Special Hardware Requirements: Note: An improved method to measure and monitor temperature (using the EM231 RTD module) is documented in Tip 47
Depending on the number of the channel used on the EM 235, the corresponding address of the analog input word has to be used in the program: AIW0 for input channel 1, AIW2 for input channel 2, AIW4 for input channel 3.
The unused EM 235 inputs are short-circuited.
This program tip shows how to measure a temperature and monitor the specified limits by using the analog expansion module EM 235. A Pt100 temperature sensor is connected to one input channel of the analog module. Digital value = value stored in AIWx (x=0,2,4). EM 235 Circuit:
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In order to convert the temperature-conditioned resistance change of the Pt100 into a voltage reading, the analog output is used as constant current source. The output supplies the Pt100 sensor with a 12.5 mA constant current. With this circuit, a linear input voltage of 5 mV per degree Celsius (°C) is generated. The EM 235 converts this voltage into a digital value, which the program reads out cyclically. From the value read, the program calculates the temperature [°C], using the following formula:T[°C] =Digital value - 0° C offset 1° C value
0° C offset = digital value, measured at 0° C. In this example, the offset is 4000.
1° C value = value difference at a temperature rise of 1° C. In this example, 1° C value = 16.
The program calculates the value to the first decimal point and writes the result in the embedded variable of Message 1: “Temperature = xxx.x°C”, which is read out with a TD 200 operator interface. In the initialization segment of the program, the user can enter a high and low temperature limit. The program monitors the measured value and displays a warning by means of the TD 200 if the measured temperature leaves the selected range. Message 2: “Temperature > xxx.x°C” appears in the second display line of the TD 200 if the measured temperature exceeds the high temperature limit; Message 3: “Temperature < xxx.x°C” is displayed if the measured temperature drops below the low temperature limit.
