Ronald Raygun wrote:

Terry Fields wrote:

Very approximately, the number of grams of water per cubic metre in
saturated air is the numerical equivalent of the temperatu at 15
degC the air can hold 15g of water per cubic metre.

Let's say your shed has small swings in temperature, from say 6 degC
overnight to 10 degC during the day. If the humidity remained constant
at 50 percent, then even if the machinery remained at 6 degC because
it warmed up slowly, there would be no condensation as then the dew
point at 10 degC and 50 percent (relative) humidity is 5 degC:

The air is 50 percent saturated (50 percent RH at 10 degC); and 50
percent of 10 grams (saturated air at 10 degC) is 5 grams. but 5 grams
is enough to saturate air at 5 degC, and so 5 degC is the dew-point.

If, however, the temperature swing was from 6 degC overnight to 13
degC all at 50 percent RH, then the dewpoint would be 6.5 degC and
condensation would take place.

How do you justify your assumptions?

Suppose your shed begins its day at 6 degC and is full of 50% RH air
(and therefore contains about 3g of water per m3), and suppose further
that by the early afternoon it warms up to 13 degC. This warming
up is going to reduce the RH of the shed air (to about 23%), and then
by the time it cools down to 6 degC again, the RH will have gone back
up to 50% (and hence still be non-condensing).

The only way to increase the 13 degC RH to 50% would be to add water
to the shed air. Where is this going to come from?

OK, so there could have been a change in the weather, increasing
the outside RH, and if the shed has been in use, with doors/windows
wide open, then the shed air would become well enough mixed with
outside air to be indistinguishable from it. But if the shed has
remained closed, and its structure and windows/doors are reasonably
well sealed, the absolute (not relative) humidity is going to remain
constant at 3g/m3.

I thought humidity worked like this until I bought a weather station
having an external temperature and humidity sensor. I found that the
humidity stays remarkable constant.

There is reason to suppose this is the case, as other postings in this
thread have shown concerning placing tools in mighty wooden cases
acting effectively as a humidity sink. Wooden floors, benches, cloth
items, and other furniture will add to this. But this works two ways,
if the humidity starts to fall as the temperature rises, the items
acting as a humidity sink will start to lower their water content
towards the new equlibrium value and tend keep the shed atmosphere in
equilibrium.

And the presence of a human being in the shed is going to be a
disaster. Let's say it's the size of a garage, 2m x 2m x 6m or 24 cu
metres. At 50 percent RH and 12 degC (robgraham's figure) this
garage/shed will hold 6 x 24 = 144 grams of water. Two hour's work by
an occupant will add about another 100 grams (figures from a
scientific paper) from respiration and sweat, for a total of 244g or
just over 10 g per cubic metre. The temperature of the shed will only
need to fall to 10 degC, a mere 2 degrees, before reaching the dew
point, if the items acting as a humidity sink don't absorb the surplus
fast enough. Three hour's work and there's every chance of
condensation, unless all that wood can take up the increase.

The only type of shed that won't have items acting as a humidity sink
will be metal or glass ones, and they are notorious for condensation
problems.

TF