DHT22 vs BME280 vs SI7021 , which can read near 100% humidity the best

i need these for a mushroom greenhouse, so, i need to make the humidity get to around 95-100%

Im quite capable of calibrating these but i need to know if they can set to reliably read out that range, 95-100%, so that every time it is exposed to say, 100%, it reads out 100%, all i really need to know is if or not the humidity in the greenhouse is 100%, this is to enable a fogger. Ideally if it can read the specific value between 85% and 100% RH, i could quantify the amount of water neccesary to deliver to the fogger. SO i guess i either need to be able to reliably read out 100% or reliable and reproducibly read out a value at or above 85% at the very least. if it ends up functioning more like a binary switch then so be it.

I shouldnt allow humidity to drop below 85% though i generally want to keep it at about 95-100%, yet not oversaturate which ends in everything getting wetter than it should be.

But sometimes ill also want it at 90%, for a particular species of mushroom.

which of these sensors would be best suited to the task (with DHT22 id be using 2-3 wheras with the others, only 1)

All thats really important is that theres one value above 85% that i can read with certainty, ideally 100%, but if something else, thats fine too, so long as i know the humidity and temperature i know exactly how much water to nebulize. or if 100% is best, then i can just go until it stops or act based on how it approaches saturation.

for any other value i dont need any real accuracy, just 1 point where i know its reliable.

The only sensor that is suited for your needs (according to the datasheet, I never built a mushroom greenhouse) is the BME280 (the SI7021 might work but has definitely the lower accuracy in the high humidity regions). Forget the DHT22, that's cheap crap, use it for the temperature but the measuring humidity more of a joke than serious work.

Keep in mind that even the BME280 has an accuracy of +/- 3%RH, so I'm not sure if that's sufficient for your mushrooms.

+/-3% is fine.

This is late information, but may be useful to others who come across this in a search.

I have been testing several AM2302 (aka DHT22). Here in the Houston, Tx area we often have long periods of very high humidity.

Thus far all the DHT22's I have placed outdoors (under a roof so they are not subject to rain or any direct moisture) have begun giving false humidity readings after a day or two. The typical day has been between 70% and 100% humidity with the temperature between 70F (21C) and 94F (34C).

[N.B., yes that is correct, our dewpoint has not been below 21C for the last 4 weeks]

In the failure mode the sensors continue to give humidity readings but the readings are about 15% RH too high.

It appears this is a saturation problem and basically renders them useless for anything except air conditioned spaces.

Caveat Emptor.

We've been struggling to find a sensor that could handle occasional 100% humidity conditions in tropical caves. So far all of the digital sensors have failed in more or less the same way: they work OK for a couple of weeks, then suffer from rising readings for a week or so till they hit their maximum and just stay pegged there from then on. What I believe is happening is that the conditions go condensing, and then the moisture simply never evaporates again from inside the sensor material. After a few days "dry" the sensors return to normal operation. So if you could find a sensor with an internal heater & condensing environment reset procedure I'd turn that feature on every couple of days.

I also tried a Masons Hygrometer:

but the DS18b20 temp sensors did not have enough resolution to measure the 95-100% range that I was after. If you used higher bit depth temperature sensors you should be able to make this old school method work for your mushroom farm provided you can keep one of the sensors dry (say inside a housing?) and the other one in a wet sleeve.

So far all of the digital sensors have failed in more or less the same way: they work OK for a couple of weeks, then suffer from rising readings for a week or so till they hit their maximum and just stay pegged there from then on.

Please provide a list of the sensors your tried and failed this way.

I'll have to go digging, as many of those failures were from the 2014-2015 time period. Most were HTU21D's with a few from the SHT series (some of those SHT's had sintered shrouds -supposedly for burial).

I've a few BME280's out now and some have already begun exhibiting the same basic problem: normal operation up till about 95% RH, then a rise to 100% and staying there.

Because they reset to apparently normal operation after being dried out. I'm not even sure this can be called a "failure" exactly. It should probably be described as using those sensors outside the environment they were designed for (ie 90-100% humidity for extended periods of time)

review - Compare DHT22, AM2302, AM2320, AM2321, SHT71, HTU21D, Si7021, BME280

I once used a dht22 for a mushroom greenhouse and I also need 95% humidity and only last one day reading % accurately. After that it only display 99%. I tried several things and nothing worked. Even I tried heating it up with a lighter and the humidity went down, but as the temperature decrease the %humidity increase until it reached 99%. I have the same problem now. Could you any solution?

hectorgm432:
I once used a dht22 for a mushroom greenhouse and I also need 95% humidity and only last one day reading % accurately. After that it only display 99%.

As you can see from several other threads on this specific sensor, that's normal behaviour.

Solution: get a better sensor.

Hey,

could you share result which sensor came out as best for humidity?
Facing similar problems...

Thanks

Maybe chilled mirror humidity sensors, but they are expensive.