Additional protection of the thermoelements is often required. Numerous materials are available for the protection of a thermocouple in various industrial applications. A table listing the more readily available thermocouple protection tube materials and application data is presented in the Industrial Sensors section of this document. Data of this nature is at best only usable as a general guide. This information cannot be taken as a guarantee of adequate or successful use of any of the listed materials in any specific application because of numerous variables possible such as impurities, concentration, temperature cycling, vibra- tion, etc.
Numerous other materials are available that provide protection for a thermocouple. Numer- ous factors such as machinability, ductility, and cost determine the availability of any of the listed materials in protection tube form. Some of these materials are available in a restricted number of tubing or pipe sizes. Others are readily available in numerous sizes. As mentioned earlier, the reference to a usable maximum temperature in no way insures satisfactory usage of a material in a specific application to that temperature.
In addition to the many considerations relating to calibration and protection tube material choice, a thermocouple must be located properly with respect to a work load and energy source being controlled whenever the thermocouple is used with controlling instrumentation. Considerations to keep in mind are the minimization of flow disruption caused by the sensor if it protrudes into a stream of liquid or gas; insertion into the medium being measured, either a fluid or a solid, to a sufficient depth to enable the measuring junction to respond to temperature changes of this medium rather than having it swamped by a temperature of a surrounding medium; and, under some conditions, awareness of heat flow into or out of the measuring junction due to the heat conductivity of the thermoelements and protecting materials.
In most industrial applications errors from heat flow through the sensor are negligible; but insertion depth, power source, sensor and work load placement are factors readily control- lable at the system design stage. A “rule of thumb”: minimum immersion of the sensor into the fluid or medium measured should be 4 to 10 times the OD of the protection tube.
Thermocouple assemblies provided with protection tubes are available with three types of junction styles: grounded, ungrounded (isolated), and exposed.
Protection Tube Thermocouple Element
Electrical Insulating Material
The grounded junction is most common. This style is available on assemblies having electrically conductive protection tubes, and means the thermocouple measuring junction is in electrical contact with the protection tube. In the ungrounded junction steps are taken in manufacture to electrically isolate the measuring junction from an electrically conductive protection tube. This junction style is required when the thermocouple is used with instrumentation which is itself not internally electrically isolated. The ungrounded style junction is slower to respond than the grounded style for a given junction end mass, but it can be the most reliable and rugged style junction. The exposed junction is where the measuring junction of the thermocouple is not protected by any material. It is the fastest responding junction, but most subject to corrosive failure.
Barber-Colman Company, Loves Park, IL.