Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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.
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 .
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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.
