Introduction
Before we look in to the installation process of RTD we will quickly go through the use of thermowell
Thermowell Use
The use of a thermowell is recommended for any pressurized application. The thermowell prevents damage to the sensor element, and also allows the removal of the element without spilling process fluid. Install the thermowell in the process to achieve an airtight and leak-free fit.
Coat the sensor element stem with a suitable heat transfer compound (e.g. A mixture of graphite and glycerin or petrolatum).
NOTE: Above 350°F, some smoking of the heat transfer compound may occur as the liquid portion of the compound vaporizes, leaving dry graphite to perform the heat transfer function.
After coating the element stem, install the sensor element. The pipe threads on the element do not have to be sealed.
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Installation
1. Insert the sensor into the thermowell. By inserting a spring loaded stem, a positive contact with the bottom of the well will be achieved. Make sure that the cables in the connection head do not twist during insertion. If the wires begin to turn, disconnect the wires from the terminals and reconnect them after screwing the assembly into the thermowell.
2. Connect the RTD with a clean copper wire. To guarantee a good calibration, all connection cables must have the same size and the same length. Connect the wires to the terminals on the connection head. The two sides of the RTD will have wires of different colors; the same colored wires are common legs.
3. Insertion: the proper insertion depth ensures the best accuracy. For best results, a minimum insertion of ten times the diameter of the stem in the process is recommended.
Maintenance
1. Calibration frequency depends on a variety of factors including temperature cycling, vibration, and shock. The frequency of calibration must be determined in each case by the user.
2. Calibration is achieved by comparing RTD resistance with a working standard. It is preferred to calibrate the RTD in its installed position. If the sensor is removed for calibration, it should be returned to the same location and immersion depth for greatest reliability.
3. Periodically check the sensor sheath or the thermowell surface for corrosion or damage. If damage has occurred, replacement may be considered.
RTD Wiring Connections
Two wire: provides a connection to each end of the element. This construction is suitable where the resistance of the conductor cable can be considered as an additive constant in the circuit, and particularly when changes in cable resistance due to changes in ambient temperature can be ignored. Two-wire RTD instrumentation is very rare and mostly obsolete.
Three wire: provides a connection to one end of the element and two to the other end of the element. Connected to an instrument designed to accept the input of three cables, sufficient compensation is generally achieved for the resistance of the conductor cable and the change in temperature in the resistance of the conductor cable. This is the most commonly used configuration.
Four wires: Provides two connections to each end of the element to fully compensate for the resistance of the conductor cable and the temperature change in the cable. Connected to an instrument designed to accept four-wire input, this configuration is used when high-precision temperature measurement is vital.
Also read
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