Si7021 acting unexpectedly

Community PCB Design
1

Hi Everyone,

I am wondering if the Si7021 breakout board is required to be positioned in any specific way as to sense the temperature and humidity most accurately?

I currently have the breakout board hot-glued to the top of a clear enclosure with a 4.5mm diameter hole directly over the sensor. White protective cover on the sensor has not been removed. See the image attached. It is set to sense every second.

There is a little bit of debris on the white cover, and it is not as perfect as it was when first installed. Will this alter the readings to the effect that they will be grossly incorrect? Should I replace the board?

Is there a way to cover the sensor to protect it, but still allow it to sense the temperature and humidity?

Is there an industry standard to the positioning of temp/humidity sensors? Should I be potting the sensor rather than exposing it raw? What’s the best thing to do here?

Before it was moved to the top of the enclosure, it was sitting on the base of the enclosure exposed to the air inside the enclosure. I imagine this is then sensing what the temperature is inside the enclosure, not the ambient air outside of the enclosure. Even though the enclosure had an opening for USB charging nearby.

The temperature was about 20°C when I got in the car, then went up to 26°C when placed on the dashboard of my car, it didn’t feel like 26°C when I put my wrist in the same position. I then moved the sensor to the centre console away from the direct sunlight, then I put the air conditioner on in the car to the coldest, and it dropped 2°C, over half an hour with the aircon on. I was freezing.

Another test showed it to be 14°C while walking in the park, the sensor is shaded by my shadow. I then placed the sensor in direct sunlight and it goes up to 32°C. I then stand in front of the sensor to create a shadow, and it goes down to 28°C not budging for about 5 minutes, then drops to 26°C. I find this quite unexpected as the ambient air is still at a low 14°C.

Can anyone offer any advice or suggestions on how I might get more accurate readings to when the sensor is put in different environments?

And, why is it that the sensor thinks it’s still quite hot and stays at this temperature for so long rather than dropping to what the temperature actually is?

Any help appreciated :pray:

Si7021.jpg
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I am wondering if the Si7021 breakout board is required to be positioned in any specific way as to sense the temperature and humidity most accurately?

Not really, but the sensor does need access to the air around it to sense humidity. If you were to seal it in a box, it would only report the humidity level inside the box.

I currently have the breakout board hot-glued to the top of a clear enclosure with a 4.5mm diameter hole directly over the sensor. White protective cover on the sensor has not been removed. See the image attached.

That should be OK. Make sure water doesn’t leak in through your hole though.

There is a little bit of debris on the white cover, and it is not as perfect as it was when first installed. Will this alter the readings to the effect that they will be grossly incorrect? Should I replace the board?

A little bit of debris on the body of sensor shouldn’t hurt it too much. Might slow down it’s response to humidity a tiny bit but I don’t see much of an obstruction in your photos. You could try a replacement to see if you get different results if you wanted too but from what I can see I think you’re OK.

Is there a way to cover the sensor to protect it, but still allow it to sense the temperature and humidity?

It’s going to sense temperature no matter what you do to it. You just need to keep liquid water off the board and sensor but still allow air to pass around the board. I’ve heard that teflon tape might work if you’re looking to cover the hole in your enclosure but that’s going to slow the reaction time of the sensor.

Is there an industry standard to the positioning of temp/humidity sensors? Should I be potting the sensor rather than exposing it raw? What’s the best thing to do here?

Not that I know of. If you pot the sensor it’s not going to accurately report humidity anymore. Whatever you need to construct to keep water off the board but still allow air to circulate around the sensor is what you’re going to want to build. It’s overkill for a single sensor, but [something like this would work. It keeps water out but allows air to circulate.

The temperature was about 20°C when I got in the car, then went up to 26°C when placed on the dashboard of my car, it didn’t feel like 26°C when I put my wrist in the same position. I then moved the sensor to the centre console away from the direct sunlight, then I put the air conditioner on in the car to the coldest, and it dropped 2°C, over half an hour with the aircon on. I was freezing.

The sun can rapidly heat the sensor, the enclosure and everything else inside the enclosure and that might take a while to cool off. Remember the sensor it taking it’s own temperature, not the temperature of the air in front of it. The entire sensor has to heat up and cool down to ambient for you to get an air temperature. While out of sunlight it should be pretty close to air temperature but it won’t respond to changes in air temp as quickly.

Another test showed it to be 14°C while walking in the park, the sensor is shaded by my shadow. I then placed the sensor in direct sunlight and it goes up to 32°C. I then stand in front of the sensor to create a shadow, and it goes down to 28°C not budging for about 5 minutes, then drops to 26°C. I find this quite unexpected as the ambient air is still at a low 14°C.

Your entire setup gets warm in the sun and then takes a while to cool down slowing the response rate of the temperature sensor. It’s like a pot of water you put on the stove. A boiling pot stays hot for a long time after the burner is off because it has thermal mass. Your setup has less thermal mass than a pot of water, but it reacts the same way.

Can anyone offer any advice or suggestions on how I might get more accurate readings to when the sensor is put in different environments?

If you need something that reacts more quickly, you might want a dedicated temperature sensor mounted several inches away from the rest of your enclosure. A [MAX31820 mounted on 15-30cm of wire hanging in free air would be a lot more responsive to change in temperature since it has very little mass to heat and cool. Just keep it out of direct sunlight so that it’s not reporting an abnormally high temp.](https://www.sparkfun.com/products/14049)](Amazon.com)