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Which Hygrometer

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Charlie Woody

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Just being reading about rust prevention on another thread and the use of a Hygrometer was recommended.

So just had a look on Google shopping and fleabay and am totally confused by the range of different types etc.

Can anyone tell recommend a make & model please for use in a reasonably insulated workshop set in a damp environment (Glorious Devon) please?
 

Lord Kitchener

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Human hair hygrometers are generally pretty good. Just buy the cheapest that is likely to be reasonably reliable, then see what it reads in your living room, then take it to the workshop and compare it. The exact numbers don't really matter, it's really for comparison.
 

MIGNAL

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If you want reasonably accurate readings then it's probably best to buy a thermometer with half degree readings. Use it as a sling psychrometer but with the aid of a vacuum cleaner to pull the air across it. Cost is very little, accuracy is high.
Make and use the sling psychrometer to calibrate one of these:




Which are surprisingly (or perhaps not) accurate. It works on a similar principle to the bi metallic strip, except we are using 2 types and 'cut' of wood (end grain and long grain) that react to humidity at different rates. Low tech they may be but they work extremely well. Somewhere on this site is a thread of someone making one. Apologies but I can't remember his user name.
 

Sawyer

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Bruce Hoadley in 'Understanding Wood' suggests a home-made balance with an oven-dried piece of wood as the weight. As the wood absorbs or loses moisture from the air the weight change shows on the balance needle. Ingenious and very cheap. Sorry, can't post any pictures, but it's on pg. 145 of this very good book.
 

Argus

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Measuring humidity is a little involved but not complex.

Direct-reading instruments calibrated in RH% are good enough for an indication +/- a few percent, but the most accurate measurement (very expensive calibrated instruments apart) are done with two ordinary identical thermometers, side by side, but not touching.

As mentioned above, you may use a Sling Psychrometer But it works like a football rattle where you have to whirl it around in the air to be measured. Not necessarily convenient in a kiln full of wood. They do come up from time to time on Fleabay.

However you can make your own with two thermometers, a piece of wood, some absorbent wick and an old film canister, if you don’t need to use the sling type.

The sensing bulb of one is bare, the other is covered with a clean, close fitting cloth wick the end of which is immersed in a container of DISTILLED water so that it is constantly wet. Don't use tap water.
Draw a constant stream of the air sample across the bulbs.

You are aiming for the two simultaneous temperatures which will be different. Termed the dry bulb and wet bulb temperatures. (DB & WB).

Next surf the web for a Psychrometric Chart. This is a graph which is a map of all the properties of air and will enable you to plot the DB and WB temperatures and read off the relative humidity. The same chart will also provide the absolute humidity of the air sample and provide you with the amount of water removed over time, taken from repeated readings provided that the air quantity remains the same and you know the volumes of air involved.

This method is very accurate.


Hope it helps.

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MIGNAL

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You can do it with one thermometer. Take the dry reading first, then use the same thermometer to take the wet reading.
The vacuum cleaner idea is good because it will move enough air across the bulb. We commonly drill a 1 1/2 " hole in a piece of wood around 1 " thick. The bulb of the thermometer is mounted across one side of the hole and the hose of the vacuum is pushed against the other side of the hole, drawing enough air across the bulb. Works much better or rather easier than a whirling psychrometer. In fact I no longer use my sling psychrometer as intended, I just stick the vacuum hose against the plastic mounting where the thermometer bulbs are.
 

Fred Page

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I have a little electronic thing from NScessity. It cost only a few pounds and serves me well in watching humidity (and temp) around a piano. I only panic when humidity drops to under 40%.
Exactly of what use it wood be in a workshop I'm not sure. You don't sound as though you are drying wood? Are you attempting to relate such information as to the possibility of condensation and therefore potential rusting of tools etc?
 

Charlie Woody

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Hi Fred
No I am not trying to dry wood ..... but as it is a workshop there is wood stored there. So I don't want it to get wet.

As you guessed my main concern is the potential rusting of tools.

So I think I need something that will indicate to me if humidity is rising so I can put the dehumidifier on.
 

Argus

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The humidity of a particular space is not any indication of whether tools will rust or not.

If surface rust is your concern, go back to measuring the relative humidity; then you need to ensure that the temperature of the vulnerable ironmongery is above the dew-point temperature of whatever RH prevails in your workshop.

Heat or a moisture proof barrier is the answer, not moisture removal, because if the dew-point temperature of the surface of your tools is lower than the dew-point (100% RH), you will get condensation, then rust

(Dew-point temperature is the temperature at which the air at a given temperature is saturated i.e. it cannot absorb any more vapour).

In other words, the RH is 100%.
As temperature rises, it is capable of absorbing and holding more moisture; as it falls the RH increases for no change in the moisture content.


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