Atmospheric Sensors in condensing conditions

I am currently using a DHT22 as part of a weather station project. However I am getting some strange Humidity readings. Generally they are higher than expected. I haven’t tested the accuracy of the sensor, however I have compared 2 DHT22’s with similar results.

I have read that the sensor can be adversely affected by condensation. In particular, it appears that should water condense on the sensor, the humidity can be thrown off somewhat permanently. Unfortunately, I cant think of any way of maintaining accurate readings, while protecting the sensor from condensation.

Can anyone recommend an alternative sensor, or a method for protecting the sensor from condensation?

One way to drive off condensation is to warm the sensor for perhaps 5-10 minutes, then let it cool and equilibrate for perhaps 10-30 minutes.

You will have to determine the appropriate temperatures and timing by experiment.

Try this one from Adafruit. It's very stable and has a built-in heater function that can be used to dry it out. It takes about 3 minutes to heat and 2 minutes to recover.

Arctic_Eddie:
Try this one from Adafruit. It's very stable and has a built-in heater function that can be used to dry it out. It takes about 3 minutes to heat and 2 minutes to recover.

Thanks I will give that one a go.
It has been a pain lately, as the system works well, then all of a sudden we get a cold night, and the DHT22 starts reading 99.9% humidity until it gets much warmer. And from there the sensor is basically stuffed - I either need to replace it, or recalibrate.

Well, condensing on an object at the same temperature with surrounding air IS an indicator of 100% relative humidity in the air.
If the sensor "sees" the clear sky it may get colder than surrounding air, that's why in certain mornings cars parked under a tree does not get covered with dew while others a few meters away do.
Same air temperature and humidity, car under tree is a bit warmer.

The SHT31 heater would allow the OP to keep it warm through the night then periodically cool down for a reading. Also, putting it inside of an inverted cup would prevent it from seeing the cold of space.

Arctic_Eddie:
The SHT31 heater would allow the OP to keep it warm through the night then periodically cool down for a reading. Also, putting it inside of an inverted cup would prevent it from seeing the cold of space.

The DHT22 is currently mounted inside a box, which has a breather hole in the bottom of it. Yet it still seems to exibit the same behaviour.

To make sure the reading was incorrect, I put 2 DHT22's side by side in the enclosure. One was ready 70% humidity, while the other was reading 99.9%. I left them both overnight outdoors (in the enclosure) and now both read 99.9% consistently. They are now both reading high all the time.

I have abandoned the DHT22 due to similar problems and now use the SHT31-D. Just a few minutes ago, I finished an experiment monitoring temperature and humidity in a bathroom closet. The data was sent by RFM69 radio to the living room and captured by another Arduino and displayed on the serial monitor then inserted into SciDAVis for plotting.

Similar sort of project to mine. Basically a weather station, but with 1 outdoor node, and 2 indoor nodes on RF24 radios, sending to another arduino, which then writes to MySQL via RPi. RPi hosts website containing all data and plots etc.

I have ordered a BME280 to give it a go, and if that doesnt work, Ill switch to the SHT31

I’ve been using the SHT31 outdoors for a few weeks with a RFM12B sending the data to an Arduino YUN in the house, then sending the data up to Thingspeak. The accuracy is within the datasheet specs, even now in the winter time. I haven’t used the heater yet, and only taking a reading every 10 minutes.

I’m using a atmega328pu chip, but looking into modifying a TinyWire version to work on a attiny85 to make a smaller footprint, use a coin cell battery and to use up some the attiny85’s I still have laying around.