Demystifying dehumidity

[RogerS]

I bought a dehumidifer to run in the background in my workshop. It's not a particularly damp workshop but having now quite a bit of cast iron plus bringing inside my wood stock I felt that I needed a bit of belt and braces.

So I bought one of the Amcor jobbies quoted at 22 litres a day..which seemed reasonable to me. Then I read the small print...22 litres at 32 degrees C and 90% ambient humidity. What? Bit of a con, I thought. Anyway, fired it up and I do regularly extract 7 litres of water out of the air over 10 hours maybe...

Now here's where I am confused. I have a separate humidity meter on top of the wood pile. The humidity swings about wildly...70% some days then goes down to about 55% (with the dehumidifier running). Then I noticed that the other day it was down at 44% without the dehumdifier and so can only assume that it is reflecting the ambient air humidity from outside? It was a warm day in the workshop (21 degrees). The workshop is fairly well sealed to the outisde elements.

So am I wasting my time with the dehumidifier?

 

[mel]

hi roger
my only thoughts on this are
1 , is the dehumidifier big enough to cope with the square footage of the workshop ???
possibly this would affect the erractic readings of the humidity meter
2 , the workshop would need an airlock for the dehumidifier to stand a chance of working properly . not letting through damp air into the workshop everytime you open the door
dont know if this helps
mel

 

[user 2070]

Hi Roger,

In answer to the question: If the dehumidifier is removing 7litres in 10 hours, it is working FOR you, but it may not be enough.

The reason you get varied results from your moisture meter is perhaps as follows.

Even if you have a fully hermetic seal on the workshop a moisture meter will (correctly) report different results due simply to changes in air temperature.

The humidity meter is reporting "relative humidity". This is the quantity (mass) of water vapour in the air expressed as a percentage of the MAXIMUM that air at that temperature could hold. (Saturated)

However, the quantity of water vapour that air can support varies wildy with temperature. So, if you change the air temperature the meter will (correctly) report a different value (without changing the air/water mix at all).

Keeping the doors shut and sealing the floors and walls would probably help control the damp, but keep the dehumidifier running anyway. It is simplest to compare only results which are taken at the same air temperature.

For the cast-iron, your target would probably be to prevent the moisture condensing on the (cold) metal, so keeping the workshop (cast-iron) from going below (say) 5C would make the target for relative humidity (approx <35%@20C) easier for your dehumidifier to achieve.

In practice, a dehumidifier alone may not be enough if you allow the workshop temperature to drop too low. Dust sheets and, over winter, patent sprays tend to work well.

Simon

 

[RogerS]

Thanks Simon for the explanation. I've got an old Blacks Mountain tent made from ventile...a breathable waterproof fabric. Reckon I may sacrifice that as I've not used it in...ooh..25 years!

 

[user 2070]

Hi Roger,

The breathable fabric might be the opposite of what you want for the workshop. I bought some heavy cotton protective sheets from Axminster, with sewn-in magnets a few years ago.

25 years! Could this be because even Hotel beds are more comfortable?

Simon

 

[ivan]

Roger,
As air warms up, it can hold more water as vapour. So if a room is sealed, just turning on a heater will lower the relative humidity. Similarly, if the temperature falls in the same sealed room, the relative humidity (RH) rises. With more cooling, eventually the air in the room will get to 100% RH and further cooling still will result in condensation (if outdoors, dew).

Generally, I gather that you risk damage to tools (from unexpectected downward temperature shifts bringing condensation) if the RH is much over 70%. My dehumidifier (cheapie from Wickes) can maintain 60% in an unheated insulated cavity wall double garage attached to house. This has keept all rust at bay for over 5 years. Cheap dial hygrometers are notoriously inaccurate, but if you target 60% RH whatever the temerature, tools should remain pristine. At times the 5l water container will need daily emptying, mostly 2 or 3 x a week.

Never heat with open flame, like one of those bottle gas heaters - burning a gallon of gas (or oil) releases 5 gallons of water into the air as water vapour.

In a workshop where doors are often opened and heating might be an afterthought, the RH can vary a lot with both weather and time of year (seasons). The dehumidifier runs a lot in wet Devon springs and autumns, less in colder wet winters (cold air has less moisture in it and RH falls when the air gets warmed inside the garage, from heat esacping from the house).

Covering up machines (apart from keeping unsused ones clean) helps to stabilise their temperature. If a machine is in an uninsulated wooden shed it can get very cold at 3am. if any warmer air later touches it whilst it's still cold you may get condensation. Outdoors you see this effect on cars left out overnight. Or on my steamed up glasses when I walk into the pub on a chilly winter's night. Covering the machine helps to stop it getting cold enough to trigger condensation later (you hope, or the cover gets damp and promotes rusting.....)

Dehumidifiers have to quote temperatures in their specs because in a hot room at 70% RH there is much more water in the air than in a cold room at the same 70% RH, . Thus dehumidifiers can extract water fastest from warmer air.

Hope this helps

 

[engineer one]

not sure about the dehumidifiers roger, but would remind you to consider some of the products offered to the motorcycle and car restoration business to control growth of the red devil.

i think that it is true that just moving through the workshop affects the humidity, and anything you can do to take some out of the air has to be a good thing, particularly if you are storing wood for a time.

one thing i learnt from my mum was to put a small container of water in the room if using a gas fire, since it seems to take moisture out of the air due to combustion, otherwise you need to keep the windows open.

i think as far as the wood is concerned, you just need to work on it in an environment which is relatively stable during the time you work the wood.

paul :wink:

 

[ivan]

Sorry to disagree with Mum, but any water with an open surface to the air in the room will add to the humidity by evaporating. It will evaporate more quickly the bigger the surface area exposed. Bodies are a good souce of water vapour from lungs and skin, so your stuff will last longer if you don't go into the workshop....

PS. I assume your dehumidifier has a 'variable dryness' control setting, to let you set a target level of drying out, and not just 'on-off'?

 

[Nick W]

... unless the water is cold, in which case it will act as a condensing surface for the water vapour in the air. :? :?:

I haven't done the maths (and I'm not going to), but there will be a point at which condensation will exceed evaporation, and I would expect that to be somewhere below ambient temp, so the water would need to be kept significantly cooler than ambient temperature for the water to act as a dryer. Perhpas best if kept at the same temp as ... oh I dont know, a dehumdifier's condensing coils maybe? :lol:

 

[engineer one]

sorry ivan i did actually mean what you said.
gas fires tend to remove moisture, which is why so many people
have breathing problems when keeping warm.

i think that the dehumidifier helps the overall atmosphere in the workshop,
whilst the wood tends to get affected by the time you are not in the shop.
paul :wink:

 

[Nick W]

Paul,

Any non-vented flame in a room will increase the humidity by producing water as one of the products of combustion. It will also make breathing more difficult by removing oxygen from the air.

A vented flame on the other hand, should have the sole effect of increasing the temperature (assuming it gets its fresh air from outside the room), which will have the parallell effect of reducing the relative humidity. The 'dryer' air will then cause some people breating problems.

EDIT: Of course the situation is more complicated for most gas fires, which vent out through the chimney but use ambient air from the room, which is then replaced by air from outside in the form of draughts. The resulting relative humidity then depends as much on outside conditions as anyting else.

 

[engineer one]

and i would love to be able to still learn :lol: :twisted: :twisted:

it is nice to be able to get such a wide range of knowledge

paul :wink:

 

[ivan]

Gas actually produces moisture when it burns. If it's a fixed fire, this goes up the chimney. If it's a portable calor wheel-about type fire, one litre of burned liquid gas releases 5l of water into the room air which can condense if the air cools down again.

In winter, any form of heating, (except bottle gas above, and I'm not doing the calcs for that either!) will definitely 'dry out the air', not because it removes any moisture, but because it reduces relative* humitiy(see post above). If the RH is low moisture evaporates more easily from the mucous membranes (mouth and lungs) and you say 'cor this air's dry'. Bodies are designed for outdoors air, which has an average RH of around 70%. Turn up the heating in winter and you'll be living in 40%RH and SWMBO will be complaing of cracked lips. Some people buy a humidifier to increase RH when the heating is on. If you turn the heating off, the temp. drops and the same air will now have a higher RH. This will not seem so dry, yet the actual amount of water in the air is just the same.

None of this is likely to be directly relevant to the workshop unless it's full of radiators or portable gas heaters. Aim for somewhere in the range of 50-60% RH and tools will stay rust free.

By the way, wood stored at 50-60% RH will have an equilibrium moisture content of around 10%. Filling the workshop with a lot of wet wood will get the dehumidifier working overtime...

* It may help to think of RH as a measuremet that tells you how easily moisture is going to evaporate or condense (low RH, fast evaporation: very high RH, condensation likely, evaporation vrey slow) It is not a useful guide to the quantity of actual water there is in the air, because this is very temperature dependent.

 

[Stoday]

You can download a free psychrometric chart here: http://www.linric.com/free4me.htm as well as notes on how to use it.

The chart gives the relationship between temperature, relative humidity and amount of water in the air. For example it shows that cold saturated winter air, if warmed from 40 deg F to 75 will reduce its RH from 100% to 30%.

 

[ivan]

Thanks for the link, Stoday, printed it out for future ref.

 

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[SVB]

So - my problem is that my garage is underground (well, cut into the side of a valley) but is not heated. I have a problem with water condensing on the ceiling (which supports the garden on top) and the inside face of the insulated roller door. This also has the effect of turning anything left in there a bit damp.

I have recently installed a dehumidifier but was surprised that after 24hrs running almost no water had been collected. Is this because
a) the garage is cold therefore the moisture will not condense in the dehumidifier
b) the dehumidifier is broken
c) ...............

Any thought appreciated.

 

[Lord Nibbo]

So - my problem is that my garage is underground (well, cut into the side of a valley) but is not heated. I have a problem with water condensing on the ceiling (which supports the garden on top) and the inside face of the insulated roller door. This also has the effect of turning anything left in there a bit damp.

I have recently installed a dehumidifier but was surprised that after 24hrs running almost no water had been collected. Is this because
a) the garage is cold therefore the moisture will not condense in the dehumidifier
b) the dehumidifier is broken
c) ...............

Any thought appreciated.

I would suspect the dehumidifier as not working, I've got one on my boat and it works right through the winter ok and it gets a lot colder than it is at present.