CA1301477C - Chilled piezoelectric hygrometer - Google Patents
Chilled piezoelectric hygrometerInfo
- Publication number
- CA1301477C CA1301477C CA 611170 CA611170A CA1301477C CA 1301477 C CA1301477 C CA 1301477C CA 611170 CA611170 CA 611170 CA 611170 A CA611170 A CA 611170A CA 1301477 C CA1301477 C CA 1301477C
- Authority
- CA
- Canada
- Prior art keywords
- crystal
- piezoelectric quartz
- temperature
- frequency
- dew point
- 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.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N25/00—Investigating or analyzing materials by the use of thermal means
- G01N25/56—Investigating or analyzing materials by the use of thermal means by investigating moisture content
- G01N25/66—Investigating or analyzing materials by the use of thermal means by investigating moisture content by investigating dew-point
- G01N25/68—Investigating or analyzing materials by the use of thermal means by investigating moisture content by investigating dew-point by varying the temperature of a condensing surface
Landscapes
- Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Chemical & Material Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analyzing Materials Using Thermal Means (AREA)
Abstract
ABSTRACT
A dew point hygrometer called a CHILLED PIEZOELECTRIC
HYGROMETER, which is disclosed herein includes a dew point sensing element consisting of a piezoelectric quartz crystal resonator mounted on a Peltier cooler.
The piezoelectric quartz crystal resonator is excited to its resonant frequency and cooled to the point of condensation of water vapor on its surface. As the condensation occurs, the mass of the condensed water vapor onto the piezoelectric quartz crystal resonator surface causes a change in the resonant frequency. A
temperature sensor mounted on the surface of the piezoelectric quartz crystal resonator is use to monitor the temperature at which a dew deposit occurs.
The digital nature of the dew point sensor allows features like autocompensation and autocalibration to be implemented in the instrument.
A dew point hygrometer called a CHILLED PIEZOELECTRIC
HYGROMETER, which is disclosed herein includes a dew point sensing element consisting of a piezoelectric quartz crystal resonator mounted on a Peltier cooler.
The piezoelectric quartz crystal resonator is excited to its resonant frequency and cooled to the point of condensation of water vapor on its surface. As the condensation occurs, the mass of the condensed water vapor onto the piezoelectric quartz crystal resonator surface causes a change in the resonant frequency. A
temperature sensor mounted on the surface of the piezoelectric quartz crystal resonator is use to monitor the temperature at which a dew deposit occurs.
The digital nature of the dew point sensor allows features like autocompensation and autocalibration to be implemented in the instrument.
Description
13()~(177 BACKGROUND OF ~3 INV~NTION
1. Field of the invention The present invention relates to a method and apparatus, there~ore called ~CHI~ED PI~ZOELECT~IC
HYGROHETER", for measuring humidity content or dew point of air or gasec which i8 particularly usefull in process measurement as described herein with reference~ to such process.
1. Field of the invention The present invention relates to a method and apparatus, there~ore called ~CHI~ED PI~ZOELECT~IC
HYGROHETER", for measuring humidity content or dew point of air or gasec which i8 particularly usefull in process measurement as described herein with reference~ to such process.
2. Description o~ prior art As well known to those skilled in the art, a chilled hygrometer comprises a chilled surface and a mean for measuring humid air condensation on such surface, Prior art devices further comprises a mirror as the chilled surface and an optical mechanism which acts as the humidity measuring component. For the purpose of measuring the dew point temperature of air or gases, this one is circulated through a closed chanel and passed by the chilled mirror. The optical mechanism which consist of an emiting ~ource of light and a light detector iB mounted a~ to utilise the mirror to re~lect the light source towards the light detector.
A cooling mechanism such as a thermoelectric cooler or a gas is used to chill the mirror. Water vapor contained into the air or gas will condensate and form a thin coat of water onto the mirror. This condition will attenuate the amount of light reflected to the light detector. This amo,unt of light re~lected to the light detector enables a heat regulator to increase the tempera,ture of the mirror to evaporate the water present onto the mirror and thereby restorinq the reflection o~ liqht to the detector. In turn, when total light iB re~lected, the heat regulator will reduce the temperature of __ __ __ the ~ Qr~_for condensation to reoccur. This process is performed until equilibrum is achieved and a thin coat of water condensation is obtained.
A temperature sensor, located underneath the mirror enables the measurement of the mirror temperature.
