Wednesday, April 25, 2018

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Temperature Sensor Accessories

One of the most important accessories for any temperature-sensing element is a pressure-tight sheath known as a thermowell. This may be thought of as a thermally conductive protrusion into a process vessel or pipe that allows a temperature-sensitive instrument to detect process temperature without opening a hole in the vessel or pipe. Thermowells are critically important for installations where the temperature element (RTD, thermocouple, thermometer, etc.) must be replaceable without de-pressurizing the process.

Thermowells may be made out of any material that is thermally conductive, pressure-tight, and not chemically reactive with the process. A simple diagram showing a thermowell in use with a temperature gauge is shown here:


If the temperature gauge is removed for maintenance or replacement, the thermowell maintains pressure integrity of the pipe (no process fluid leaking out, and no air leaking in):

Photographs of a real (stainless steel) thermowell are shown here, the left-hand photo showing the entire length of the thermowell, and the right-hand photo showing the end where the temperature sensing device is inserted:


A photo of a complete RTD assembly (connection head, RTD, and thermowell) appears in the next photograph:


Another photo shows an RTD installed in a thermowell on the side of a commercial freezer, using a Rosemount model 3044C temperature transmitter to output a 4-20 mA signal to an operator display:


As useful as thermowells are, they are not without their caveats. First and foremost is the first order time lag they add to the temperature measurement system by virtue of their mass and specific heat value. It should be intuitively obvious that one or more pounds of metal will not heat up and cool down as fast as a few ounces’ worth of RTD or thermocouple, and therefore that the presence of a thermowell will decrease the response time of any temperature-sensing element.

A potential problem with thermowells is incorrect installation of the temperature-sensing element. The element must be inserted with full contact at the bottom of the thermowell’s blind hole. If any air gap is allows to exist between the end of the temperature element and the bottom of the thermowell’s hole, this will add a second time lag to the measurement system1. Some thermowells include a spring clip in the bottom of the blind hole to help maintain constant metal-to-metal contact between the sensing element and the thermowell wall.


1The air gap acts as a thermal resistance while the mass of the element itself acts as a thermal capacitance. Thus, the inclusion of an air gap forms a thermal “RC time constant” delay network secondary to the thermal delay incurred by the thermowell.

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