【2025 Temperature Meters】How to determine whether a temperature measurement system meets the technical requirements for process measurement

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Even when thermocouples, thermal resistors, and bimetallic thermometers are of good quality, proper installation procedures are essential. To obtain as accurate measurement results as possible and minimize errors, it is crucial to pay attention to the depth at which these devices are inserted.

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①The insertion depth of the temperature sensor with a fixed threaded mounting is calculated as half the diameter of the pipe plus the size of the mounting boss for the sensor plus the thickness of the insulation layer. ◆ Thermocouples and thermal resistors have external sealing; taking the insulation layer thickness into account is necessary to ensure easy installation or removal without damaging the structure of the insulation layer. The common lengths for the mounting bosses of temperature sensors are 60 mm and 120 mm (they can also be manufactured to meet specific requirements), while the typical thickness of the insulation layer is 120 mm. ◆Bimetallic thermometers are hermetically sealed; the method for calculating the insertion depth is the same, but 30 mm should be subtracted from the size of the temperature sensor’s protrusion (this is related to the structure of the protrusion).

②The insertion depth of the temperature sensor installed on the fixed flange is calculated as half the pipe diameter plus the size of the flange connection pipe plus the thickness of the insulation layer

③For temperature sensors without fixed fixtures, with movable flanges or movable threads, the insertion depth must be adjusted by the user. For example, in the case of thermocouples used to measure temperature inside a furnace, the insertion depth is typically calculated as the actual length inserted into the furnace plus the thickness of the furnace body (including the thickness of the furnace walls) plus the thickness of the insulation layer.

②For different devices, the insertion depth of the temperature sensor varies due to differences in installation location. In design, we often use the centerline of the pipe as a reference to determine the insertion depth of the temperature sensor. This method is suitable for most applications, but it may not be appropriate for temperature measurement in large-diameter pipes, tower equipment, furnaces, etc. Firstly, it could lead to waste, and secondly, it makes it difficult to secure the sensor in place.

For temperature measurement in ordinary tower equipment, when the temperature sensor is installed horizontally, an insertion depth of 300–400 mm is sufficient. For temperature measurement in boiler furnaces, temperature sensors are usually installed horizontally; after accounting for the thickness of the refractory bricks in the furnace, the thermocouple only needs to be inserted 150 mm into the furnace.