HUMIDITY MEASURING APPARATUS

Abstract: A hygrometer utilises a capacitive humidity-responsive sensor (10) as the active input device in a low frequency oscillator (11). The non-uniform temperature response of the sensor (10) is compensated by a temperature compensation circuit (12) which controls the supply voltage of the low frequency oscillator. The output from the low frequency oscillator (11) gates the signals generated by a high frequency oscillator (13) for a predetermined period which is derived from the the signal generated by the low frequency oscillator signal. The gated signal is counted by a counter (16), the output count of which is converted by a programmable read-only memory circuit (17) into a signal which is directly proportional to the relative humidity at the sensor (10). This last signal can be used to drive a digital display unit (18).

Description: TITLE: "HUMIDITY MEASURING APPARATUS" TECHNICAL FIELD This invention concerns instruments for measuri the moisture content of air and other gases. Su instruments are called hygrometers. If they use wet a dry bulb thermometers, they are known as psychrometers. BACKGROUND ART Probably the best known form of pεychrometer is th combination of wet and dry bulb mercury in glas thermometers. In this form of hygrometer, evaporation o moisture from the region of the wet bulb causes a lowerin of temperature of that bulb, which is registered by th wet bulb thermometer. From the observed difference i temperature recorded by the two thermometers, the absoiut or relative humidity of the gas in which the instrument i located (usually air) can be determined. For accurate results with the pεychrometer, the ai or gas that is being monitored should have a velocity o about 5 to 7 metres per second past the wet bulb Accordingly, these instruments are commonly provided wit a tube through which a small axial fan draws the gas at rate which ensures that the gas flows past the wet bulb a the required velocity. Normally the wet bulb is surrounde by a wick which is continually moistened by a wate reservoir. Attempts have been made to improve the accuracy o psychrometers by using electronics. One example of thi approach is provided by the pεychrometer described in th specification of Australian patent applicatio No 74448/81. That instrument employs two platinu resistive elements as the temperature sensin thermometers. Each platinum resistive element forms on arm of a respective DC-balance bridge. In order to achieve acceptable accuracy and to render the response of the instrument linear, an electronic circuit models a pεychrometric equation (relating the relative humidity t the wet and dry bulb temperatures) and converts the voltage difference between the two bridges into an output signal which indicates the relative humidity. The output signal is designed to vary substantially linearly with relative humidity. This electronic pεychrometer has been found to be suitable for use in a wide range of applications where the accuracy of the measurement of relative humidity need not be better than 2 to 3 percent, and when the relative humidity is in the range from about 20 percent to about 90 percent over a dry bulb temperature range of from 0 degrees C to 50 degrees C. A better accuracy caι be obtained when this instrument is not required to operate at temperatures lower than 20 degreeε C. In a departure from wet and dry bulb thermometry, humidity-dependent capacitive probeε have been uεed to construct hygrometers. Such probeε are described by S. Takeda in the article entitled "Capacitive humidity element using polystyrene thin film formed by plasma polymerisation", which appeared in volume 20, No 7 (July 1981) of "Japanese Journal of Applied Phyεicε", at pages 1219 to 1224. Briefly, this probe comprises a capacitive sensor in the form of a thin polymer diaphragm that is sandwiched between water permeable contacts. The response time of this type of probe is about 1 to 2 εecondε. Such probeε, however, have a non-linear response to changes in humidity and in addition their response varies according to the ambient temperature. The hygrometers using εuch probeε use sophisticated and expensive circuitry to overcome these diεadvantageε of εuch probeε, or they incorporate one or more capacitors with a capaciti temperature coefficient which is oppoεite to that of t probe. Although the capacitor approach iε leεε expensiv it is extremely difficult to find a combination capacitors of exactly the right oppoεite coefficient an the addition of extra capacitance further degrades th linearity of the hygrometer. DISCLOSURE OF THE INVENTION It iε an objective of the present invention t provide a humidity measuring device which haε an accurac which is at leaεt as good as presently used instruments which haε a rapid reεponεe to variationε in relativ humidity, which haε a stable operation over an acceptabl wide ambient temperature range, and which is relativel inexpensive to construct. This objective iε achieved by adopting a humidity dependent capacitive probe and compensating for it variation in responεe with temperature by using temperature compensation circuit which varies the suppl voltage of a low frequency oscillator which has the prob as its active element. In addition, digital linearisatio circuitry iε provided, thus producing a compact inεtrumen which haε a linear response to variationε in relativ humidity. According to the preεent invention, a hygromete comprises: a) a capacitive humidity-responεive εenεor; b) a low frequency oscillator; c) a temperature compensation circuit; and d) a high frequency oscillator; characterised in that i) the low frequency oscillator i