This temperature is assumed to be the dew point temperature. In some earlier chilled mirror hygrometers, electronic circuitry enables the regulation o~ the mirror temperature and the generation o~ a signal whi¢h is outputed and which corresponds to the dew point temperature. Since -- the-4et~3r-mination of the dew point temperature is based on optical reflection by use of a mirror and that the mirror is exposed to the flow stream, the accuracy of such a device is impaired by the impurities that are present into the air or gas to be measured. Dirt and impurities deposlt onto the ,, mirror and cau~e eronous measurements. Noreover, ','~'~ 70 the optical system o~ such devices are fragile and require freguent calibration especially when used as airborne hygrometers or in industrial processes whPr~ ~nv~ ronment ~ e ~avare .
.,, .~
. ~ ~
.
130~7 SUHHARY OF THE INVENTION
The present invention measures humidity or dew point temperature not by reflecting a source of light onto a chilled mirror as in the case of the aforedescribed prior art systems but rather by detecting the change in frequency output of a chilled piezoelectric quartz crystal when condensation occurs onto its surface and by maintaininq its temperature untill equilibrium is obtained and a thin coat of water condensation is present.
In accordance with the principles of the present invention, a piezoelectric quartz-crystal is mounted onto a cooler which for the purpose of this invention is a thermoelectric or mechanical cooler. The piezoelectric quartz-crytal is excited to its resonnant frequency and cooled to the point of condensation of water vapor on its surface. As the condensation occurs, the mase of the condensed water vapor onto the piezoelectric quartz-crystal surface causes a change in the resonnant freqUenCY-If we assume that the addition of a coat of watervapor to the piezoelectric quartz-crystal surface produces the same effect as the addition of an equal mass of piezoelectric quartz-crystal, the following equation could be used to relate the coat of water thickness to the change in the piezoelectric quartz-crystal frequency:
= ~ * Pq(t-to) (1) Pm where : Nq : 1668 m sec i.e. frequency constant for an AT cut piezoelectric quartz-crystal vibrating in the thickness shear mode Pq : density of quartz (gm cm) to : period of the piezoelectric quartz-crystal with no coating (sec) t : period of the piezoelectric quartz-crystal with coating on it (Rec) : thickness of vapor coat Pm : density of the coating material water vapor in our case Equation 1 shows that it is possible of detecting the presence of a water vapor coating by monitoring the oscillation frequency of the chilled piezoelectric quartz-crystal. This is accomplished by an electronic circuitry which measures the frequency change and outputs an error signal to the temperature regulator. A
temperature regulator controls the temperature of the piezoelectric quartz-crystal to the temperature where water condensation occurs. The electronics measures the temperature at the surface of the piezoelectric quartz-crystal. This is the dew point temparature. The electronics also measures the ambiant temperature, this is the dry bulb temperature. From these two temperatures, the electronics calculates the relative humidity and absolute humidity. A signal processing and scaling electronics output~ the humidity data which consist in : the dew point temperature, relative humidity and ab~olute humidity ln the form of voltage, current loop or digital data.
H~IFF D~SCRIPTION OF TH~ DRA~ING
Figure 1 i8 a block diagram of the lnvention.
~' i 13~1~77 ION OF 1~113~ INV~NTION
Figure 1 depicts the chilled piezoelectric hygrometer block diagram. The piezoelectric quartz-crystal is mounted onto a thermoelectric cooler 3 and excited at its resonnant frequency by a piezoelectric S quartz-crystal driver 7. The resonnant frequency is measured by a resonnant freguency measurement circuit 8 providing an indieation of the frequency change to the signal processing and sealing eircuit 9. The frequency changes when water vapor eondenaates onto the piezoelectrie quartz-erystal 1, this ehange i8 perceived by the signal proeessing and sealing eireuit 9, whieh then aetivates the temperature regulator 6 to maintain the temperature at the temperature eorresponding to the occurance of the fregueney ~ehange. The temperature regulator 6 reeeives a #ignal from the signal proeessing and sealing eireuit g whieh is an indieation of the eooling or heating aetion to be taken by the eooler driver 5. The temperature regulator 6 monitors the temperature of the piezoelectr~c quartz-crystal to acheive the dew point temperature based on the oecurence of a signal indicating water vapor condensation onto the piezoelectric quartz-crystal by the signal proeessing and scaling cireuitry. The cooler driver 5 will supply more or less heat to the thermoeleetrie cooler 3 in order to maintain th~ piezoeleetrie quartz-erystal 1 at the dew point temperature. The temperature sensor 2 enables the temperature regulator 6 to measures the piezoelectric quartz-erystal temperature. The temperature sensor 4 enable~ the temperature regulator 6 to measure the ambiant temperature. The dew point temperature and ambiant temperature are regi~tered by the signal proeessing and sealing eireuit 9 to provide analog or digital output of the humidity in the form of dew point, relati~e humidity and absolute humidity. The power supply unit 10, provides all the voltages neeessary for the operation of the hygrometer.
