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