[SOLVED] measuring humidy using thermistor

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Mithun_K_Das

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Can anyone tell me how to measure humidity using a thermistor? DHT11 or similar module is too costly. Also I found some Chaina products which is using thermistor but they are measuring humidity.
 

DHT11 does not measure humidity with a thermistor. It uses a capacitive sensor. I don't think that DHT11 is particularly costly, but that's a different thing.

There is a method to reduce humidity measurement to temperature measurements with a wet and dry thermometer. But it's not maintenance free.
 
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is there any way to use LM35 or ds18b20 temperature sensor to measure humidity?

I saw some circuits which is using these sensors inside a cotton ball. And also they recommend to change this cotton ball periodically after 3months.
 

The psychometer's wet thermometer is in permanent contact with water, so it must be sufficiently sealed.

I already mentioned maintenance requirement of psychometers. The first point is to refill the distilled water reservoir.
 
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Not sure what level of accuracy you are trying to accomplish but it seems unlikely that setting up a wet bulb measurement would be either cost effective or even very practical... Do you have a budget in mind? Have you checke major distributors like digi-key for humidity sensors?
 
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Price is not the main factor. The main factor is DHT11 is not available in our local market.
 

Mithun_K_Das said:
Price is not the main factor. The main factor is DHT11 is not available in our local market.
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Than you can use any other what you have locally, huge number of humidity sensors are in that range 10-30eur. I'm sure you will find at least one sensor.

You can search newer sensors.


Check for DHT22
https://www.adafruit.com/products/385

3 to 5V power and I/O
2.5mA max current use during conversion (while requesting data)
Good for 0-100% humidity readings with 2-5% accuracy
Good for -40 to 80°C temperature readings ±0.5°C accuracy
No more than 0.5 Hz sampling rate (once every 2 seconds)
 

Apparently the specification exists as an array of curves only. What I would do is to fit polynomials to the curves or set up table interpolation.
 
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As long as the sensor characteristics are stable over time, a calibration is basically possible. I agree that the large temperature dependency is problematic and strictly requires requires an additional temperature measurement. In should be mentioned that other humidity sensors don't have that strong temperature dependency.
 
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I found that HS12P has an internal resistance of 100KOhm at normal condition, but an infinity resistance at factory condition. And resistance gets down with humidity increases. But still don't have any calculation for his.
 

I don't know what you mean with "factory condtions". Are the sensors selaed under dry atmosphere (0% RH)?

I presume you know that the sensor resistance must be measured with AC only.

As previously mentioned, a "calculation" scheme has to derived from the curves, either by a polynomial fit or table interpolation.
 
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I measured the resistance just as we measure normal resistance. Is this wrong for this sensor?
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Yes, because it applies DC voltage to the sensor. Doing so for a few seconds probably won't hurt, but DC must be strictly avoided in the measurement circuit.
 
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