The signal proeessing and sealing eireuit 9 provides also the means of displaying the dew point, relative humidity or water vapor eoneentration.
The signal processing and sealing cireuit 9 ean eonsist of a progra~mable digital eounter that can be preset to a given value eorresponding to a dew point, relative humidity or water vapor eoneentration value.
If the preset value is exeeeded a digital eomparator aetivates an alarm. The alarm ean be a relay eontaet elosure or visual like a lit lamp.
; 50 The signal proeessing and sealing eireuits 9 ean also eonsist of a mieroproeessor with a program stored ln his memory. The software eontained in the me ory ealeulate the dew point and relative humidity from the imputs of the resonant frequeney measurement eireuit 8, the temperature sensor 2 and temperature sensor 4. The mieroproeessor ean output data in the form of a dlgltal word, a frequeney through a frequeney eonverter or a 4-20 mA eurrent loop. The mleroproeessor ean also dlsplay the dew point, relative humldity or water vapor eoneentration on a display that the user ean read.
A cooling mechanism such as a thermoelectric cooler or a gas is used to chill the mirror. Water vapor contained into the air or gas will condensate and form a thin coat of water onto the mirror. This condition will attenuate the amount of light reflected to the light detector. This amo,unt of light re~lected to the light detector enables a heat regulator to increase the tempera,ture of the mirror to evaporate the water present onto the mirror and thereby restorinq the reflection o~ liqht to the detector. In turn, when total light iB re~lected, the heat regulator will reduce the temperature of __ __ __ the ~ Qr~_for condensation to reoccur. This process is performed until equilibrum is achieved and a thin coat of water condensation is obtained.
A temperature sensor, located underneath the mirror enables the measurement of the mirror temperature.
This temperature is assumed to be the dew point temperature. In some earlier chilled mirror hygrometers, electronic circuitry enables the regulation o~ the mirror temperature and the generation o~ a signal whi¢h is outputed and which corresponds to the dew point temperature. Since -- the-4et~3r-mination of the dew point temperature is based on optical reflection by use of a mirror and that the mirror is exposed to the flow stream, the accuracy of such a device is impaired by the impurities that are present into the air or gas to be measured. Dirt and impurities deposlt onto the ,, mirror and cau~e eronous measurements. Noreover, ','~'~ 70 the optical system o~ such devices are fragile and require freguent calibration especially when used as airborne hygrometers or in industrial processes whPr~ ~nv~ ronment ~ e ~avare .
.,, .~
. ~ ~
.
130~7 SUHHARY OF THE INVENTION
The present invention measures humidity or dew point temperature not by reflecting a source of light onto a chilled mirror as in the case of the aforedescribed prior art systems but rather by detecting the change in frequency output of a chilled piezoelectric quartz crystal when condensation occurs onto its surface and by maintaininq its temperature untill equilibrium is obtained and a thin coat of water condensation is present.
In accordance with the principles of the present invention, a piezoelectric quartz-crystal is mounted onto a cooler which for the purpose of this invention is a thermoelectric or mechanical cooler. The piezoelectric quartz-crytal is excited to its resonnant frequency and cooled to the point of condensation of water vapor on its surface. As the condensation occurs, the mase of the condensed water vapor onto the piezoelectric quartz-crystal surface causes a change in the resonnant freqUenCY-If we assume that the addition of a coat of watervapor to the piezoelectric quartz-crystal surface produces the same effect as the addition of an equal mass of piezoelectric quartz-crystal, the following equation could be used to relate the coat of water thickness to the change in the piezoelectric quartz-crystal frequency:
= ~ * Pq(t-to) (1) Pm where : Nq : 1668 m sec i.e. frequency constant for an AT cut piezoelectric quartz-crystal vibrating in the thickness shear mode Pq : density of quartz (gm cm) to : period of the piezoelectric quartz-crystal with no coating (sec) t : period of the piezoelectric quartz-crystal with coating on it (Rec) : thickness of vapor coat Pm : density of the coating material water vapor in our case Equation 1 shows that it is possible of detecting the presence of a water vapor coating by monitoring the oscillation frequency of the chilled piezoelectric quartz-crystal. This is accomplished by an electronic circuitry which measures the frequency change and outputs an error signal to the temperature regulator. A
temperature regulator controls the temperature of the piezoelectric quartz-crystal to the temperature where water condensation occurs. The electronics measures the temperature at the surface of the piezoelectric quartz-crystal. This is the dew point temparature. The electronics also measures the ambiant temperature, this is the dry bulb temperature. From these two temperatures, the electronics calculates the relative humidity and absolute humidity. A signal processing and scaling electronics output~ the humidity data which consist in : the dew point temperature, relative humidity and ab~olute humidity ln the form of voltage, current loop or digital data.