responsive to changes in the capacitanc of said εenεor; ii) the temperature compensation circuit iε adapted to control the supply voltage of the low frequency oscillator and thereby compensate for variations with temperature of the response of said εenεor; iii) a gate iε provided to gate the signals from the high frequency oscillator for predetermined periods derived from the signal from the low frequency oscillator; iv) a counter iε adapted to count the number of oεcillationε in the signal from the high frequency oscillator during each εaid predetermined period; and v) an adjusting circuit is adapted to convert the output signal from fiaid counter into a signal which iε indicative of the relative humidity in the region of εaid εenεor. The signal produced by the action of the adjuεting circuit may be used to actuate a diεplay device, εuch as a visual indication of relative humidity, and/or may be stored or recorded in a suitable manner. These and other featureε of the present invention will become more apparent from the following description of an embodiment of the invention, in which reference will be made to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 iε a block schematic diagram of a measurement arrangement which incorporates the present invention. Figure 2 iε a graph showing the relationship between capacitive εenεor reεponεe and relative humidity that iε uεed for calibration purposes. O-.-PI WHO ■ DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS The measurement system illustrated in Figure 1 deεigned to provide information about both ambie temperature and relative humidity. The relative humidi measurement part of the illustrated system utilises t present invention, the humidity-sensitive element of t circuitry being a Vaiεala 6101 capacitive probe 10, whi is similar to the Takeda probeε which have been describ above. The εenεor 10 forms the active input device for low frequency oscillator 11, which iε preferably a CM oscillator. The output frequency of oscillator 11 varie as the capacitance of εenεor 10 varieε, so the outpu frequency of oscillator 11 alters with changes of humidit in the vicinity of sensor 10. As noted above, the response of εenεor 10 * i temperature-dependent. The temperature compensatio circuit 12 iε therefore uεed to control the output of th oscillator 11 so that the variation of response wit ambient temperature iε avoided. The compenεation i effected by varying the supply voltage (VCC) of th oscillator 11. Although any suitable temperatur compensation circuit may be uεed for thiε purpoεe, tha illuεtrated in Figure 1 uεes a two-terminal integrate temperature tranεducer 12A with a low-drift operationa amplifier, connected aε shown in the drawing. Th transducer 12A iε positioned close to sensor 10. The temperature compenεation iε achieved a follows. The gate capacitance of oscillator 11, over small range (typically 6 to 15 volts) iε a linear functio of the supply voltage VCC. If the supply voltage iε varie over thiε range, the effective gate capacitance of th oεcillator 11, and hence its total capacitance, is made t vary, resulting in a proportional change in the outpu frequency of the oscillator. The relationship betwee supply voltage and frequency can be determined b experiment. The gain of the amplifier in the temperatur compenεation circuit 12 can be adjusted to provide th initial compenεation. The variation of VCC that reεult from changes in the temperature around transducer 12 ensures that the compenεation iε maintained aε the ambien temperature alters. In a typical circuit arrangement, the capacitanc of the sensor 10 will vary from 107 pF to 122 pF aε th relative humidity changes from 0 percent to 100 percent In a prototype of the preεent invention, with eac resistor R of oεcillator 11 being 200 Kohmε, th oεcillator 11 had an output frequency which varied fro 24409 Hz to 27950 Hz. The output signal from the low frequency oεcillato 11 iε fed to a divide-by-100 circuit 14, the output o which iε connected to one input of gate 15. The othe input to gate 15 is the output signal of a high frequenc oεcillator 13. The high frequency oεcillator 13 i typically a crystal-controlled CMOS oscillator (such a oεcillator iε εhown in Figure 1) having an outpu frequency of about 4 MHz. (The prototype inεtrument tha waε mentioned above had a high frequency oεcillator tha operated at 3.86 MHz.) The output from gate 15 iε a - stream of high frequency oscillations gated through during each positiv half-cycle of the output of the diviεion circuit 1 . Theε high frequency oεcillationε are counted by counter 16, which iε preferably a εerial-in, parallel-out, binary coded decimal counter. The output count from counter 16 i directly proportional to the capacitance of εenεor 10, an thuε varieε aε the relative humidity in the vicinity o εenεor 10 changes. "ξfURE Becauεe the εenεor 10 doeε not have a linea reεponεe to changeε in humidity, it iε neceεεary t convert the output count from counter 16 into a valu which is linearly dependent on relative humidity. Thi conversion iε effected by an adjustment circuit 17, whic iε typically a programmable read-only memory (PROM) . Th adjuεtment circuit 17 iε essentially a correction table o "look-up table", stored in the PROM. The output count fro counter 16 forms the addresε code for the PROM and th PROM provideε an output εignal which iε directl proportional to relative humidity which corresponds to th "address code". The specific relative humidity signal that are generated by the PROM 17 have to be programme into it uεing a calibration proceεε, which involve monitoring the output of counter 16 when the probe 10 i immediately above a