Datasheet

TEMPERATURE AND HUMIDITY COMPENSATION METHOD
As was previously mentioned,
Sensor resistance (Rs) is dependent upon the
ambient temperature and humidity. Accordingly, this phenomenon will result
in a
fluctuation of the alarming Point.
Possible variations in the effects
of temperature and humidity changes are illustrated in TableV.
This Variation in Sensor Performance, however, is primarily related to the
measure of Absolute Humidity in the detecting area.
It is therefore rec-
ommended that you determine the mean or
average temperature and humidity
values in your projected
detector sales or distribution area, to be able to
compensate for seasonal variations in the alarming
Point.
The most effi-
cient and economical way we have found to achieve this compensation is to
carefully control the temperature dependency of the circuit.
This Point is
discussed below.
Table V illustrates possible variations in alarming Points in both compen-
sated and non-compensated circuits.
Another way of interpreting these varia-
tions is to say that if R/Ro
= 1 at 20°C (65% R.H.) then at -10°C (65%
R.H.) R/Ro is 1.6 and at 35°C
(65% R.H.) is 0.9.
The actual VRI can.be
expressed as:
2.5 V at ZOOC,
1.9V at-10°C and 2.6 V at 35OC.
To compensate for these variations in temperature and humidity we suggest
using a negative characteristic thermistor (Rt).
In this example circuit,
the Vr is automatically adjusted as a result of temperature changes. As
was mentioned, Rt is set at
2.5V for 20°C in this circuit.
At -10°C Vr
will self-adjust to 2,1V, while at 35°C
it will go to 2.7V.
The differente between Vm and Vr is
detected by the following Comparator
12.
Therefore you tan get rid of the seasonal Variation of alarming Points
hich are caused by temperature and humidity fluctuations by adjusting Vr.
With the application of a temperature compensation circuit you will be able
to control the alarming Points as illustrated in Table V,
The temperature coefficient of the thermistor is larger than that of the
Sensor, therefore we have to adjust the coefficient of the thermistor by
adjusting RZ and RS.
A final important consideration is the actual place where the thermistor is
placed in the circuit.
You should not
install it near heat dissipating
components such as the transformer or the Sensor.
Also it should not be
installed in a Position where it is
likely to receive a strong wind, as
this will also effect the temperature characteristics of the Sensor.
(17)