Calibration of temperature transmitters should be checked on a periodic basis.
Input and Output Measurement Standards
The temperature transmitter discussed here receives an input signal from a thermocouple. A millivolt input signal will be needed for calibration, so a millivolt source can be used for the input standard. A milliammeter can be used to measure the transmitter output. Use a standard thermometer to calculate the input signal compensation for the ambient temperature. Finally, a power supply for the transmitter is necessary.
To calibrate a temperature transmitter with a millivolt meter as an input standard, you must compensate for any reference temperature other than 0 Deg. C ( 32 Deg. F.).
To make the input connections, the location of the reference junction must first be determined. When thermocouple wires are used to connect the millivolt source to the transmitter, the reference junction is at the transmitter connection. So, ambient temperature is measured in the transmitter housing. If copper wires are used, the reference junction is at the connection to the millvolt source, so measure ambient temperature at the millivolt source. Always observe the polarity of the leads. Connect the negative output from the millivolt source to the positive transmitter terminal. Connect the milliammeter in series with the transmitter and the power supply.
Setting the Equipment
Adjust the millivolt source and the milliammeter to the proper values as required, turn on equipment and begin calibration.
Five Point Check
Perform a five point check to determine if the transmitter is accurate according to specifications.
Accuracy of the Instrument
Adjust the zero shift first. It should be set with an input value of 10%. With the zero properly set, a 10% input results in a 10% output. Adjust the span using a 90% input. The zero and span may interact, check and readjust as required.