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Forum Index : Microcontroller and PC projects : Humidity Calculation
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palcal Guru  Joined: 12/10/2011 Location: AustraliaPosts: 2029 |
Does any one have a formula for Humidity calculaton from wet and dry bulb temperatures. My outdoor sensor doesn't last long, the humidity here varies from over 80% early morning dropping to 65% to 70% during the day. At the moment it is 31 Deg. C and78% humidity, very uncomfortable for some but I'm used to it. I want to use a wet and dry bulb which should last indefinately. I'm thinking of topping up the wet bulb tank with a feed from my garden watering system I tried a couple of formulas I found online but they weren't very accurate when compared to a psychrometric chart. Edited 2025-11-28 14:16 by palcal "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6443 |
Try here https://www.thebackshed.com/forum/ViewTopic.php?TID=15917&PID=203964#203964#203964 Good luck finding the BOM website that I reference! Jim Edit: have you tried the HDC3022? https://www.thebackshed.com/forum/ViewTopic.php?TID=17688&PID=234524#234524#234468 You should be able to dry it out once a week or whatever. Edited 2025-11-28 17:03 by TassyJim VK7JH MMedit |
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| Pluto Guru Joined: 09/06/2017 Location: FinlandPosts: 409 |
Link to a few older posts about Humidity Measurements. Pluto |
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| zeitfest Guru Joined: 31/07/2019 Location: AustraliaPosts: 655 |
Maybe just use a lookup table - kludgy but effective.. data |
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| zeitfest Guru Joined: 31/07/2019 Location: AustraliaPosts: 655 |
A formula Edited 2025-11-28 19:28 by zeitfest |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5648 |
Hi Pascal, Picomite Code for Pascal I seem to have shown you a demo program before. Time flies Volhout PicomiteVGA PETSCII ROBOTS |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 2029 |
Thanks for the answers. @Voltout I vaguely remembered asking before and did a search but missed it, my memory is not what it used to be. Thanks, that's a big help. @TassyJim..I did try a HDC3022 with bad results, I was in a hurry to get the weather station up and running again so used another HTU21. I will have another look at the HDC3022 today. From a quick look at your code it seems like you have to turn the heater on manually, I was thinking of doing it once per week with an interrupt somehow. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6443 |
In your high humidity I would run the heater whenever the humidity has been over 95% for more than 1 day. Or something similar, it is a long time since I have lived in the deep north. Jim VK7JH MMedit |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 2029 |
Jim, I am using the code for HDC3022 as posted but my sensor is reading -45 Deg and 0%, so I can't really do any more with it exept order another one. I'm comming down your way in January for a bit of relief from this weather, I have a granddaughter who lives in Hobart. Just had a look, I paid $30 with postage for this one, too expensive to play with. Edited 2025-11-29 08:24 by palcal "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6443 |
If you want to post it to me, I can have a look at the HDC3022. Having a working unit to compare things with does make life easier. Jim VK7JH MMedit |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 2029 |
Thanks for that Jim, PM me with details. Paul. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 2029 |
I cobbled together a rough wet/dry bulb setup and Voltout's code seems to work well. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 2029 |
Can anyone point me to Psychrometric Tables, in table form not a graph, in Celcius. All I can find are tables in Fahrenheit. I asked AI and got all sorts of rubbish. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2964 |
Here is one I found . Only goes up to 40°C so you may have to extrapolate when it gets hot. relative-humidity-table.zip |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6443 |
This is one I found in '23  Using the formula I used back then, it would be easy enough to produce a table to cover your temperature range. I will give it a go later when I have finished my 'real' jobs for the day. Jim VK7JH MMedit |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 2029 |
Thanks Jim, that's ok for now. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6443 |
Had a break from the shovel rh1.pdf 3 different formula giving slightly different results Was 45 degrees high enough? Jim VK7JH MMedit |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 2029 |
That's great Jim, it's just to test the readings I'm getting from Voltouts code. So far it looks good, been running all afternoon displayin RH at 1 minute intervals and seems accurate. Now to make up the hardware for the sensors, and the water to keep it topped up. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 2029 |
I got my HDC3033 working at last. I had it connected to a PicoW, when I gave up on it I connected a couple of DS18b20's to it and they didn't work either. I then connected the DS18B20 to a Pico2040 and they worked ok. I just connected my DS3022 to a Micromite and it works. Maybe a dud PicoW although it worked with a HTU21D. Just one of those things. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6443 |
That's good news. It will be interesting to compare it with the wet and dry bulb method in your high humidity. VK7JH MMedit |
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