The issue of measuring fluid temperature
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I would like to ask the experts: is there any difference between the temperature measured using a \"glass mercury thermometer\" on the outer wall of a stainless steel process pipeline and the actual temperature of the fluid flowing inside the pipeline? If there is such a difference, will it be more than 10 degrees Celsius? (assuming the influence of ambient temperature is ignored)
It is not feasible to estimate the actual temperature of the fluid inside a pipe by measuring the wall temperature of the pipe; it’s better to install a thermometer if possible.
If it’s truly not possible to install one, insulation can be placed outside the pipe wall; by placing a thermometer inside this insulation layer, relatively accurate readings can be obtained, though there will be a delay.
If the influence of ambient temperature is not taken into account, the temperature error should be small, as the highest temperature of the fluid is at the center of the pipe, while the temperature of the pipe walls is relatively lower; the difference is probably only one or two degrees. Glass mercury thermometers have relatively high precision, with an error of no more than ten degrees.
Glass mercury thermometers are basically not used in industry anymore. It should be avoided as much as possible. Its temperature changes relatively slowly; generally, the sensing element of a glass mercury thermometer does not come into direct contact with the medium, as there is a sleeve and some air in between. It is still usable if the measured temperature is relatively stable. It’s not good when the temperature changes significantly. However, if the measurement conditions are good (good contact and high accuracy of the thermometer), the error will not be large, and it will not exceed ten degrees.