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    • CommentAuthorandyman99
    • CommentTimeMar 4th 2014
     
    Is it possible to get current humidity level data? On the met office site, if you select UK observations, then it doesn't appear to have a humidity column. If you select forecast then it does, but I presume that is just an educated guess. I want to see if any of my devices that measure humidity are accurate, as they give vastly differing readings. Otherwise any ideas about how to calibrate to a rough level?
  1.  
    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
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 4th 2014
     
    Find a local WeatherUnderground station as they usually have RH
    • CommentAuthorSprocket
    • CommentTimeMar 4th 2014 edited
     
    > 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%
    • CommentAuthorPeterStarck
    • CommentTimeMar 4th 2014 edited
     
    I use this site which has the current humidity and pressure readings for any UK area using the closest weather station which for me is Manston.

    http://www.bbc.co.uk/weather/6695249
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 4th 2014
     
    When I looked your RH was 56%, mine was 90%
  2.  
    It's been dry here for a couple of days, sunny all morning.
  3.  
    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.
    • CommentAuthorbarney
    • CommentTimeMar 4th 2014
     
    Just get a sling psychrometer and determine RH from that ?

    Regards

    Barney
  4.  
    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.
    • CommentAuthorandyman99
    • CommentTimeMar 4th 2014
     
    Thanks for all the comments, currently having broadband and now phone problems, so will follow links when restored. Essentially looking to understand humidity levels in various parts of the house before deciding how to control it. The 3 devices I have read 20% (obviously broken), 70% and 90% (hopefully inaccurate), I thought if I could get a close "official" reading and try all the devices outside, I might have 1 that was close enough, but I guess I this is not going to be possible. Will have to read up and experiment! Thanks Andy
    • CommentAuthorRobL
    • CommentTimeMar 4th 2014
     
    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.
    • CommentAuthorborpin
    • CommentTimeMar 4th 2014 edited
     
    Maplin have these http://www.maplin.co.uk/p/black-usb-wireless-touchscreen-weather-forecaster-n96gy give you indoor & Outdoor temp and hum (plus some other bits.

    Only £50 and may get one as my wind and rain has stopped working.

    Edit, if you want to get the data into a RPi use pywws.
    • CommentAuthorEd Davies
    • CommentTimeMar 4th 2014
     
    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
    • CommentAuthorcar-mark
    • CommentTimeMar 5th 2014
     
    Hi there

    I bought 3 elcheapo items from ebay, usual month to deliver. Total cost £12
    All read %humidity to within 1 digit of each other, all approach 90% plus in prolonged rainy conditions in outdoor conditions. all approach 95% in shower/wet-room until the fan has coped with it.
    What gets interesting is the heated rooms and how low the humidity can go.

    Yes I did use the wet bulb dry bulb calculations and these cheap beasts are still within 5 digits at the ends of scale

    Yours Mark
  5.  
    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.
    • CommentAuthorEd Davies
    • CommentTimeMar 5th 2014
     
    I think he wants to check the calibration by taking the sensors outside on days with different, but known, humidities.

    With mobile broadband, the most accurate scheme would probably be to park near an airport and get the METAR for that. Without mobile broadband, get an accomplice to log the METARs as they're produced then compare afterwards. Here's Heathrow recently:

    http://www.wunderground.com/Aviation/index.html?query=EGLL#METAR

    METAR EGLL 051050Z 22005KT 190V260 CAVOK 10/03 Q1021 NOSIG

    Lots of explanations around the net of the coding but the key bit is 10/03 meaning temperature 10 °C, dew point 3 °C.
    • CommentAuthorandyman99
    • CommentTimeMar 5th 2014
     
    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
    • CommentAuthorcar-mark
    • CommentTimeMar 6th 2014
     
    Hi there

    Yes Andy the very same, very happy with the figures.

    Warning it does take time to understand how the humidity changes with ventilation and heating.

    yours Mark
  6.  
    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.
    • CommentAuthordazdread
    • CommentTimeMar 6th 2014
     
    This is excellent stuff, I just invested in El Cheapo from eBay to as our house has suffered terribly over the last two years from mould and I have not changed anything that might have altered air flow in that time... apart from a Door Stop door 6 months ago.

    It 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?

    EWI is on the to do list when the money gods answer our prayers!
    •  
      CommentAuthordjh
    • CommentTimeMar 6th 2014
     
    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.
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