In many temperature measurement applications, the temperature element cannot be placed directly into the media it is to measure. In these instances a thermowell must be used to protect the temperature element.

MATERIALS
The materials selected for service is governed by the environment the thermowell will be subject to. Corrosion resistance is one of the major concerns in regard to material selection for a particular application.

CONNECTIONS
Connections on the thermowell to the process may be made in any manner approved by the ASME Boiler and Pressure Vessel Piping Code.

INSERTION LENGTH

The insertion length should be long enough to allow the total temperature sensitive portion of the element to project into the media to be measured.

A thermometer bulb should be placed into liquid an amount equivalent to the bulbs sensitive length plus a minimum of one inch. In gas or air, the bulb should be placed a minimum of three inches plus the bulbs sensitive length.

Thermocouples have short temperature sensitive lengths and can be used with the smallest of insertion lengths.

Bimetal thermometers, RTD elements and liquid-in-glass thermometers have sensitive lengths of approximately one to two inches.

Filled system thermometers have various sensitive lengths. The sensitive length must be determined before deciding on the proper insertion length.
You must make sure that you include any fittings or walls that the thermowell must pass through to reach the temperature sensing area.

BORE SIZE

Select a bore diameter that is compatible with the temperature sensing element being used.

DESIGN (Pressure and Velocity Ratings)

The design requirements that provide increased thermowell strength reduces the temperature sensing accuracy and response time. Becaues of this the Thermowell designs must carefully balance the thermowell construction and the sensing requirements.

ASME PTC19.3 offers guidance in determining which thermowell will be suitable for your specific application. Thermowell failure canb be caused by vibration, pressure and the fluid flow. Failures of a thermowell are not normally due to pressure or temperature.

Vibrational is the most prevanent cause of thermowell failure. Fluid flowing by the thermowell forms a turbulent area behind the thermowell that causes the vibration. This is vibration has a frequency determined by the construction of the thermowell and the speed of the fluid. The design of the thermowell must have a natural frequency that will never match the frequency of the vibration caused by the fluid.