Instructions

UM-0085-B09 DT80 Range User Manual Page 323
RG
Calibration
For LM135 series sensors, the channel factor specifies a voltage divider ratio, which can be used to correct a slope error
based on a single point calibration. For example, suppose an LM135 is connected using a 2:1 voltage divider as shown
in the wiring diagram. If the channel then reads 301.0K when the actual temperature is 302.4K (an error of -1.4K) then
the required scaling factor would be 2 – (–1.4 / 302.4) = 2.0046, which would be applied as:
2LM135(2.0046)
Most LM135 series devices also provide an ADJ pin which allows the connection of an optional external trimpot.
IC4 – 4-Wire LM135-Series Inputs
In this wiring configuration the DT80’s 4.5V voltage excitation is used to power the sensor (via an internal current limiting
resistor). The output voltage then feeds a 2:1 voltage divider and is measured using the DT80’s + and – inputs.
Figure 149: Wiring for LM135 series input
To measure
Use the command
temperature
1LM335
Humidity Sensors
Relative humidity is commonly measured by the "wet bulb depression" method. Two temperature sensors are required,
one to measure air temperature and the other the cooling effect of a wetted surface. Usually a temperature sensor is
encased in a wick extending into a reservoir of distilled water. The temperature difference between the two sensors is the
wet bulb depression.
The choice of temperature sensors is critical if reasonable accuracy is required at high relative humidity where the wet
bulb depression is small. If platinum RTDs are used they should have good accuracy or matching (0.2°C).
Good accuracy can also be achieved by use of a temperature difference sensor such as a thermocouple or thermopile.
Measure the dry bulb with a standard grade temperature sensor and subtract the difference sensor reading to obtain the
wet bulb temperature.
The sensors are normally placed within a radiation screen to prevent radiant heat affecting the readings. This is
particularly important for outdoor applications.
 Example — Humidity Measurement
The following program reads two RTDs and calculates the relative humidity with an accuracy of a few percent for
temperature above 5°C and over most of the relative humidity range (the algorithm assumes that the sensors are
ventilated but not aspirated).
BEGIN"STICKY"
Y1=6.1,0.44,0.014,2.71E-4,2.73E-6,2.75E-8 'SVP polynomial
RA5S
1PT385("Dry bulb",4W)
2PT385("Wet bulb",4W)
3CV(Y1,W)=&"Dry bulb"
4CV(Y1,W)=&"Wet bulb"
5CV("RH%",FF1)=(4CV-0.8*(1CV-2CV))/3CV
END
Frequency
The frequency of an analog input signal can be measured using the F channel type, which returns a value in Hz.
Any of the Voltage wiring configurations may be used (P303).
Note: the default threshold point is 0V, so the input signal must have zero crossings in order to be measured. If this is not the case (e.g.
for a logic signal), the
2V channel option can be used to change the threshold point to +2.5V.