Humidity Sensor Reset Time

sunshine0893

I am using a sensirion SHT15 humidity/temp sensor to measure enclosed humidity. While the sensor works in 8 seconds as specified for increasing humidity, it stays at the elevated humidity for a while. I need it to be reset to ambient between measurements. I tried using a hand pump with ambient air to direct at the sensor, but so far, nothing has worked. Is this a limitation to humidity sensors in general? Has anyone found a good way to push humidity sensors back to ambient or know of one that has a shorter response time downward?

Yanomani

Hi sunshine0893

Welcome to the forum

According to the datasheet, those sensors feature an internal heater element, whose on/off status could be commanded by writing to the status register.
When on, the heater may increase the temperature of the sensor by 5 – 10°C beyond ambient temperature, helping drying it towards ambient humidity conditions.
Some experimentation should be made, by cycling the heater on/off trying different duty cycles, until you find the best compromise between consumed power x measurement readiness.
You should be judicious when using the heating element, the minimum, the best.
These sensors are not constructed nor intended to be operated with the heating element continuously on, so care must be exercised during the process of searching for the suit spot between functionality and measurement readiness.

Further info could be found on device's datasheet:

sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/Humidity/Sensirion_Humidity_SHT1x_Datasheet_V5.pdf

Hope it helps

Henrique

LoopyByteloose

Humidity is indeed problematic to measure.

One should refer to Dew Point charts to see how complex it is. If the ambient temperature or the barometric pressure is changing quickly while measuring humidity, one can cross the line of 100% saturation and not be aware of it.

The SHT15 has the heater to attempt to dry the sensor, but there remain situations where it may just be overwhelmed.

Other humidity sensors don't even attempt to clear the sensor... so they actually may not be as useful. It is important to NOT have the sensor configured so that water collects on the surface, maybe an upside down position will work best.

Mark_T

I suspect the only fast + accurate humidity sensors are ones that measure the OH infra-red absorption line,
which is probably expensive and impractical in small sample cells. Any interaction between water vapour
in the air and a surface is likely to be much more complicated and sensitive to other factors. As I understand it
cheap relative humidity sensors use a special polymer that absorbs moisture, so must be very slow as it is
limited by the diffusion rate in a solid.

Tracy Allen

You can speed up the response by providing air flow with a fan or whatever.

I like the SHT71 or SHT75, which are the ones mounted on a narrow stalk. Both the back and the front of the sensor are perforated, so they allow better flow-through and faster diffusion.