ASTM E220 Calibration of Thermocouples By Comparison Techniques

Scope:

The ASTM E220 test method is used for only unused thermocouples. This test method does not apply to used thermocouples due to their potential material inhomogeneity. Inhomogeneity is a property that cannot be identified or quantified by standard calibration techniques. Thermocouples with large-diameter thermoelements and sheathed thermocouples may require special care to control thermal conduction losses.

Test procedure:

There are three test procedures described in ASTM E220:

Automated Method

In this method, any number of thermocouples can be tested rapidly. The reference thermometer may be of any type meeting the uncertainty requirements for the calibration, but it is necessary that the data acquisition system can accommodate the corresponding type of signal. A scanning unit sequentially connects each thermocouple to the input of a voltmeter. In the time interval between a reading of the reference thermometer and of a test thermocouple, the temperature of the calibration zone will vary. Measurements shall be made rapidly enough so that this variation in temperature is small in comparison to the uncertainty of the temperature. The EMF-measuring system consists of a voltmeter, a display, and a data storage system (Figure 1). It may be desirable to provide a calibrated EMF source and a zero (or shorted) input to the data acquisition system as references to improve the accuracy of the measurement.

Figure 1: Automated thermocouple data acquisition system layout – with thermocouple reference

Manual Method:-

In this method, each thermocouple is connected to the voltmeter in sequence. The reference thermometer should be read just before and just after the reading of each thermocouple under calibration. After measuring the EMF of each test thermocouple, the entire sequence should be repeated at the same temperature. This testing method described in ASTM E220 is used when one or more thermocouple is calibrated with manually operated switches.

Differential method:-

In this method, the EMF of the reference thermocouple is measured. The differential EMF between the thermocouple being calibrated and the reference thermocouple is directly measured by connecting the thermocouples in series, with the polarity of one of the thermocouples reversed.

Test Specimen:

ASTM E220 test involves calibration of unused thermocouples for a temperature range of -195 to 1700ºC.

Data:

The EMF difference for each calibration point is calculated in ASTM E220 by using the following equation:

ΔE = Er – E

Where,

Er is the table value of EMF 

E is the EMF of the test thermocouple at the temperature of the calibration point.

Conclusion:

The ASTM E220 test method is used for only unused thermocouples. This test method does not apply to used thermocouples due to their potential material inhomogeneity.