PRT COMPARISON SYSTEM

The low thermal scanner is an excellent tool for comparing platinum resistance thermometers (PRTs). Accurate measurements require the ability to detect differences of a fraction of a milliohm. For example, a 25 ohm laboratory reference PRT operating at 1 milliamp has a sensitivity of about 0.1 ohm per degree Celsius. Thus in order to observe a 1 millidegree temperature change the system must be able to resolve 0.1 milliohm.

PRT SYSTEM RESULTS

Both systems enable the performance of PRTs to be automatically verified in a systematically controlled process. The Series Connection system has been used to measure R-zero of 100 ohm test probes to approximately 0.3 milliohm at the one standard deviation confidence level.

This corresponds to a voltage change of only 100 nanovolts across the PRT. The low thermal scanner has typical thermal offsets of only 10 or 20 nanovolts so it will allow sub-millidegree accuracy with out introducing significant switching errors. By automating these measurements many readings can be taken rapidly for improved results.

Following are two setups that work well for measuring PRTs. Both methods can improve measurement results through the advantages of automation. They also enable you to compare several reference PRTs or a number of production PRTs at one time.

FOUR-TERMINAL CONNECTION
Because a PRT is essentially a resistor, the normal resistor connections can be used. The four-terminal connection on the right above shows how PRTs can be connected to be compared using a high resolution digital multimeter. Some meters such as the Hewlett-Packard Model 34420A have a math module built in that will allow entry of the PRT calibration coefficients and will display the corrected temperature directly.

SERIES CONNECTION
The diagram shown on the right below shows the connection using an external current source. In this case the multimeter is set to read volts. This method has several advantages. The most important advantage is that you can control the current to the level specified for the PRTs. Also the current source and the scanner output can be reversed separately so that four different measurements can be made on each unit to reduce errors in the meter circuit.

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