Sensor step time or time constraints refers to the amount of time it takes for a temperature sensor to adapt to a new environment or change to the conditions. This doesn’t happen immediately, in the same way if you open a fridge door, the food temperature doesn’t rise immediately. On the other hand, the air temperature inside the fridge changes quickly because air has a low thermal mass and heats up or cools down quickly. It’s all about the thermal mass!
Typically any changes in a normal environments doesn’t happen immediately and it will take time before environment is adapted to the new temperature. Therefore measurement of rapidly changing air temperature would lead to a situation where sensor values would jump up and down all the time, and it would not correlate of real environment temperature changes.
Mass of the RuuviTag is 25 grams (RuuviTag in 4in1) and 38 grams (RuuviTag Pro). If you measure fridge temperature changes with RuuviTag, typically you will notice a immediate change in the measurements when the door is opened and it will recover when the door is closed. Mass of the RuuviTag is very small compared for example to mass of a one liter milk package. Therefore it’s safe to measure changes with RuuviTag. Typically high accuracy drug fridge temperature sensors are calibrated with an additional 10 gram mass to simulate real measurement situation inside the fridge.
RuuviTags step time (0°C->20°C) in a free air:
- Only sensor chip: Sensirion SHTC3: 1 τ (tau) 63% = ~8s, 5 τ (tau) ~99% = ~40s
- External sensor board with TMP117 temperature sensor (RuuviTag Pro with external TMP117 sensor): 1 τ (tau) 63% = 162 s (2 min 42 s)
- RuuviTag without enclosure (circuit board+battery): 1 τ (tau) 63 = 586 s ( 9 min 46 s)
- Standard white RuuviTag 4in1 SHTC3: 1 τ (tau) 63% = 674 s ( 11 min 14 s) (note that top enclosure doesn’t reduce time very much)
- RuuviTag Pro 2in1 or 3in1: 1 τ (tau) 63% = 910 s (15 min 10 s)
If RuuviTags are attached to surfaces and mass around the device is increased, also step time will increase. If sensor is placed in water (or other liquid), thermal conductivity around the device is better and step time will be less.
Step time also applies also to air humidity and air pressure measurement.
More information about the time constraint https://en.wikipedia.org/wiki/Time_constant