For gas actuated temperature gauges there is no limit. The longer the stem or capillary the bigger the ambient temperature deviation will be. For Bi-metal type of temperature gauges Badotherm advices 1 meter maximum stem length.
To make the right selection of a temperature gauge, the operating conditions on site must be considered. This means operating conditions, ambient conditions, mounting types are to be considered. There are mainly 2 types of mechanical temperature gauges:
The measuring element of a thermometer is located within the stem. Therefore modifying of the stem is not available. However, by selecting an adjustable connection thread, the thermometer can be immersed shorter.
The temperature range indicates the scale range of a thermometers. However, measuring range is equivalent to the range in which the error limits are applicable. The measuring range is marked by arrows at the outer side of the scale.
Thermowells separate the temperature sensor (stem) from the process medium and provide protection against mechanical and corrosive effects. Depending on the construction type, thermowells also allow an easy replacement of the temperature instrument.
Thermowell might be needed:
i. Medium is corrosive
ii. Static pressure is >25 bar
iii. Sanitary applications
iv. Easy maintenance / test requirements
For bimetallic thermometers bottom / back / every angle mount are available. For gas filled thermometers, additionally remote mount with a capillary tube and a surface mount option are available.
This is very logical. Thermometers always indicate the ambient temperatures therefore pointers are not at zero.
