Scope:

When subjected to high temperatures, thermocouples produced from thermocouple cable that has been polluted by moisture or other impurities may experience significant changes in thermoelectric qualities or fail catastrophically. Manufacturers and users of mineral-insulated, metal-sheathed thermocouples or thermocouple cables use ASTM E780 – 06 to verify the measured insulation resistance values to meet the prescribed minimum value.

Procedure

Measuring the insulation resistance between all thermoelements in a thermocouple necessitates removing the hot junction, testing is limited to measuring the insulation resistance between thermoelement pairs where possible, and between the thermoelement pairs and the thermocouple’s sheath.

Specimen size

An instrument such as a megohm bridge or megohmmeter is used to measure resistance.
Room temperature (22± 5°C (72 ± 10°F)) dc electrical insulation resistance
(l) Length of thermocouple cable, between each of the thermoelements and between the thermoelements and the sheath.

Data

Test voltage range= 50 – 500 VDC

Measurement ranges= 1 × 10⁵ Ω- 1 ×10¹²Ω

Accuracy of reading= ±10%