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THERMOCOUPLE CALIBRATION SYSTEM

The measurement of temperature is a basic parameter in the control of many processes in both research and industry. Thermocouples are used extensively for many higher temperature measurements. Manual calibration of thermocouples is a tedious and time consuming process. With the aid of a low thermal switching system, with thermal emfs in the nanovolt region, automation with high accuracy is possible.

The following system is being used by Brad Mellons and Will Schneider of Lockheed Missiles and Space Company in Sunnyvale, California for the calibration of thermocouples in their Standards Laboratory. They are using this system to calibrate all types of base metal junctions as well as platinum-rhodium types. The most critical are the type S units, which have an output sensitivity of only 8 to 12 microvolts per degree Celsius.

SYSTEM DESCRIPTION

At Lockheed, thermocouple leads are connected to ice point references that are connected to scanner input channels (see figure below). The appropriate ice point references are selected for the type of thermocouple under test.

An alternative configuration would be to connect the thermocouples directly to the scanner input channels and connect the ice points between the scanner output

terminals and the DVM. With this method only two pairs of ice points are required. A small additional error is created by the temperature difference between the input and output terminals. With insulation this error can be reduced to less than 0.2 degrees Celsius.

Once the connections are made the measurement process is completely automated. At Lockheed the computer controls the oven and waits until the oven reaches equilibrium as measured by the standard thermocouple. The scanner is then switched to take a series of readings to compare the test thermocouples to the standard, with half the readings taken in the reversed direction. When all test units have been checked the computer will set the oven controller to the next temperature.

THERMOCOUPLE SYSTEM RESULTS
The major source of error in a temperature calibration is caused by gradients in the oven. At Lockheed the stability of a measurement for a type S thermocouple is about 1.5 microvolts. The day-to-day repeatability of the same thermocouples in the same holes in the oven is about 2.5 microvolts. This corresponds to about 0.2 to 0.3 degrees Celsius. For other types of thermocouples with higher outputs, the errors caused by the measuring system are negligible.