Immersion depth and stem conduction
A probe conducts heat along its stem. Insufficient immersion makes the sensor read closer to room temperature than to the medium.
The sensing element of a probe sits some distance from the tip, and the stem connects it thermally to the outside world. If the probe is not immersed deeply enough, heat flows along the stem and the element does not reach the medium temperature. The error grows with the temperature difference between medium and ambient.
Checking it
Immerse to the working depth, record the reading, then immerse a further 20–30 mm and record again. A change indicates stem conduction; increase immersion until the reading is stable. Record the depth used for calibration and use the same in service.
Rules of thumb
- Immersion of at least ten to fifteen times the probe diameter plus the element length is a common starting point, but verify it.
- Short probes and dry blocks are the highest-risk combination.
- Insulating the exposed stem and the top of the block reduces the effect.
Frequently asked questions
- What is the minimum immersion depth for a temperature probe?
- There is no single figure. A common starting point is 10–15 times the probe diameter plus the sensing-element length; the immersion test (extra 20–30 mm, no change) is the real criterion.
- Does immersion depth matter at room temperature?
- Less, because the stem-conduction error scales with the difference between the medium and ambient — but it still matters for high-accuracy work and grows rapidly at 100 °C or −40 °C.
References
- [1]EURAMET cg-13: Calibration of Temperature Block Calibrators
- [2]EURAMET cg-8: Calibration of Thermocouples
General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.
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