Common Thermocouples Application Problems and their Remedies ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Common Thermocouples Application Problems and their Remedies

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Just like RTDs, thermocouples when applied for temperature measurement present their own peculiar application problems. To understand and be able to solve these problems requires a comprehensive understanding of thermocouples as temperature measurement device. You can start with the basics of temperature measurement with thermocouples here.

To help resolve some of the problems you might need to tackle when using thermocouples, here is a list of the common problems encountered. This is not a comprehensive list anyway:
Thermocouple Problem Possible Cause(s) Possible Remedies
Temperature indication too low with a very thin thermocouple Instrument with a low input or internal resistance; high connection lead resistance (1) Adjust sensor lead lengths.
(2) Select an indication instrument with a higher input resistance 
Varying temperature indication with proper operation Reference junction temperature or electric simulation not constant (thermal/electrical reference junction) Reference junction temperature or reference junction simulation must be held constant
Temperature indication error increases with increasing temperature (indication too low) Decreasing insulation resistance which acts as a shunt path decreasing the EMF of the thermocouple. (1)Minimum recommendations for insulation resistance for thermocouples  according to IEC 60751 are:
(a) Insulation resistance at 20°C (68°F) must be greater or equal to 100MΩ
(b) Insulation resistance at 500°C (930°F) must be greater or equal to 2MΩ

(2) Exchange thermocouple measuring inset , then seal against moisture.
Large deviations of the temperature indication from the values in the table (1) Parasitic voltages - thermal and galvanic voltages
(2) Incorrect material combinations.
(3) Incorrect linearization applied
(4) Poor electrical contact
Check thermocouple and leads. Exchange if necessary.

Incorrect compensating cables or their polarity is reversed (1) Check if the correct compensating cables have the correct polarity.
(2) If a compensating cable is used:
Temperature of connection terminals should be maximum 200°C (392°F). If temperature exceeds 100°C, then connection terminals should be maintained at the same temperature
Temperature indication changes over the course of time Chemical effects on the thermocouple especially at higher temperatures (1) Exchange defective sensor with a suitable thermocouple
(2) Install thermocouple deeper. Consider air purge(oxygen addition)
Thermal aging of the thermocouple (1) Select larger wire size in order to slow down the aging process - generally an aged thermocouple indicate lower temperature than a new one.
(2) Check critical measuring locations regularly
(3) Do regular re-calibrations
Temperature indicating instrument shows room temperature Thermocouple Lead wire break Check continuity
Negative temperature indication Incorrect polarity at Thermocouple Reverse Thermocouple polarity
Temperature indication error in the range of 20 - 25°C Thermocouple type L linearized as type J or reverse Correct linearization
Temperature indication even when thermocouple is disconnected (1) Pick up on the compensation cable due to electromagnetic noise
(2) Parasitic galvanic voltage due to moisture in the compensation cable
Dry compensation cable

Thermocouple Test:
Two basic tests can be carried out on thermocouples to ascertain if they are still good. They are:
(a) Continuity test with a multimeter
(b) An insulation resistance test with an insulation tester.

When these tests are done at room temperature, we can safely conclude that a thermocouple is good if:
(1) After a continuity test, the sensor resistance is less than 20Ω for thermocouple sensor with wire diameter less than 0.5mm. The resistance of the thermocouple sensor is a function of the wire diameter and length.
(2) After an insulation resistance test, the insulation resistance reads 100MΩ or greater.