Problems with thermocouples
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If a regular glass thermometer is inserted into the thermocouple socket of a process pipeline, which one will provide a more accurate reading of the temperature inside the pipeline: the one read by the thermometer or the one transmitted to the DCS? Are both affected to the same extent by the ambient temperature? Does the thermocouple have any mechanism to account for the influence of ambient temperature?
Both are used to measure the temperature of the pipe sleeve wall (close to the temperature of the fluid inside the pipe); glass thermometers are relatively more accurate (provided that such thermometers have been calibrated), while thermocouples are affected by the ambient temperature, hence temperature compensation is necessary in DCS systems.
I agree with the view on the second floor: as long as a glass thermometer has been calibrated, it provides more accurate measurements than thermocouples. The temperature measured with a glass thermometer should be the same as that displayed by the DCS, which indicates that the compensation is working.
That’s the thing – glass thermometers installed on-site need to be more accurate!
Is a glass thermometer a bimetallic thermometer used for on-site indication?
No, the gauge of a bimetallic thermometer has a pointer, just like the gauge of a pressure gauge.
A glass thermometer is likely more accurate; thermocouples require temperature compensation