Not Quite a Sensor (but an important accessory)
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 or solids (like rocks in a roadstone cooler exit chute).
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 consists of:
1. Added purchase and installation costs,
2. Slower temporal response to temperature changes and,
3. Possible 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.
The most expensive, complex thermowells that we have ever seen were made from drilled molybdenum (moly) 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, directly into the molten glass. It was 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.