Careful consideration must be given to the location of temperature sensing probes in order to obtain accurate temperature measurement. Large measurement errors in service may be due to the wrong choice of instrument, but more frequently the error is due to the incorrect location of the measurement points.
There are types of equipment used to protect the sensors from the surroundings which are listed below:
Immersion Probes:
To minimize errors in the measurement of the temperature of process fluids, whether liquid or gas, it is preferable to insert the sensor so that it is directly immersed in the fluid.
There are probes used in open vessels and pipelines.
Measurement of Liquid in Vessels:
Measurement of Liquid in Vessels:
Consider a vessel filled with liquid and stirred by a double-anchor agitator. The ideal place to measure the temperature would be somewhere near the centre of the mass at, say, T1.
A dip probe at T2 would seem to be the best arrangement. But while the liquid’s design level is at A, the liquid level may fall as low as B in operation, leaving the T2 probe dry. T3 is the only remaining option.
Probes in Pipes or Ducts:
Three possible configurations for insertion into a pipe:
Figure A shows an arrangement to ensure that the probe is inserted far enough to fully immerse the sensitive length in the fluid to minimize thermal conductivity from the sealing coupling to the sensor.
Figure B shows the problem that can arise in small pipes where the probe can cause serious obstruction to the flow.
Figure C shows to put the thermometer at a bend in the pipe, it can be inserted radially, provided that the pipe is big enough. Great care should be taken to ensure complete immersion of the sensitive portion of the probe.
Thermometer Pockets and Thermowells:
The use of a thermometer pocket does degrade the measurement accuracy of the instrumentation. Thermometer pocket mounted in the wall of a steam-jacketed process vessel is shown:
The thermometer probe receives heat from the pocket wall through conduction where it touches it and from other places through radiation. The pocket’s inner wall receives heat from the process fluid and, in this case, conduction from the vessel’s steam jacket.
Using a thermowell or pocket will also slow an instrument’s response speed to changes in temperature. Typically, a directly immersed thermometer sample will reach a thermal balance within 30 to 90 seconds.
Effect of Process Fluid Flow Rate:
Fractional Heating:
Where the process fluid flows past a high - speed probe, a frictional heating effect occurs, especially in the case of gases. It is not easy to evaluate the magnitude of the effect, but it is advisable to locate the probe at a location where the fluid speed is low.
Conductive Cooling
Where the process fluid flows past a high - speed probe, a frictional heating effect occurs, especially in the case of gases. It is not easy to evaluate the magnitude of the effect, but it is advisable to locate the probe at a location where the fluid speed is low.
Cavitation
Liquid flowing past a high - speed thermometer probe may cause cavitation on the probe’s downstream side. In addition to any heating effect of the high flow rate, the cavitation creates noise and causes the probe to vibrate.