Green Building Forum - In situ RH sensors

In situ RH sensors

piersadler — Thu, 09 Feb 2012 13:46:22 +0000

Can anyone suggest an in situ humidity sensor and data logger for putting into my wall and underfloor space. I want to assess the effects of external wall insulation (and repair of defective render on my wall.

In situ RH sensors

Robur — Thu, 09 Feb 2012 13:57:59 +0000

If you like tinkering then the Seeedstudio Stalker is worth a look - basically a data logging Arduino. There are other Arduino solutions too including nanode and various shields. You can then take your choice of RH/Temp sensors for the job in hand.

For a plug and play solution take a look at TinyTag.

In situ RH sensors

SteamyTea — Thu, 09 Feb 2012 14:01:19 +0000

Go for an iButton and holder, then you can read it without removing it. About 50 quid.

In situ RH sensors

JSHarris — Thu, 09 Feb 2012 15:19:06 +0000

As I'm currently finding out, RH sensing isn't as straightforward as it seems, unless you want to spend a lot of money on sensors.

I've bought three different sensors so far. One reads RH accurately (it's laser trimmed and calibrated) as a voltage, which is fine except it won't drive long cables directly; it costs around Â£20. One will drive long cables and outputs a frequency proportional to RH, but needs some calibration constants read out from a chip before first use (offset and gain, in essence) and these then need to be accounted for in the logger somewhere; it costs around Â£17. Finally I've been looking at using raw, non-calibrated capacitive RH sensors and making up tiny boards to read the capacitance change, perform the required calibration calculations and output RH in a meaningful and accurate data steam that will work over long cables. The raw sensors are less than Â£5, but the small ucontroller to do the calibration and data transmission stuff is another Â£2.50 or so, plus I've only started playing with it this afternoon, so I'm not absolutely certain I can get it to work properly.

The latter looks as if it might work, and accurately calibrating RH sensors looks easy enough. 0% can be obtained by sticking the sensor in the space above a sealed jar of good desiccant and 75% RH can be obtained by doing the same except using a thin layer of moist salt in place of the desiccant in the jar.

As for data logging, then as others have suggested there is a wealth of DIY type solutions around but they all need a fair bit of work to get them to do something useful. The logger I've been working on will record RH as well as temperature, and is in this thread: http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=8133&page=2#Item_4

There is a dearth of affordable logging and sensing units around, AFAICS, unless you don't mind paying out Â£100 or more for a setup, or unless you only want to monitor a single point.

In situ RH sensors

piersadler — Thu, 16 Feb 2012 15:25:34 +0000

Thanks for suggestions. I've looked at other threads too. Much as I'd like to go for a bit of a DIY multi-channel system, I think it would be beyond me, so i shall go for ibuttons. I didn't realise you could do RH with them as well as temp.

SteamyTea you mentioned a holder - what's it like? Would I be able to place the ibutton in a drill hole and withdraw it when I wanted to download with such a holder or will I have to make something for this/

In situ RH sensors

SteamyTea — Thu, 16 Feb 2012 15:29:15 +0000

Never used the holder, look at the www.homechip.co.uk website. You can just try and attached 2 wires to it and then attach them (the right way around) to the reader. Works for me

In situ RH sensors

djh — Fri, 17 Feb 2012 14:27:53 +0000

Just in case somebody may say "a-ha", there's another cheap way of measuring RH. You start by understanding that the moisture level in a piece of wood is directly related to the RH of the air around it (obviously with a very slow averaging time constant). Then add that there are tables that will convert the wood moisture to RH or vice-versa for each particular species (see American or Australian forest products organizations, for example). Finally add that moisture level in wood can be measured by resistance measurements to fairly good accuracy (stick two pins in a piece of wood and measure resistance in between).

And that's the basis of a cheap system. Some small bits of wood, twice as many pins, some lengths of two-core wire and a resistance meter/sensor.

In situ RH sensors

SteamyTea — Fri, 17 Feb 2012 16:02:32 +0000

May give that a go and compare it to my other sensors. I like tabletop science.

In situ RH sensors

JSHarris — Fri, 17 Feb 2012 16:19:13 +0000

Posted By: SteamyTeaMay give that a go and compare it to my other sensors. I like tabletop science.

If you want really simple RH sensors then just hang bits of seaweed around the house and give them a feel every now and again to see how soggy they are..............

In situ RH sensors

SteamyTea — Fri, 17 Feb 2012 16:21:07 +0000

Then wash fingers for an hour.
Off to the beach now.

In situ RH sensors

slowbuilding — Sun, 18 May 2014 15:13:23 +0100

Piers - I'm guessing you may have had a lot more experience with sensors since this thread? I'd love to pick your brains over this, as I'd like to have sensors in my IWI and my solid floor, but have no clue where to start and definitely don't feel up to DIY on this, since I barely grasp what they do let alone how they work and how one would build your own!