Multipurpose sensor temperature slow to respond

I installed the ST multipurpose sensor in a room that is unheated. It was reporting 64 degrees inside the house (correct), and now it is VERY SLOWLY dropping. The room is at about 50 degrees, but it took 7 minutes from putting it out there to drop to 57 degrees (and still dropping). Open/Close and vibration sensor are reporting correctly.

Is the slow ramp of the temperature sensor normal?

That is normal, if you put a normal thermometer in there think about how long that would take to change…

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A normal thermometer is a mechanical device, with a significant mass of differing metals expanding at different rates (or with just mercury expanding inside non-expanding glass) to indicate temperature.

If you look at the datasheet of a silicon temperature sensor like the TMP35/36, they measure response time in still air in hundreds of milliseconds, even for temperature changes of 100%.

If this is normal for the multi-purpose sensor, I’m surprised but it’ll do. I was just wondering if I got a defective one that I needed to return.

I would say it would take some time. The sensor is inside a plastic container so the temp of the plastic and any air trapped in there can cause deviations.

There could also be battery throttling so it may not be sending up to the minute reports.

My best guess it will normalize within 30-60 minutes.

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Silicon diode sensors were developed specifically for the cryogenic temperature range. Essentially, they are linear devices where the conductivity of the diode increases linearly in the low cryogenic regions. To make a temperature measurement system with a diode requires excitation, offsetting, and amplification. The actual sensing element in a silicon sensor is a simple P-N transistor junction. The voltage across a regular P-N transistor junction has an inherent temperature dependency of about 2 mV/°C and this fact may be used to develop a temperature measuring system which is why they are used in these. Unfortunately the manufacturers use rejects from cryo industry because they are practically free , so that junction is most likely defective or precautions havent been taken to prevent oscillation both of which lead to slow or innacurate readings.

It also might just be delayed reporting between sensor and interface.

Yeah, I think the low thermal conductivity of the plastic case is the right answer here.

As long as I don’t have a defective device, then I’m happy with it. If my porch door temperature changes by 50 degrees in a minute, then I expect the smoke detector to catch it, not the door open/closed sensor :slight_smile: