Automatic Test System for Thermocouple Monitor Calibration

摘要

Thermocouple monitors are used for spacecraft thermal testing purpose in large numbers. In industry also they are widely used for myriad applications. This work describes the design and development of an automatic calibration system for a 16 channel thermocouple monitor using voltage simulation method. By adopting automated computer controlled testing of the instrument over GPIB instrumentation bus the calibration test time and human errors in logging of test data are aimed to be reduced. Thermocouple sensors are able to cover a wider temperature range with a single thermocouple type compared to thermistors or platinum resistor sensors. But these sensors require further signal conditioning and room temperature offset compensation called cold junction compensation for proper use. The thermocouple monitor provides the signal amplification and cold junction compensation. They may also contain multiplexer for multi-channel operation. Accuracy of the instrument is ensured by periodically calibrating it against a known traceable standard. The calibration consists of two parts, calibrating the actual thermocouple sensors at a known temperature using a thermal bath or dry well type of sources. The second part is calibrating the read out part or monitor using a thermocouple voltage simulator. These voltages will be of very low order, for example for a T type thermocouple which can measure between -270℃ and +400℃ the voltage produced is just -5.603 mV to 20.872 mV respectively. Thermocouple sensor calibration is not covered in this work.