A temperature sensor reports its own temperature, not the air’s. Before the two agree, heat has to flow between them through the sensor’s housing, and that takes time proportional to the sensor’s mass and the insulation around it. A tiny bare thermocouple settles in seconds; a sensor sealed inside a plastic logger or display can take 15 minutes or more. Reading before it has settled gives a number somewhere between where the sensor came from and where it is now.
Thermal mass and the time constant
When a sensor is moved into air at a new temperature, its reading approaches the new value along an exponential curve. The time constant τ is how long it takes to cover 63 percent of the gap; after 3τ it has covered 95 percent; after 5τ, 99 percent. Manufacturers quote either τ or T90 (time to 90 percent, about 2.3τ). A sensor with a 4-minute time constant moved from 10 °C to 20 °C reads about 16.3 °C after 4 minutes, 19.5 °C after 12 minutes and 19.9 °C after 20.
Typical settling times
| Sensor | Time constant (still air) | Time to 99% |
|---|---|---|
| Fine bare thermocouple | 1 to 5 s | Under 30 s |
| Probe in stainless sheath (3 mm) | 10 to 30 s | 1 to 3 min |
| Chip sensor on a small open board | 30 to 90 s | 3 to 8 min |
| Sensor inside a plastic logger case | 3 to 6 min | 15 to 30 min |
| Sensor inside a display thermometer or weather station | 5 to 10 min | 30 to 60 min |
| Liquid-in-glass or dial thermometer | 1 to 5 min | 5 to 25 min |
Moving air cuts these dramatically; a fan across a logger can halve its settling time. Still air in a drawer or box doubles it.
Why it matters in practice
- Spot checks. A meter carried from a warm van into a cold store reads high for the first ten minutes. Cold-chain audits that read immediately are systematically wrong.
- Logger deployment. The first half hour of a logger’s record is its own settling, not the room. Discard it or start the logger after it has been in place.
- Comparing two sensors. If one has been in the room an hour and the other just arrived, the gap is settling, not calibration. See why two monitors show different readings for the same logic applied to CO₂.
- Self-heating. Sensors in powered devices warm up from their own electronics. A Wi-Fi monitor can read 1 to 2 °C above the air after it has been on for a while; the “stable” reading includes that offset.
Humidity has the same problem, slower
Capacitive humidity sensors rely on a polymer film exchanging water with the air. Diffusion is slow, especially in still air and inside a housing, and the film’s response also depends on the sensor reaching thermal equilibrium first, since RH is temperature-dependent. After a move, expect humidity to take at least as long as temperature and often twice as long. See what relative humidity is.
Practical rules
- Give any sensor at least five time constants after moving it; when in doubt, 20 to 30 minutes for enclosed devices.
- Watch the reading; when it stops changing over two successive intervals, it has settled.
- Ignore the first 30 minutes of any logger record.
- For fast spot checks, use a small probe rather than a boxed meter.
- Keep sensors out of direct sun and away from the heat of their own electronics where the design allows.
Related: why sensor placement changes measurements and how often sensors should record data.
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Frequently Asked Questions
How long should I wait before trusting a temperature reading?
A bare thermocouple or small probe settles in seconds; a probe in a metal sheath in a minute or two; a sensor inside a plastic logger or weather-station housing in 10 to 20 minutes; a large dial thermometer longer still.
What is a time constant?
The time for a sensor to cover 63 percent of a step change. Reaching 99 percent takes about five time constants. Datasheets sometimes quote T90 instead, the time to 90 percent.
Do humidity sensors have the same delay?
Yes, and often a longer one. The polymer film must absorb or release water, and it does so slowly in still air. Humidity after a move can take 20 to 30 minutes to settle.
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