Thermocouple Calibration (Temperature Sensor Calibration): Thermocouples measure temperature and are often used in process control.
How do they work?
When the wires of two thermally homogeneous materials are connected at one end and placed in a temperature gradient, a thermoelectric voltage (EMF) is observed at the other end. The connection is called the measurement connection. Red lead is negative on all thermocouples. The color of the other wire indicates the type of thermocouple. For example, on a J-type thermocouple, the positive wire is white. The tables for each type of thermocouple list the voltages generated at different temperatures. Thermocouples should be checked whenever there are no signs of output accuracy. It may also be necessary to test a thermocouple used for a measurement standard.
Thermocouple calibration input and output measurement standards
The temperature bath provides a controlled temperature sensor for testing. A valve in the temperature bath is used to hold the sensor during close inspection. Another well is used to hold a standard measuring thermometer, to verify the actual bath temperature. The standard second measurement thermometer is used to read the ambient temperature at the reference junction. The signal and output amplitude of the standard sensor determine the output of the thermocouple. Because the output is measured in millivolts, millimeters are used to read the output.
Set the temperature bath as the standard temperature input to the thermocouple. Select the output standard with the appropriate reading range in millivolts. Connect the red lead to the negative input of millivolts and the white lead to the positive input of millivolts.
Should we check the temperature of the thermocouple at three points?
Since no adjustment is possible, only the calibration of a thermocouple sensor can be checked. This test is generally performed at three test point inputs: ambient temperature, average temperature, and high range of application. Recall that in a thermocouple, it is the temperature difference between the reference and the junction that produces the millivolt output. Before placing the thermocouple in the bath, determine the ambient temperature, which indicates the temperature at the junction of the thermocouple measurement. You need to compensate for the ambient temperature when using search tables that are set to 0 degrees. The amount of millivolts in the tables is added to the ambient temperature from the value of the sensor.
This compensated millivolt value is used to determine the correct temperature from the tables.
Calibration of temperature transmitters (temperature sensor calibration) should be checked periodically.
Input and output measurement standards
The temperature transmitter discussed here receives the input signal from a thermocouple. A millivolt input signal is required for calibration, so a millivolt source can be used for the input standard. One millimeter can be used to measure the output of the transmitter. Use a standard thermometer to calculate the input signal compensation for ambient temperature. Finally, a power supply is required for the transmitter. To calibrate a millimeter-volt temperature transmitter as an input standard, you must compensate for any reference degree other than 0 degrees.
To create input connections, the reference connection location must first be specified. When thermocouple wires are used to connect a millivolt source to a transmitter, the reference connection is at the transmitter connection. Therefore, the ambient temperature is measured in the transmitter chamber. If copper wires are used, the reference connection is in the millivolt source, so measure the ambient temperature in the millivolt source. Always observe the polarity of the lead. Connect the negative output of the millivolt source to the positive transmitter terminal. Connect the millimeter in series to the transmitter and power supply.
Adjust the millivolt and millimeter source to the appropriate values, turn on the equipment and start calibrating
Check five points
.Do a five-point check to see if the transmitter meets the exact specifications
First set zero. Must be set with an input value of 10%. With the correct zero setting, a 10% input leads to a 10% output. Adjust the opening using the 90% input. Zero and aperture may be interacted, checked and reset if necessary.
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