H~IFF D~SCRIPTION OF TH~ DRA~ING
Figure 1 i8 a block diagram of the lnvention.
~' i 13~1~77 ION OF 1~113~ INV~NTION
Figure 1 depicts the chilled piezoelectric hygrometer block diagram. The piezoelectric quartz-crystal is mounted onto a thermoelectric cooler 3 and excited at its resonnant frequency by a piezoelectric S quartz-crystal driver 7. The resonnant frequency is measured by a resonnant freguency measurement circuit 8 providing an indieation of the frequency change to the signal processing and sealing eircuit 9. The frequency changes when water vapor eondenaates onto the piezoelectrie quartz-erystal 1, this ehange i8 perceived by the signal proeessing and sealing eireuit 9, whieh then aetivates the temperature regulator 6 to maintain the temperature at the temperature eorresponding to the occurance of the fregueney ~ehange. The temperature regulator 6 reeeives a #ignal from the signal proeessing and sealing eireuit g whieh is an indieation of the eooling or heating aetion to be taken by the eooler driver 5. The temperature regulator 6 monitors the temperature of the piezoelectr~c quartz-crystal to acheive the dew point temperature based on the oecurence of a signal indicating water vapor condensation onto the piezoelectric quartz-crystal by the signal proeessing and scaling cireuitry. The cooler driver 5 will supply more or less heat to the thermoeleetrie cooler 3 in order to maintain th~ piezoeleetrie quartz-erystal 1 at the dew point temperature. The temperature sensor 2 enables the temperature regulator 6 to measures the piezoelectric quartz-erystal temperature. The temperature sensor 4 enable~ the temperature regulator 6 to measure the ambiant temperature. The dew point temperature and ambiant temperature are regi~tered by the signal proeessing and sealing eireuit 9 to provide analog or digital output of the humidity in the form of dew point, relati~e humidity and absolute humidity. The power supply unit 10, provides all the voltages neeessary for the operation of the hygrometer.
The signal proeessing and sealing eireuit 9 provides also the means of displaying the dew point, relative humidity or water vapor eoneentration.
The signal processing and sealing cireuit 9 ean eonsist of a progra~mable digital eounter that can be preset to a given value eorresponding to a dew point, relative humidity or water vapor eoneentration value.
If the preset value is exeeeded a digital eomparator aetivates an alarm. The alarm ean be a relay eontaet elosure or visual like a lit lamp.
; 50 The signal proeessing and sealing eireuits 9 ean also eonsist of a mieroproeessor with a program stored ln his memory. The software eontained in the me ory ealeulate the dew point and relative humidity from the imputs of the resonant frequeney measurement eireuit 8, the temperature sensor 2 and temperature sensor 4. The mieroproeessor ean output data in the form of a dlgltal word, a frequeney through a frequeney eonverter or a 4-20 mA eurrent loop. The mleroproeessor ean also dlsplay the dew point, relative humldity or water vapor eoneentration on a display that the user ean read.
Claims (10)
1. A method of determining the concentration of water vapor in a gaseous fluid comprising the steps of:
cooling a piezoelectric quartz-crystal until a thin coat of water is deposited on it monitoring the deviation of the frequency of the piezoelectric quartz-crystal base-line frequency monitoring the temperature of the piezoelectric quartz-crystal ambient air generating, using the frequency deviation and the temperature a numerical value indicative of the dew point or relative humidity of the said fluid.
cooling a piezoelectric quartz-crystal until a thin coat of water is deposited on it monitoring the deviation of the frequency of the piezoelectric quartz-crystal base-line frequency monitoring the temperature of the piezoelectric quartz-crystal ambient air generating, using the frequency deviation and the temperature a numerical value indicative of the dew point or relative humidity of the said fluid.
