Hi Andy,
The humidity can vary quite a lot depending on wind/rain/rivers etc. So it would be difficult to give a local result that is meaningful for a large area. The one I have takes minutes to respond to any cooking in the kitchen. The highest reading I have ever seen is just over 70% so maybe it's not that accurate. I am not sure how to get a known humidity in a room so it can be calibrated/checked.

Richard

> I am not sure how to get a known humidity in a room

Not convenient on the scale of a room but in a suitable container you can easily generate a known humidity above a saturated salt solution. I haven't tried it myself recently and I can't remember the details exactly but was able to google these examples/refs which look like the right kind of thing so they might help:-


http://ambientweather.wikispaces.com/Humidity+Calibration,+Saturated+Salt+Test

http://exoticpets.about.com/od/herpresources/ss/hygrometer.htm

http://www.omega.com/temperature/z/pdf/z103.pdf

(that last document looks particularly handy - I have saved myself a copy for 'lab' use)

However, you should know that cheap electronic hygrometers are pretty inaccurate near the extremes of the RH range so certainly don't expect accurate readings much below 20% or above 80%

When I looked your RH was 56%, mine was 90%

Posted By: SprocketHowever, you should know that cheap electronic hygrometers are pretty inaccurate near the extremes of the RH range so certainly don't expect accurate readings much below 20% or above 80%

Based on my experience with weather station equipment (Davis, Oregon & 1-wire) even within those limitations RH units seem to fail[1] sooner than temp sensors etc., especially so if regularly used in very humid conditions. I guess that's maybe inherent - after all you can't fully waterproof a humidity sensor!


[1] As in become even more unreliable.

I have both expensive ones made by renowned instrument makers and very cheap ones off Ebay. The latter seem to work pretty well. In the British Isles you are unlikely to get a reading below 50% unless in an air-conditioned place. Ambient RH can change rapidly - either because air temperature is changing or rain fronts are approaching.

You don't state why you need to know RH. if it's protection of possessions eg furniture and clothes against damp/mould, you should be safe if RH is not above 70% for prolonged periods - say 5days +.

If you are trying to control RH in an enclosed space, a dehumidifier is by far cheaper than heating.

If you have a IR thermometer, and a bit of dew on a window, you can do it:

Measure the room temp, and the temp of the window on the edge of the dew - right where its starting to form. Wiki can convert the two numbers for you into a relative humidity:

http://en.wikipedia.org/wiki/Dew_point

This technique is what some pricey RH meters use- they point a laser at a peltier chilled mirror and the light back indicates the onset of the dew-point.

Posted By: RobL: “If you have a IR thermometer, and a bit of dew on a window, you can do it: …”

Clever. Only really works as the dew is forming - might be a bit misleading if the water's there but not yet evaporated as the window warms up or something.

Still, if you have an IR thermometer you ought to be able to use any old damp rag as a wet-bulb hygrometer:

https://en.wikipedia.org/wiki/Wet_bulb

Have to admit I'd have to read that carefully and do some head scratching with a psychrometric chart to work out what the humidity is.

https://en.wikipedia.org/wiki/Psychrometric_chart#Psychrometric_charts

Posted By: andyman99Essentially looking to understand humidity levels in various parts of the house before deciding how to control it.

At the risk of stating the obvious, if it's indoor humidity in your own house you're interested in then seeing other peoples figures, no matter how current, isn't going to help.

Thanks Ed, excellent idea, will be near Twickenham this Sunday, so will give it a go.

Mark are these he HTC-1 devices I have read about on other threads.

Thanks Andy

The level, of humidity indoors, especially in a standard modern house, is mainly the product of the occupiers. Moisture from their breath but vastly more from their lifestyles: showers, cooking etc. Plus extreme stupidity of drying clothes on radiator/by open fires/airing cupboards and employing portable gas heaters! So it can fluctuate very rapidly, leading to condensation and mould on cold surfaces. There is some influence from outside humidity levels, but less than you think. This is mainly because modern houses are very much sealed units, and windows only opened in hot (dry) weather in summer.

Posted By: dazdreadIt reads as between 55 and 65% which I take it is within norms, could it be that the house interior is being kept warmer due to our three year old and 4 month old children which encourages condensation on the solid walls?

Yes, 55-65% is perfectly normal. What would be interesting to know is the temperature as well. Both of the room where you are measuring the humidity, and of the wall or wherever you're finding mould.

EWI is on the to do list when the money gods answer our prayers!

That's the best idea to minimise energy use but you might find that some of the thin internal insulation (insulated wallpaper type of stuff) is enough to lift the internal surface temperature of the walls enough to stop mould.