εerieε of εtandard εaturated 'sal solutions. One example of the readings that are obtaine by thiε proceεε iε εhown in Figure 2. The curve drawn i Figure 2 thuε forms the baεiε for the "look-up" table tha iε stored in the PROM 17. The output εignal from the adjuεtment circuit 17 i used to drive a digital diεplay 18, but it could also b stored in any suitable manner (for example, by printing o by recording on paper or magnetic tape, or by recording o a magnetic disc, or by a trace on a chart recorder) o uεed to activate an alarm (in a εyεtem where action needε to be taken if the relative humidity exceedε or falls below a predetermined value). In the embodiment εhown in Figure 1, a timer 19 iε provided to update the count rate from counter 16 every 4 seconds (other periods could be used, of course). Thiε meanε that the digitε displayed by display 18 are also updated every four seconds, thus providing a flicker-free indication of the relative humidity at εensor 10. Another feature present in the embodiment that i illustrated in Figure 1, but which does not form a eεεential part of the present invention, iε th simultaneous display of both humidity and ambien 5 temperature. The temperature display iε provided b display unit 20, which iε controlled by the output from εuitably scaled analogue-to-digital converter 21. Th εignal to the converter 21 iε derived from the output o the temperature compensation circuit 12. 0 An instrument having the features of the εyεte illuεtrated in Figure 1 waε constructed and found t produce readingε of relative humidity which were accurat to within 2 percent over a relative humidity range of 1 to 90 percent, while the ambient temperature varied fro 5 10 degreeε C to 60 degreeε C. Over the increaεe temperature range of 0 degreeε C to 80 degreeε C, : th accuracy of the relative humidity indicated waε within percent. INDUSTRIAL APPLICABILITY 0 The hygrometer of the preεent application may b uεed in airconditioning plantε and in any other locatio where a conventional hygrometer or pεychrometer would b uεed. It iε particularly uεeful in εituationε where rapi - reεponεe iε required, and where it iε inconvenient t locate a wet and dry thermometer arrangement. Induεtrie which will adopt the preεent invention include the bread making and plaεticε industries. If a humidity-dependen capacitive probe capable of high temperature uεe i produced, the present invention can also be used in th monitoring of motor vehicle exhaustε. Theεe application are only exemplary and are not limiting the application o the present invention.

Claims: CLAIMS 1. A hygrometer comprising: a) a capacitive humidity-reεponεive εenεor (10); b) a low frequency oεcillator (11); c) a temperature compenεation circuit (12); and d) a high frequency oεcillator (13); characterised in that i) the low frequency oscillator (11) i responεive to changeε in the capacitanc of εaid sensor; ii) the temperature compensation circuit (12 is adapted to control the supply voltag of the low frequency oεcillator (11) an thereby co penεate for variationε wit temperature of the response of .sai εenεor (10) ; iii) a gate (15) is provided to gate th signals from the high frequenc oεcillator (13) for predetermined period derived from the εignal from the lo frequency oεcillator (11); iv) a counter (16) iε adapted to count th number of oεcillationε in the εignal fro the high frequency oεcillator (13) durin each εaid predetermined period; and v) an adjusting circuit (17) iε adapted t convert the output εignal from εai counter (16) into a signal which iε indicative of the relative humidity in the region of εaid εenεor (10). OM?I / V/IPO 2. A hygrometer as defined in claim 1, furthe characterised in that εaid temperature compenεatio circuit (12) comprises a two-terminal integrate temperature transducer (12A) and a low-drif operational amplifier. 3. A hygrometer aε defined in claim 2, in which th output of εaid temperature compenεation circui (12) is also connected, via an analogue-to-digita converter (21), to a temperature display unit (20). 4. A hygrometer aε defined in claim 1, claim 2 o claim 3, further characterised in that a) the output of the low frequency oεcillato (11) iε εupplied to a frequency dividin circuit (14) and the signal from the frequenc dividing circuit (14) forms a first input t the gate (15); and b) the high frequency oscillator (13) comprises crystal controlled CMOS oscillator, the outpu of which forms a second input to the gat (15). 5. A hygrometer aε defined in any preceding claim, further characterised in that: a) the counter (16) iε a serial . in, parallel out, binary coded decimal counter; and b) εaid adjuεting circuit (17) iε a programmabl read only memory circuit containin calibration information that has bee experimentally determined, the output coun from the counter (16) providing the addreε code for the memory and the memory adapted t generate an output εignal having a magnitud which iε directly proportional to relativ humidity at εaid sensor (10). 6. A hygrometer aε defined in claim 5, in which timing circuit (19) iε adapted to control th update rate of the output count from the counte (16). 7. A hygrometer aε defined in any preceding claim further characterised in that the output εigna from εaid adjuεting circuit (17) is connected t one or more of: a) a digital display unit (20); b) a recording tape or magnetic disc; c) a chart recorder; and > d) an alarm.

Inventors: BENDELI ALEXANDER, WARD BARRY KENDRIC, BENDELI, ALEXANDER, WARD, BARRY, KENDRIC
Applicants: COMMW SCIENT IND RES ORG, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR
Application year: 1983
Main classification: G01N27
IPC codes: G01N27