Where do I get an ACCURATE humidity meter?

[Tough Guy no. 1265]

I've got a cheap digital humidity meter, and a cheap analog one, and an expensive dehumidifier with a humidistat. Before running the dehumidifier, the humidity was around 70-80% according to all three. I ran the dehumidifier flat out for a couple of days and it removed huge quantities of water, and I can feel the air is dry in my throat. Its humidistat reads 31%, which I can believe - everything is bone dry, no condensation, clothes hung indoors dry rapidly, yet the 2 meters are reading 65%! At various times the 2 meters sometimes agree and sometimes are out by 15-20%. I've calibrated them using the damp salt method to 75%, but they suck when it changes from that. Where can I get one that actually works?

[harry]

xpensive dehumidifier with a humidistat. Before running the dehumidifier, the humidity was around 70-80% according to all three. I ran the dehumidif ier flat out for a couple of days and it removed huge quantities of water, and I can feel the air is dry in my throat. Its humidistat reads 31%, whic h I can believe - everything is bone dry, no condensation, clothes hung ind oors dry rapidly, yet the 2 meters are reading 65%! At various times the 2 meters sometimes agree and sometimes are out by 15-20%. I've calibrated t hem using the damp salt method to 75%, but they suck when it changes from t hat. Where can I get one that actually works?

You probably can't. They are notoriously inaccurate.

[stuart noble]

Will this fit in your Xmas stocking?

[Chris Hogg]

Pine cone? Seaweed? Seriously though, hygrometers can take time to reach equilibrium with the surrounding air and 'instantaneous readings' of a changing humidity can be inaccurate. Have a read of this

. The traditional way of measuring humidity is the wet and dry bulb psychrometer, but perhaps not really appropriate in your situation. But at my last place of work, knowing the humidity of the air in the lab was important for some tests they were doing on paper printability, and not only did they have quite complicated air conditioning to control it, but you frequently saw a technician walking around whirling a little portable psychrometer like a football supporter's rattle, something like this . But anything that is going to be reasonably accurate is going to be costly, like most of those here .

See also

for general information.

[Martin Brown]

The simplest thing that is reasonably accurate is a wet and dry bulb thermometer and a lookup table to convert the difference into RH.

This is a DIY group so two matched thermometers that graph and a small bottle of water and a length of shoe lace as a wick will do it. But you are unlikely to get measurements better than +/-10% this way without using expensive matched thermometers reading to 0.2C or better.

Otherwise expect to pay about £100 for something digital that actually works reliably over a decent range and to within 3% or so.

Basically RH varies rapidly with ambient temperature and the old analogue meters tend to get stuck and are slow to respond.

[rick]

I think you can't get an exact humidity reading, you can get reading from various sources (wet, dry) and then calculate a relative humidity.

That said large clean room & precision engineering rooms have to stringently control humidity - the AC units accurate control humidity - so it must be possible to read electrically or mechanically for control purposes ... but other than the bulb/dry bulb approach I don't know how.

Try Google

[Robin]

That link's cruel to the afflicted! I saw "$4-Tools" and thought "well at that price..." before the "if something's too good to be true..." filter cut in.

[pamela]

How about one of these?

moisture-meters/

thermohygrometers/

Don't forget that most of these devices measure relative humidity. 65% is on the upper limit of comfortable but would still be dry although the actual water content in the air depends on temperature.

I had an analogue hair hygrometer and it seemed more responsive than the digital types and wasn't prohibitively expensive but it seemed to need zeroing quite frequently and that took a bit of time.

[Alan Dawes]

If the sensor for the humidistat is in the body of the dehumidifier it is sensing the humdity of the stream of dehumidified air leaving it not the room as a whole, which explains the difference between its reading and that of the 2 meters that you have placed in the room.

Alan

[Tough Guy no. 1265]

That sounds like the best idea, although I may just rely on the dehumidifier's humidistat, which appears to be showing something sensible. It's operated by presumably a bimetallic strip or some such thing, as like with a thermostat, you can turn it up and down with the power off and it clicks.

[Tough Guy no. 1265]

I don't find humid air uncomfortable, I was drying the air to prevent condensation and mould. I thought only overly dry air was uncomfortable (it causes dry throats or skin in some people).

[Tough Guy no. 1265]

I assume it's in the stream of air BEFORE the dehumidifier, otherwise it's a pointless reading. The dehumidifier needs to know how damp the room is, so it knows whether to turn off yet. How dry the air is coming out of it is completely irrelevant.

[pamela]

I've never had overly dry air with a domestic dehumidifier even if I left it on for most of the day but I suppose it must be possible - espeically if the dehumidifier is used on a cold winter's day when humidity is aleady low.

If you have condensation and mould then your air is starting off damp and although a dehumidifier removes impressive amounts of water, in my experience it doesn't drop humidity down by much more than 20 percentage points. In a small to medium lived-in room (where washing, cleaning, cooking, breathing, etc add a lot of humidity) a regular dehumidifier might struggle to keep up. YMMV.

I may be unusual in not encountering indoor dry air because I've never needed a humidifier to add moisture although I see them widely sold.

[Tough Guy no. 1265]

Depends how powerful a unit you've got, this one is removing 2 litres a day. The condensation just vanished. I'm in Scotland and it's rained nearly every day for the last 2 months, so it's very damp. The house is 17C. If its dial is to be believed, it reduced 75% to 31%, and it certainly feels very dry now.

[newshound]

That would be my suggestion too. You could always get a non-whirling one.

[Tough Guy no. 1265]

Nice price. Ordered.

So what advantage would the whirling have given me?

[tabbypurr]

With the common plastic strip mechanical types it goes on the output side. The dehumidifier function thus gets it to move much further, so it moves far enough to switch the thing off.

Once the dehumidifier stops then its exposed to damp room air again, and eventually switches back on.

NT

[Chris Hogg]

On Tue, 08 Dec 2015 23:15:10 -0000, "Tough Guy no. 1265"

Whirling ones reach equilibrium more quickly, and because you can walk around in the space you're testing, whirling as you go, they potentially sample a larger volume of air and so are more representative of the whole.

To get a reliable reading with any hygrometer, you really need to take repeat measurements until they become stable. But I imagine that will be more effort that you're prepared to put in for the importance of the result you want (it would be for me!).

[harry]

Re air conditioning. The really accurate ones knock all water out of the air by chilling and then add a measured amount afterwards.

[Tough Guy no. 1265]

That would be as stupid as having a thermostat on a convector heater above the heating element.

Mine is definitely not, as it clicks at the same point when it's running as if I switch it off for an hour.