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.
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Threaded-Straight
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Weldable
- Tapered | Flanged-Tapered
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Socket-Tapered
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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.
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