Nutech Engineers
143-D Government Industrial Estate, Kandivli West, Mumbai - 400 067
Phone : 91-22-28683549 / 28684580,  Fax : 91-22-28674580
email : nutechen@vsnl.com , nutechen@bol.net.in,
info@nutechengineers.com,  sales@nutechengineers.com

Temperature Sensing Devices

Material -- The longevity Factory

In general, the Thermowell material chosen for the installation is governed mainly by the corrosion conditions of the well environment face.  Materials for various services are given in the corrosion table in this bulletin.  The high mirror polish given to all stainless and monel wells provides maximum corrosion resistance.

Occasionally, the material consideration is one of strength rather than corrosion.  For example, a stainless steel well may be required for high pressure water service where otherwise a brass well would be satisfactory from a corrosion standpoint.  Consult the pressure-temperature ratings given for each well type.

The standard materials listed for each well series will cover most requirements.  A stock of material is carried at all times to insure fast delivery on wells to be made of standard material.

CONNECTION - The installation Factory

Threaded wells are made in readily welded or brazed materials.  This is important for installations requiring seal welding or brazing.  The pipe thread provides the mechanical strength; the weld merely seals.

INSERTION LENGTH -- The Accuracy Factor

The distance from the end of the well to the underside of the thread, or other connection means (designated "U") , is the insertion length.  for best accuracy, this length should be long enough to permit the entire temperature sensitive part of the thermometer bulb to project into the temperature medium being measured.  A properly installed thermometer bulb will project into liquid an amount equal to its sensitive length plus at least one inch.  In air or gas, the bulb should be immersed its sensitive length plus at least three inches.

Thermocouples and thermistors have short sensitive lengths and therefore can use the smallest insertion lengths.

Bi-metal thermometers, resistance temperature detector, and liquid-in-glass thermometers have bulbs with sensitive portions between one and two inches long.  Therefore, the minimum standard insertion length of 2-1/2" must be entirely immersed in liquid for proper accuracy.

Filled system thermometer bulbs may have sensitive portions from one to several inches in length.  Determine the sensitive length of bulb before choosing an insertion length.

Above all -- be sure that dead length, i.e. -- that required to pass through walls, pipe fittings, etc., is taken into account when choosing the necessary well insertion length.

BORE SIZE - The interchangeability Factor

Almost any installation uses several types of temperature measuring instruments.  The selection of a standard bore diameter can produce extreme flexibility within the plant.  The same well can accommodate either thermocouple, resistance temperature detector, bi-metal thermometer, or test thermometer.

TAPERED OR STRAIGHT SHANK - The Velocity Rating Factor

Tapered shank wells provide greater stiffness for the same sensitivity.  The higher strength to weight ratio gives these wells higher natural frequency than for equivalent length straight shank wells, thus permitting operation at higher fluid velocity Refer to "Velocity Ratings of Wells".