Thermometers: Installation Length and Active Length
Measuring systems have very different “active zones” in which the actual measurement takes place. Sensors, for example, measure directly at the tip of the immersion tube, at the point at which the sensor is installed. This is due to the size of the sensor element, and means the area at which the medium is actually measured is very small.
In the case of industrial measuring instruments, this is a little different: when the measuring takes place with a liquid or a gas, the temperature naturally affects the measuring instrument wherever it encounters it. Therefore, deviations due to ambient temperature influences, the heating of the housing, etc. must be taken into account in the design of the device.
Industrial thermometers
In the case of industrial thermometers, we have designed the reservoir at the lower end of the glass insert so it is as large as possible, so that the majority of the measuring liquid is in the immersion tube, and is therefore exposed to the measured medium. For a correct measurement, the industrial thermometer should be immersed in the process as far as possible over installation length l1 (measured from the sealing collar, including the thread, to the end of the immersion tube).
Gas filled dial thermometers (local)
In the case of gas filled dial thermometers, on the basis of the measurement principle (expansion of a gas), additional errors occur due to environmental reasons if the temperature on the casing deviates from the reference temperature (23 °C ± 2 °C). The ambient temperature influence on the measurement result is kept low, as the active gas volume (vessel content) is very large in comparison with the inactive gas volume (capillary line and measuring element).
The entire length of the temperature sensor should be immersed in the measured medium. It must always be ensured that the expansion vessel with its ET length is completely immersed in the measured medium.
Gas filled dial thermometers with capillary line
As with locally indicating gas filled thermometers, in the case of dial thermometers with a capillary line, additional environmentally-related errors may arise on the basis of the measurement principle if the temperature on the casing or the capillary line deviates from the reference temperature (23 °C ± 2 °C).
For remote reading dial thermometers, the longer the capillary line is, the larger the vessel volume has to be. Standard values for the attainable minimum immersion tube length, the active ET length and the capillary line length are available on our data sheets. The ambient temperature influence on the measurement result can be optimised if the active vessel volume is chosen as large in comparison with the inactive gas volume (capillary line and measuring element). We can manufacture designs whose vessel volume is adapted to the specific application upon request.
When fitting the capillary line, it is necessary to ensure that it is not guided past cold sources or sources of heat. In these cases, the thermal insulation of the capillary line is a good idea. The bending radius amounts to at least 30 mm.
Bimetal dial thermometers
A significant advantage of bimetal dial thermometers is their insensitivity to ambient temperatures, as the temperature only affects the actual measuring instrument (bimetal coil).
Bimetal dial thermometers can be used, transported and stored at ambient temperatures of -40 to 60 °C (for casing filling of -20 to 60 °C). Bimetal dial thermometers are therefore suitable for outdoor use, in direct sunlight, for example.
