Immersion depth (how deep you insert the sensor into temperature source) is one important consideration when calibrating temperature sensors.
Our temperature calibration lab people gave this rule of thumb when using a stirred liquid bath:
- 1% accuracy – immerse 5 diameters + length of the sensing element
- 0.01% accuracy – immerse 10 diameters + length of the sensing element
- 0.0001% accuracy – immerse 15 diameters + length of the sensing element
Heat conduction in a stirred liquid bath is better than in a dry-block and the required immersion depth is smaller.
For dry-blocks, there is a recommendation that you should immerse 15 times the diameter of the sensor added with the length of the sensor element. So, if you have a 6 mm diameter sensor, which has a 40 mm element inside, you immerse it (6 mm x 15 + 40 mm) 130 mm.
Sometimes it is difficult to know how long the actual element is inside the sensor, but it should be mentioned in the sensor specifications.
Also, you should be aware of where the sensor element is located (it is not always in the very tip of the sensor).
The sensor to be calibrated and the reference sensor should be immersed into the same depth so that the middle points of the actual sensor elements are in the same depth.
Naturally with very short sensors, it is not possible to immerse them very deep. That is one reason for the high uncertainty when calibrating short sensors.
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