Humidity - Page 1
Forum section: electronics / projects

author: DavidDLC
author_rank: Frequent Contributor
timestamp: March 23, 2013, 09:48:30 pm
content: I don't know much about humidity, I started a project with the DHT11 ( which later will replace with the DHT22 ), the DHT11 temperature and humidity measurements are shown on a display.I also have two Acu-Rite temperature and humidity sensors.The humidity measurements on the Acu-Rites and the DHT-11 are way off, for example the current measurements are:Acu-Rites: Around 50%DHT11: 34%I already checked for correct decode of the DHT11 data with my scope, I also have 3 DHT11 and all of them measures about the same value of 33-34%.Can somebody explain if there are different ways to measure humidity ? Why the Acu-Rite and DHT11 values are off ?Or if I need to make additional operations to the DHT11 raw data ?Thanks.David.

author: Rerouter
author_rank: Super Contributor
timestamp: March 23, 2013, 09:57:21 pm
content: the DHT11 is about 4% accurate on the specs, so to me would be the more trusted device, the Acu-rite even if the name implies its accurate or that its reading is correct may not be the case,

author: DavidDLC
author_rank: Frequent Contributor
timestamp: March 23, 2013, 10:07:32 pm
content: But even with the 4-5% the difference is too much.

author: mzzj
author_rank: Super Contributor
timestamp: March 23, 2013, 10:22:19 pm
content: First Acu-rite that I found on google search looked like worst kind chinese crap. +-10% accuracy and insane response time is what I would expect in the best case..I would get humidity calibration salt solutions to figure out which one is egg and chicken...33% and 75% salts should get you going.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: March 23, 2013, 10:27:32 pm
content: Kiitos mzzj.Yes the Acu-Rite are cheap Wal-Mart units.I was wondering if there is like Relative Humidity and other way to express humidity that I may need to calculate something with the data coming from the DHT11.

author: mzzj
author_rank: Super Contributor
timestamp: March 23, 2013, 10:57:40 pm
content: Quote from: DavidDLC on March 23, 2013, 10:27:32 pmOnly one way to express relative humidity and Dozens of ways to express (absolute)humidity. But I really doubt that DHT11 would use any of these alternative ones. Other forum members probably have more experience with these sensors.One important thing to note is that relative humidity measurement is very sensitive for temperature. 3 °C deviation in temperature causes 10% measurement error in relative humidity. (When talking about roughly 30-50% relative humidity at room temperature )

author: nukie
author_rank: Frequent Contributor
timestamp: March 24, 2013, 06:42:45 am
content: Sensirion.

author: qno
author_rank: Frequent Contributor
timestamp: March 24, 2013, 12:38:41 pm
content: There are numerous ways to check you RH meter.Check the web.Put it in a small container wrapped in a damp cloth should bring the relative humidity to 99%.This would get you the first point of the graph.The second point in the graph can be found with the 2 thermometer measurement.Take 2 alcohol thermometer tubes and wrap one in a small wet cloth.wave the thermometers around and the difference in temperature will tell you the rh of the room by reading the table.Check the web for "RH measurement wit 2 thermometers" for the table.

author: kripton2035
author_rank: Super Contributor
timestamp: March 24, 2013, 12:44:51 pm
content: Quote from: nukie on March 24, 2013, 06:42:45 amyep I would say that too, but it's clearly not the same price !!! some 5x ?

author: mzzj
author_rank: Super Contributor
timestamp: March 24, 2013, 02:05:34 pm
content: Quote from: qno on March 24, 2013, 12:38:41 pmI think DHT-11 is specified up to 80% or something like that so checking it at 100% relative humidity wont help much. In my experience anything above 90% relative humidity is difficult for the cheapo sensors.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: March 24, 2013, 05:30:30 pm
content: 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 internetDefenetly 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 pinkWell 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 - wellI 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.

author: mzzj
author_rank: Super Contributor
timestamp: March 24, 2013, 06:14:59 pm
content: Quote from: Mr Smiley on March 24, 2013, 05:30:30 pmSounds like good reason to spend few bucks more on more reasonable humidity sensorI know a few really good capacitive sensors based meters/probes that are accurate to 1-2% in real humidity cal lab tests but you guys are going to need medication when I tell the price of those thingies

author: kripton2035
author_rank: Super Contributor
timestamp: March 24, 2013, 08:11:51 pm
content: so the sensirion sht11 is really not that expensive after all you say Mr Smiley !

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: March 24, 2013, 08:31:57 pm
content: At the time, sensirion were giving away free samples of the sht11, i requested some samples, waited, requested some samples, waited and so on, never got a single one, so thought stuff them, i had the time and the interest so i went back to basics and discovered more than just soldering three wires onto three pins. Also, if you want a reasonably priced accurate RH meter, build your own, calibrate it using the old school ways of measuring humidity and have confidence. Otherwise spend an absolute over the top for your needs fortune and have a little confidence in it's accuracy. Or spend a few quid and live in the land of the blue pill.The number of people here obsessed with having their meter read 5.00000v with a source of 5.000000v, doesn't mean others are daft wanting their RH meter to read 48% when it is 48%, how many people would buy a fluke with a stated accuracy of +-5%,RH is not easy to measure, the number of variables is quite extreme, but it can be done accurately with limited funds and old school methods, such as two DS18b20's, a cotton shoe lace some distilled water a small fan,some look-up tables and an 8 pin pic or atmeg.

author: kripton2035
author_rank: Super Contributor
timestamp: March 24, 2013, 08:43:21 pm
content: I asked for sht11 sample too but mine arrived to me. I build a meter with a pic and a lcd displaybut I didnt manage to calibrate it the way you do. also I only have one.may be I may buy another a sht11 and compare them ?would be a good start to see if they are really calibrated at the factory or not.

author: C
author_rank: Super Contributor
timestamp: March 24, 2013, 11:41:09 pm
content: On sparkfun in thehttp://www.sparkfun.com/news/1090If you have soldered them, they need some steps done as listed in data sheetC

author: nukie
author_rank: Frequent Contributor
timestamp: March 27, 2013, 11:24:44 pm
content: The Sensirion is calibrated that's why it cost $$ it has none of your DHTxx issue. You can buy one sensirion and calibrate your DHTs devices. The tiny temperature & humidity dials you see in museums display are actually very accurate.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: April 01, 2013, 07:13:09 pm
content: I have replaced the DHT11 with the Sparkfun RHT03 and now both temperature and humidity are on spot with the Acu-Rite units.Acu-Rite: 37%, RHT03: 36.9%David.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: April 01, 2013, 07:27:05 pm
content: Still need to put the decimal point on the RHT03 readings

author: nukie
author_rank: Frequent Contributor
timestamp: April 02, 2013, 05:17:37 am
content: Is this from another manufacturer? If yes then that's great to see a good product from the Chinese.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: April 02, 2013, 04:58:45 pm
content: Yes, the humidity/temperature sensor is from Maxdetect