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Thread: Humidity question

  1. #1
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    Default Humidity question

    According to the local university weather report the outside Temp is 17 C and the RH is 90%...according to the gauge in my unheated insulated workshop the Temp is 23C and the RH is 36%....Question is my gauge faulty ?
    I have now placed the same gauge outside to take another reading..I will report back later.
    Last edited by Timbuck; 09-05-2017 at 12:44 AM.
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  2. #2
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    A warm volume of air is able to hold more water than an equal volume of cold air before saturation. So your warmer shop should have a lower RH. I don't know what the formula for interpolating 90% at 17 to 23C, but I'll take a guess that your gauge is ok.

    Nelson

  3. #3
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    To calibrate/check a hygrometer put it in a confined space with a saturated salt solution. Something like a tupperware box with salt and water slurry in the bottom and some sort of shelf for the meter to sit on. It should read 75% +/-1% under those conditions.

  4. #4
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    36% anywhere in the UK today seems like wishful thinking Ken! My insulated heated workshop is registering 23 C and 53% RH ... but the dehumidifier has been running most of the day to keep it there.
    Cheers
    Paul

  5. #5
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    Quote Originally Posted by Wildestcat View Post
    36% anywhere in the UK today seems like wishful thinking Ken! My insulated heated workshop is registering 23 C and 53% RH ... but the dehumidifier has been running most of the day to keep it there.
    yes my gauge seems ok after a while outside..it soon rose up to 79%.. put it back in the workshop and now it's down to 38%. But the temp has gone up a bit...turned out to be quite a nice day in the end , after a lot of rain this morning.
    Last edited by Timbuck; 09-05-2017 at 06:32 AM.
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  6. #6
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    Hygrometer gauges are notorious for being misleading and inaccurate. I read somewhere that to measure true humidity it was necessary to use something call a a "sling hygrometer" or some such sort of medieval device. A device which one spins through the air. I don't think so. I don't have the time. Would make my arms tired. Humid is humid. Dry is dry. And then there is in between. Then there is the psychrometer which is a true Boy Scout lab set up type device. .. Interesting that there was such a change from outside to inside your shop. Maybe it is all that old wood soaking up the moisture?

  7. #7
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    It's a sling psychrometer. I have one. It's just two thermometers, one has the bulb end covered in wet cloth, the other is 'dry'. You don't have to spin it in the air, you can use a vacuum cleaner to pull air across both the bulbs.
    Common household salt isn't the best for testing a hygrometer, it's too high for our range. Potassium carbonate is much better at 43% RH. Ours is a small range - between 35% and say 55% - everything out of these should be of no relevance. So you use the hygrometer like a traffic signal, once it starts to approach 35% RH you better start doing something about it.

  8. #8
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    Quote Originally Posted by sequoia View Post
    Hygrometer gauges are notorious for being misleading and inaccurate. I read somewhere that to measure true humidity it was necessary to use something call a a "sling hygrometer" or some such sort of medieval device. A device which one spins through the air. I don't think so. I don't have the time. Would make my arms tired. Humid is humid. Dry is dry. And then there is in between. Then there is the psychrometer which is a true Boy Scout lab set up type device. .. Interesting that there was such a change from outside to inside your shop. Maybe it is all that old wood soaking up the moisture?
    The shed is lined wth 25mm polystyrene on all four walls plus the ceiling and the window is shuttered and there is a thick fitted carpet on the floor...The RH is nearly almost always just below 40% if I want to increase it I just leave the door open. I've noticed that when I'm heat bending with a water spray it starts to increase pretty quick.
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  9. #9
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    I would be a little suspicious Ken. I have a fully insulated (50 mm kingspan) heated workshop with thick carpets, but I have to run a dehumidifier to maintain humidity between 50% and 55%. Very rare for a day when it doesn't need to run. I have two good quality humidity gauges, and I periodically calibrate them both against a Casella National Physical Laboratory approved aspirated hygrometer (wet/dry bulbs with controlled airflow via clockwork motor & fan).
    40% in the UK on a regular basis, even indoors, sounds too low too me. The Casella was the bargain of the century - I used one for test work in my previous life and knew it cost the company well in excess of 500 ... I got one in mint condition for 35 from eBay .

    This one is a bit more expensive ... http://www.ebay.co.uk/itm/like/22263...7602469&crdt=0
    Last edited by Wildestcat; 09-06-2017 at 06:09 AM.
    Cheers
    Paul

  10. #10
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    Quote Originally Posted by Wildestcat View Post
    I would be a little suspicious Ken. I have a fully insulated (50 mm kingspan) heated workshop with thick carpets, but I have to run a dehumidifier to maintain humidity between 50% and 55%. Very rare for a day when it doesn't need to run. I have two good quality humidity gauges, and I periodically calibrate them both against a Casella National Physical Laboratory approved aspirated hygrometer (wet/dry bulbs with controlled airflow via clockwork motor & fan).
    40% in the UK on a regular basis, even indoors, sounds too low too me. The Casella was the bargain of the century - I used one for test work in my previous life and knew it cost the company well in excess of 500 ... I got one in mint condition for 35 from eBay .

    This one is a bit more expensive ... http://www.ebay.co.uk/itm/like/22263...7602469&crdt=0
    You were right Paul the gauge is duff....I ordered two more gauges...they arrived this morning I set them up alongside my original ..after a while I checked the RH, The original read 39% the two new ones read 59% & 60% the temps were all the same tho'...The original one is now in the bin.
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