Characteristics of thermocouple
Tolerances on Temperature Reading
Tolerance denotes the maximum allowable value obtained by subtracting the temperature reading or the temperature at the hot junction from the standard temperature converted from the applicable temperature EMF table.
Maximum Operating Temperature
Operating temperature limit means the upper temperature where thermocouple can be used continuously in air. Maximum limit means the upper temperature where thermocouple can be used temporarily for short period of time owing to unavoidable circumstances.
Principal factors that affect the life of a thermocouple are:
- Temperature: Thermocouple life decreases by about 50% when an increase of 50 ºC occurs.
- Diameter: By doubling the diameter of the wire, the life increases by 2-3 times.
- Thermic cycling: When thermocouples are exposed to thermic cycling from room temperature to above 500ºC, their life decreases by about 50% compared to a thermocouple used continuously at the same temperature.
- Protection: When thermocouples are covered by a protective sheath and placed into ceramic insulators, their life is considerably extended.
Thermocouple Response Times
The response time for a thermocouple is usually defined as the time taken for the thermal voltage (output) to reach 63% of maximum for the step change temperature. It is dependent on several parametersincluding the thermocouple dimension, construction, tip configuration and the nature of the medium inwhich the sensor is located.
Thermocouple assemblies are’’ tip’’ sensing devices which lend them to both surface and immersion applications depending on their construction. However immersion type must be used carefully to avoid error due to stema conduction from the process which can result in a high or low reading respectively. A general rule is to immerse into the medium to a minimum of 4 times the outside diameter of them sheath; no quantitative data applies but care must be exercised in order to obtain meaningful results.
Surface Temperature Measurement
Although thermocouple assemblies are primarily tip sensing devices, the use of protection tubes renders surface sensing impractical. Physically, the probe does not lend itself to surface presentation and steam conduction would cause reading errors. If thermocouple is to be used reliably for surface sensing, it must be either exposed, welded junction from with very small thermal mass or be housed in a construction, which permits true surface contact when attaching to the surface.