2. A method of compensating a piezoelectric quartz-crystal for contamination deposition comprising the steps of:
Heating a piezoelectric quartz-crystal above the dew point so that only the contamination mass will contribute to the frequency shift measuring a piezoelectric quartz-crystal oscillation frequency once the temperature has stabilize Storing this frequency value in memory using this new frequency as the base-line frequency to monitor the frequency deviation to detect the dew point.
Heating a piezoelectric quartz-crystal above the dew point so that only the contamination mass will contribute to the frequency shift measuring a piezoelectric quartz-crystal oscillation frequency once the temperature has stabilize Storing this frequency value in memory using this new frequency as the base-line frequency to monitor the frequency deviation to detect the dew point.
3. An apparatus for measuring humidity content in air or gases using a chilled piezoelectric quartz crystal mean for sensing the presence of water vapor condensation and means for measuring the frequency change of a chilled piezoelectric quartz-crystal as an indication of water vapor condensation presence with circuits means comprising;
Means for measuring the temperature of a piezoelectric quartz-crystal and means for measuring the ambient temperature coupled with regulator means to control a piezoelectric quartz-crystal temperature to the dew point temperature means for cooling a piezoelectric quartz-crystal using a thermoelectric cooler electrically driven by a cooler driver Means for generating and controlling the frequency of a piezoelectric quartz-crystal and means for measuring changes of such frequency and frequency amplitude as means to detect the presence of water vapor condensation onto a piezoelectric quartz-crystal Means for generating a frequency change signal and means for amplifying the generated frequency change signal as to enable a chilled piezoelectric quartz-crystal temperature regulator to regulate the temperature of a thermoelectric cooler.
Means for measuring the temperature of a piezoelectric quartz-crystal and means for measuring the ambient temperature coupled with regulator means to control a piezoelectric quartz-crystal temperature to the dew point temperature means for cooling a piezoelectric quartz-crystal using a thermoelectric cooler electrically driven by a cooler driver Means for generating and controlling the frequency of a piezoelectric quartz-crystal and means for measuring changes of such frequency and frequency amplitude as means to detect the presence of water vapor condensation onto a piezoelectric quartz-crystal Means for generating a frequency change signal and means for amplifying the generated frequency change signal as to enable a chilled piezoelectric quartz-crystal temperature regulator to regulate the temperature of a thermoelectric cooler.
4. A device as defined in claim 3, further including alarm means responsive to a second signal indicating a dew point, relative humidity or water vapor concentration exceeding a predetermined value.
5. A device as defined in claim 3, said circuit means including a microprocessor.
6. A device as defined in claim 3, said circuit means including a display for displaying under the control of said microprocessor a numerical value of the dew point, relative humidity or vapor concentration in said fluid.
7. A device as defined in claim 3, said circuit means including 4-20 ma current loop transmitter for transmitting dew point, relative humidity, temperature or water vapor concentration in said fluid.
8. A device as defined in claim 3, said circuit means including frequency converter for transmitting dew point, relative humidity, temperature or water vapor concentration in said fluid.
9. A device or apparatus as defined in claim 3, wherein the piezoelectric quartz-crystal temperature is regulated and controlled at the point of ice forming and means for controlling the mass of ice onto a piezoelectric quartz-crystal.
10. A device or apparatus as defined in claim 3, further including autocalibration for automatic calibration of the apparatus means and means compensates for contamination of the piezoelectric quartz-crystal.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 611170 CA1301477C (en) | 1989-09-13 | 1989-09-13 | Chilled piezoelectric hygrometer |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 611170 CA1301477C (en) | 1989-09-13 | 1989-09-13 | Chilled piezoelectric hygrometer |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1301477C true CA1301477C (en) | 1992-05-26 |
Family
ID=4140587
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 611170 Expired CA1301477C (en) | 1989-09-13 | 1989-09-13 | Chilled piezoelectric hygrometer |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1301477C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2024100364A1 (en) * | 2022-11-10 | 2024-05-16 | Centre National d'Études Spatiales | Device for measuring air moisture content |
-
1989
- 1989-09-13 CA CA 611170 patent/CA1301477C/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2024100364A1 (en) * | 2022-11-10 | 2024-05-16 | Centre National d'Études Spatiales | Device for measuring air moisture content |
| FR3142006A1 (en) * | 2022-11-10 | 2024-05-17 | Centre National d'Études Spatiales | Device for measuring humidity in the air |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKLA | Lapsed |