Temperature Sensors

The SOR Temperature Assemblies can be a RTD (Resistance Temperature Detector) or Thermocouple as the primary temperature sensing device. They can also be supplied with a Thermowell that protects the Temperature element from the process and allows for removal without shutting down and draining the pipe or vessel for replacement. RTDs provide a resistance output that can be packaged with a Temperature transmitter that converts the resistance input into a 4-20 mA or other industry standard digital output. A Thermocouple generates an mV output as temperature changes and can also be supplied with a Temperature Transmitter. 

SOR recognizes that there is no industry convention with respect to terminology and definitions pertinent to temperature sensors. This glossary applies to SOR Temperature Sensors.

The following are some common industry terms and reference standards for RTDs and Thermocouples.

Lagging Length Additional length of Bar stock on the back side of the Thermowell for the instrument connection.  Industry standards are to add the lagging length to the base length of the Head stock of the well.  For threaded wells this is usually 1.75” and for flanged wells it is 2.25”.  The lag extends the instrument connection to get through piping insulation or heat considerations for the connection head typically at the end of the temperature assembly.

RTD Utilize the basic characteristic that the Electrical Resistance of Metals increases with temperature. The resistance value is measured electronically using the voltage generated across the element by an electrical current (about 1 milliamp).

RTD Accuracy The industry has standardized on two types of accuracy for Platinum 100 ohm RTD Elements. They are Class B (+/- 0.8oC at 100oC), the industry standard and the higher accuracy Class A (+/- 0.35oC @ 100oC). Other elements for even higher accuracy are available upon request.

Repeatability Defined as the ability to repeat the same output value at a given temperature point in a spanned temperature range. Depending on the process conditions, RTDs typically are repeatable to +.14o or .05%, whichever is greater.

Response Time Measured as the time necessary for a sensor to report a 63.3% step change in temperature in water moving transverse to the sensor sheath at 3 fps.

Thermocouple Two dissimilar metals that are joined together, if the temperature at each end of the wire is different, a current will flow, known as EMF (Electro Motive Force).

Thermocouple Accuracy The Industry has published accuracy specs for the different Thermocouple Calibrations – Type J, K, T, N and the Nobel Calibrations (Platinum) R, S & B.  Typical Industrial grade accuracy is +/- 2.2oC  or 0.75% of reading whichever is greater.  Higher purity T/Cs – call special limits are specified in the +/- 1.1oC or 0.4% of reading

Thermowell  A pressure-tight receptacle that protects and extends the life of a temperature sensor in processing applications where the sensor is not mechanically or chemically compatible with the process environment.  Also allows for the installation and removal of the temperature element without shutting down the process to replace.

Stability Defined as the ability of a sensor to maintain its stated accuracy over an extended period of time, usually one year, at its rated temperature. RTDs when used properly maintain stability much greater than thermocouples.

U length Length of the Stem of the thermowell – as measured from the bottom (Tip) to the flange face or start of the process Threads/Head on Screw Wells.


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