Immersion Length (“U” Dimension”- For best teamperature measurement accuracy, the “U” dimension should be long enough to permit the entire temperature sensitive part of the measuring instrument to project into the medium bearing measured.
Liquid temperature measurement: A properly designed thermowell will extend into the fluid an amount equal to the length of the temperature sensitive zone plus one inch or greater.
Gas temperature measurement: A properly designed thermowell will extend into the fluid an amount equal to the length of the temperature sensitive zone plus three inches or greater.
The temperature sensitive zone for thermocouples and thermistors is short, enabling measurement accuracy with limited immersion into the process fluid.
Bi-metal thermometers, resistance temperature detectors (RTDs), and liquid in glass thermometers have bulbs with temperature sensitive zones between one and two inches.
Process Connection- External means to connect thermowell to process system. Wells can be threaded, bolted, clamped or welded into place.
Instrument Connection- Internal threads to connect temperature instruments to thermowell.
“U” Dimension-Length of the thermowell immersed into process sytems. Measure from the base of the process connection to the end tip of the well.
“T” Dimension- Also called “lag length” or lagging extension. Extends length between the instrument and process connections to accomodate vessel or piping insulation.
“A” Dimension- Instrument insertion length into thermowell. Equal to bore length.
“D” Dimension- Also called the “tip diameter”. Diameter of thermowell shank at the end of the tip of the thermowell.
“Q” Dimsension- Also called “root diameter”. Diameter of thermowell shank below the process connection. This dimension may vary with process connection.
Bore Diameter- Dimension of internal bore to match the diameter of the instrument inserted into the thermowell.
Shank Style- Tapered shank wells provide greater stiffness for the same sensitivity.