Hi,
I have project witch need read temperature and humidty but in wet condition in greenhouse. I using DHT22 but I dont know how exact is. In the greenhouse is humidity around 80% and have sensors on 1.5 m cable. Can anybody help choose the best sensor for this situation? I find some sensors as SHT10,20 ... with waterproof sesign but it is much expensive and I dont want buy it if not working correct.
Many thanks for help.
There are sensors for hobby and toy applications and there are sensors for industrial and commercial applications. When choosing a sensor (any sensor) we consider the application and the environment it must function in as well as the uncertainty / accuracy we need or want. The SHT 10 series at about $50 USD are hardly what I would call expensive. The word "expensive" is a relative term and my expensive and your expensive are not always going to be the same. How accurate of measurement do you need for your application?
I also suggest you read up on how humidity sensors can be checked periodically for accuracy using saturated salts slurry.
Ron
You might consider two DS18B20, one as a wet bulb - $5 each, complete with 2m shielded cable.
Nick_Pyner:
You might consider two DS18B20, one as a wet bulb - $5 each, complete with 2m shielded cable.
Another good option, read about wet bulb, dry bulb.
Ron
Another option is the Bosch BME280. It will give temperature, humidity and barometric pressure. Investigation of the data sheet will tell if it will work in your environment and have the accuracy that you need.
When shopping for a module, make sure that it is BME280, not BMP280. The BMP280 is cheaper but lacks the ability to measure humidity.
Polymer based humidity sensors saturate at high humidity, wet/dry bulb is more likely to be reliable for
high humidity work I suspect.
Method dry/wet bulb is interesting. If I understand correct I put one DS18B20 to wet condition(can be on water?) and second to dry condition on my greenhouse. The I read both values and calculate humidity using some table. Is that correct?
Jurkylius:
Is that correct?
Essentially. You use a wick in water for the wet one, rather than dipping it in water. You are measuring the cooling due to evaporation from the wick. I'm sure you can Google your way through this. I raise the matter because the DHT22 seems to be a mickeymouse device, and I don't understand why wet/dry DS18B20s isn't common practice. They are so cheap and reliable, but you do have to maintain the "wetness".
The wick is normally cotton wick material and slipped over the temperature sensor and the water is distilled water. You look at the wet and dry bulb temperatures and do a look up on a nomograph table. The method is generally used in the calibration of humidity sensors but not used as a daily method of measuring relative humidity. The method was also popular with liquid in glass thermometers and thus the term "bulb" is used.
So, I could for example, setup a DHT 22 or similar RH sensor in the same thermal and RH environment as a wet and dry bulb. I use my wet and dry bulb as my standard and compare my numbers to my sensor under test. I can then adjust my code to offset any error.
The downside is I only get one reading and if we look at the nomograph RH is not linear.
This is why when people wanting to measure RH accurately rely on much more accurate instruments than a sensor like the DHT 22. Good sensors which are durable and accurate carry a price tag. 
Ron
And this sensor is not good?
https://www.ebay.com/itm/162776413150
What do you want to measure, soil moisture or relative humidity of air?
Jurkylius:
And this sensor is not good?
https://www.ebay.com/itm/162776413150
I have no idea. I normally look for:
Accuracy (25°C): ±2.5% RH, ±0.3°C
Long-Term Stability: Better than 1%
RH per year (typical)
I also buy from a reputable source. I would guess for hobby applications the Ebay sensor is fine. For a commercial application I buy from a reputable source. Here in the US I did buy from Omega Engineering and again it depends on what uncertainties you can work with. Getting long term stability is difficult with RH sensors.
My best guess for a simple green house application the Ebay linked sensor may be fine. I just have no experience with that specific sensor.
Ron