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si7021 problems
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Please be positive and constructive with your questions and comments.

si7021 problems

by Jayveesea on Tue Jan 08, 2019 11:08 pm

I'm having some difficulty with my si7021 temp/humidity sensors (ordered back on black friday). I have each sensor connected to it's own Huzzah ESP8266 and periodically posting temp/humidity via MQTT. I've placed both systems next to my Nest and an Accurite temp/humidity sensor. The Nest and Accurite agree on humidity (within 1% RH) but both si7021's are reading about 10% RH higher than the other two (temperature seems ok) and they've all been sitting there for several days. Is there any tricks to getting better accuracy or do I have some bad sensors?

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Re: si7021 problems

by Jayveesea on Mon Jan 14, 2019 10:13 pm

The trend continues with these devices over the last few days. The si7021's consistently read 10-12% RH higher than my other devices.

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Re: si7021 problems

by adafruit_support_carter on Wed Jan 16, 2019 3:02 pm

Which library are you using with the Si7021?

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Re: si7021 problems

by millercommamatt on Wed Jan 16, 2019 3:29 pm

How to the temperatures compare? If your reference sensors are reading warm, the reported RH could be low.

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Re: si7021 problems

by Jayveesea on Thu Jan 17, 2019 9:01 am

the reference temps read about 1.5degF higher than the si7021s.

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Re: si7021 problems

by Jayveesea on Thu Jan 17, 2019 9:09 am

adafruit_support_carter wrote:Which library are you using with the Si7021?


I'm using Adafruit's "Adafruit_Si7021".

Here's my code.

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Re: si7021 problems

by adafruit_support_carter on Thu Jan 17, 2019 1:09 pm

What are the RH and temperature levels you are operating at?

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Re: si7021 problems

by millercommamatt on Thu Jan 17, 2019 3:52 pm

An air temperature of 24 C over a dew point of 10 C equals an RH of 41.7%. An air temperature of 26 C over the same dew point equals an RH of 36.5%. The difference in RH could just come down to a different in temperature. For the same absolute humidity, the relative humidity is a function of air temperature.

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Re: si7021 problems

by Jayveesea on Sat Jan 19, 2019 2:37 pm

adafruit_support_carter wrote:What are the RH and temperature levels you are operating at?

Temperatures right now are about 65 to 70degF, RHs right now are about 30-45%.

millercommamatt wrote:An air temperature of 24 C over a dew point of 10 C equals an RH of 41.7%. An air temperature of 26 C over the same dew point equals an RH of 36.5%. The difference in RH could just come down to a different in temperature. For the same absolute humidity, the relative humidity is a function of air temperature.

Thanks, I think this must be a source of some of the error, but I've been seeing about 10-12% difference between my Nest and Acurite and the si7021's. Considering the sensor accuracy for each of them might account for the remaining discrepancy.

That being said, the Acurite is reading 66degF and 37% RH, and the si7021 is reading 66degF and 46% RH right now and they are sitting right next to each other... so I'm not totally convinced that this thing is operating correctly.

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Please be positive and constructive with your questions and comments.