Hi,
My mother bought a caravan last year in the UK.
Every winter you have to remove any bedding and objects that can hold the damp, and place all mattresses and cushions and seats in the middle of the rooms away from the walls to try and protect them from the damp that runs down the inner walls during the winter months.
Ahhh, i thought, I’ll make a data logger for each room, leave them over winter and see what happened humidity and temperature wise in each room so see if one side or room was effected more than the other.
So off i went looking at measuring humidity. Cheapest option was, as you have found the DHT11's. They claim to be calibrated and accurate. Mmmmmmm. They use a thermistor for temperature and a resistive sensor for humidity.
I got five of them, time for testing, single rig, pic, all in a line along a dil header and a 20x2 lcd display.
Expectations, all reading the same temperature, all reading the same humidity. Same batch so supposedly all calibrated next to each other.
NOPE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Temperature wise, between the lowest reading and the highest reading, there was a 12 degree difference, yes 12 degree C.
Humidity wise, well i was looking at a difference from lowest to highest of 40%RH.
No two units were even close to each other.
This is where the journey of insanity began.
How to accurately measure humidity.
I put them all together in the 'bag of salt',,,, none of them reached 75%RH, none of them went above 64%RH, and that was just one of them. The others were well below.
Then came the search for anybody in the area i knew who had anything that measured humidity.
I had a table full of digital humidity meters, all were giving a different reading, all together were well outside the +-5% error margin.
This lead to the home made wet/dry bulb Masons Hygrometer and lots and lots of Relative humidity and psychrometric charts.
Problem with the tables is almost all of them only show dry-wet bulb differences in 1'C steps, and as an example say you have dry bulb is 16'c and the wet bulb is 11'c, a difference of 5'c, from the tables RH is 54%, but 1 degree step either side gives 62%-54%-45% so a +- 1'c difference in the wet bulb temperature can give a RH change of 17% difference in RH. Even the psychrometric charts give wide differences for just 1'C change. I needed 0.1'C steps, like finding a living do-do on the internet
Defenetly not good enough for a data logger, especially when trying to compare several areas at the same time.
I could have taken one with a middle readings and then biased all the others to give the same reading, but why, they claim these devices are factory calibrated in a humidity chamber and the firmware uploaded with accurate calibration data for each device.
So, how can someone, joe blog, accurately read RH humidity without pressure chambers and black magic to check what they are looking at is correct with a reasonable accuracy and confidence.
Well the secret is the UK pound shops, i was in one one day and saw on the shelf for £1 a lump of plastic that contained a 1" clock, a 1" dial thermometer and yes, a 1" dial humidity gauge. Well i thought, no harm in testing it.
Salt bag, exactly 75%RH, just got some Calcium Carbonite, 42%RH, well bugger me, and all for a quid.
It's only downside is it's housing is bright pink
Well this really got me thinking.
Ok, then came the home made slingshot psycrometer. confirmed the quid dial and the Masons, but all the digital ones were nowhere close.
I needed some other way.
Next thing to play with was the good old glass of water, bag of ice cubes, face mask and a thermometer.
Put water in glass, put ice into water, put mask over mouth to stop breath touching side of glass, stir well with thermometer and wait for condensation to form on outside wall of glass. At the moment condensation forms, read thermometer and use dew look-up tables to give RH.
And so the obsessive madness continued, rip apart an old solid state fridge, remove peltirer plate, stick to back of mirror, heat, cool, look for condensation, measure glass temperature, gives dew temperature and - well
I gave up last year, winter came and went ( not this winter )
I got some of the Capacitive sensors, looking at me now, gonna give them a try, also at this very moment coding for a DS18b20 digital masons hygrometer with massive look-up tables. (want to do something that will really send you barmy, program in asm to read the unknown rom serial numbers of an unknown number of daisy chained DS18b20 devices )
The next version of the DHT11 is the DHT22, which is supposed to use the DS18b20 and capacitive sensor.
When the price comes down to a reasonable level, might get one to try.
ALSO, they say the supply range for the DHT is 3-5v, well change the supply and you change the humidity readings.
I wish you luck.
If i had just bought ONE, none of this would have happened, i would have assumed what it was telling me was close and within the manufacturers spec.
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