If the system has a lot of heat, then it may not be subject to rapid temperature changes; it may, however, contain pressures and flow rates that mandate increasing the sensor assembly mass to withstand the forces applied by the process fluid. In many real-world cases, the increased mass makes it difficult to achieve the response speed necessary for process control.
In chemical plants, great quantities of liquids and gases may flow through pipes at pressures and velocities that would destroy most sensors. Typically the temperature sensor is mounted in a thermowell to provide physical protection for the sensor and to facilitate its removal for calibration or repair without interruption of the process. The speed of response is slowed substantially by the structure required to withstand the process conditions.
To minimize the loss of response speed, the thermal probe is usually spring loaded for positive contact at the bottom of the thermowell bore. In some cases, thermal grease is added inside the well tip to improve thermal conductivity.
With the above considerations in mind, what other factors determine the sensor technology most suited to the task? Lets consider the four most popular types of electrical contact thermometers in use today: