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About Thermowells

 

Thermowells are among the simplest yet least well publicized accessories used in industrial temperature measurement applications. There are many variations of two basic kinds; low pressure and high pressure. They are used to provide an isolation between a temperature sensor and the environment, either liquid, gas or slurry. A thermowell allows the temperature sensor to be removed and replaced without compromising either the ambient region or the process. The price paid for such luxury is made up of:

1. Added purchase and installation costs,
2. Slower temporal response to temperature changes and,
3. Increased temperature measurement error, due mostly to stem heat loss down the length of the thermowell.

Illustrations of generic types of metal thermowells are shown below, courtesy of RÜEGER S.A. one of the world's largest makers of high quality bimetallic, gas pressure and thermoelectric temperature sensors (and thermowells-of course.

Threaded-Straight

Weldable - Tapered

Flanged-Tapered
Socket-Tapered


The most expensive, complex thermowells that we have ever seen were made from drilled molybdenum rods with an internal sheath of high purity alumina. The annular space between the alumina and metal had a very slow gas purge of nitrogen+hydrogen to prevent oxidation of the moly surface. The well was inserted into the bottom of an electrically heated glass melting furnace and used to help measure the molten glass temperature. An IR radiation thermometer mounted on an electrically insulating holder was sighted into the tube to read the temperature indicated by the alumina, essentially under blackbody conditions.

Low pressure, moderate to high temperature environments are routinely provided with a thermowell variant called a protection tube that can be made of metal or high temperature glass or ceramic, again according to he conditions. Most high temperature industrial furnaces use ceramic or metal protection tubes, according tho the conditions.

Mineral-Insulated-Metal-Sheathed (MIMS) thermocouples have replaced many protection tube systems at temperatures up to about 700°C, although some newer sheath materials claim capabilities up to 1100°C or so. In base metal assemblies, these newer MIMS devices offer some improvement in response at moderate cost and are replaced themselves when they fail.

While the majority of uses of thermowells involve the more popular temperature sensors, such as thermocouples and RTDs, there is no fundamental technical reason why radiation thermometers can not be used to measure the temperature of the inner portion of a thermowell or protection tube and infer the process temperature on the other side. Not only is this a very common practice at high temperatures in process furnaces like glass melting tanks in the example described above, but it is used at lower temperatures using low temperature, low cost radiation thermometer and ones with fiber optics as well.

In fact, several well-known infrared radiation thermometer manufacturers offer devices called by such names as "Infracouple™" and similar-sounding terms to imply a device like a thermocouple. The height of such technology was reached about 20 years ago when a USA West Coast company, Luxtron, was formed to commercialize the device patented by Dr. R.R. Dils formerly of NIST and Pratt & Whitney Aircraft Engines.

Dils' device was based on a single crystal, 1mm diameter solid sapphire rod with a vacuum deposited, opaque thermowell on the sensing end. It was so rugged and thermal shock resistant that it could be repeatedly plunged directly into a Mach 0.8 air stream at 1700 °C and withdrawn within the space of a few seconds. This company has transferred many of its products to Englehard Corp's Temperature Sensor Division in California.


Tell your new product and application stories at The Temperature Community website: www.tempsensor.net or feedback to us and we'll consider adding it here with your byline!

Good luck and best wishes. If you have some interesting successes, let us know and we'll help you share that with others who visit these pages.

Here are some useful technical links relating to thermowell design and applications. A seperate vendor's page is available listing some of the many makers of thermowells around the world.

One of the most significant complications of thermowell design and use is the problem of vibrations and potential damage due to high velocity/high turbulent flow of even common fluids like steam or saturated air. Our special page on Thermowell Wakes provides an insight into the problem along with some resources and free software downloads.

 

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