used in the circuit of Fig. 7 and the type designation of its other components.

R1 = 350 ohms C1 = 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 R9 = 1200 ohms, 0.1% C9 = 1 F, tantalum R10 = 2 ohms R11 = 100 ohms TR1 = 2N 3227 R12 = 4698 ohms TR2 = 2N 3227 R14 = 4110 ohms TR3 = 2N 3904 R15 = 47 kohms RS=Pt100, Heraeus IC1 =LM 358 T1 = YS1 thermistor 44202 T2 = The following resistors are equivalent in Figs. 5 and 7: R3 = R19, R4 = R22, R5 = R18, and R6 = R20.

In the embodiment of Fig. 7, the operating temperature range has been chosen to be - 5 C < Ta < 45 C. The pick-up temperature Ts is so controlled that the humidity 'seen' by the pick-up is 0.75 times the humidity of the fluid under measurement. Furthermore, correction is applied for the temperature dependency of the humidity pick-up, which is assumed to be + 0.07% RH/1 C. For the temperature-dependent part of RAT there has been chosen the thermilinear thermistor package 44202 of YSI (Yellow Springs Instrument Co., Ohio), comprising the thermistors T1, and T2 and the resistors R3 = 5700 ohms and R5 = 12000 ohms. RST is a Pt100(DIN 43760) platinum temperature pick-up.

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 Ta = f,(T") which accounts for the- temperature correction of the humidity pick-up 10. With the aid of the equilibrium equations of-the bridge and the resistance values of thermistors T1 and T2,values are calculated for R4 and R6 which meet the required temperature dependence Ts = f1(Ta).

When for the resistors R4 and R6 the values of, respectively, 4698 ohms and 2 ohms are chosen and when the product of R1 and R2 is 411037 ohms, the theoretical deviation #Ts from the desired temperature Ts = f1(Ta) shown in the graph of Fig. 6 is obtained at different values of ambient temperature Ta in the range from - 5 to 45 C.

In the following, the claims are presented, various details of the invention being allowed to vary within the scope of the inventive. idea defined by them and to deviate from that which has been presented hereinabove.

Claims

1. A control means in a. humidity pick-up (10) of which the operation is based on change of impedance of the pick-up, in particular in a capacitive humidity pick-up having as its insulator material a polymer of which the dielectric constant varies as a function of humidity, and the pick-up (10) being under control by said control means heated with electricity directly or indirectly to a temperature (tea) which is higher than the ambient temperature (Ta) of the pick-up (10) with a view to improving the accuracy of measurement of the pick-up (10) or to increasing its life span, and said control means comprising a bridge circuit (RAT, R1, R2, RST) or equivalent containing temperature-dependent resistance elements (RAT., RST) with the aid of which said ambient temperature (Ta) and the pick-up temperature (Ts) are observed and the error voltage of said bridge circuit (AU) being used as feedback signal by which the electric power heating the pick-up (10) is controlled, characterized in that as heating resistance for the pick-up is used such a resistance element (RsT) with positive temperature coefficient (PTC), for instance a platinum resistance, which at the same time serves as sensor of the pick-up's temperature, and that as sensor for the ambient temperature (Ta) is used such as resistor/thermistor assembly (RAT) that a given function Ts = f(Ta) peculiar to each pick-up type is realized with the air of the control means.

2. Control means according to claim 1, characterized in that said resistor/thermistor assembly (RAT) has been dimensioned to realize a given function Ts = ft(Ta) by applying which correction is made for the temperature dependence of the pick-up.

3. Control means according to claim 1 or 2, characterized in that the conductance (6AT) af the resistor/thermistor assembly is a substantially linear function of the pick-up's ambient temperature (GAT = 1 /RAT = K, X Ta + C,, where K, and C, are expediently selected constants).

4. Control means according to claim 1, 2 or 3, characterized in that in the bridge circuit comprised in the means the combination (RsT) of heating resistance for the pick-up (10) and sensor for the pick-up temperature (tea) and said resistor/thermistor assembly are incorporated in different arms of the bridge opposing each other.

5. Control means according to claim 4, characterized in that the impedances of the different bridge arms are so chosen thst the current (I+) in that arm of the bridge which includes the heating resistance/sensor combination (RsT) is of a greater order of magnitude than the current (I,) in the opposite arm (l2 > > l1).

6. Control means according to claim 1, 2, 3, 4 or 5, characterized in that for modifying the humidity pick-up (10) to operate as a sensor of absolute humidity and/or dewpoint, said resistor/thermistor assembly (RAT) has been replaced with a given fixed, substantially temperature-independent resistance and that the pick-up (10) is heated to a given constant temperature (Ts1) which is higher than the absolute humidity and/or dewpoint observation temperature (Ta).

7. Control means for a humidity pick-up substantially as described herein with reference to Figs. 1-7 of the drawings.