condensation

[DMBParticipant @dmb]

My findings over a long period of time, are that sheds don't normally have waterproof wall covering, only the roof. So I have cut large squares of felt nailed and glued to the walls and door so whole outside is waterproofed, which drastically cuts the damp. Insides of the walls and roof insulated to keep cold out, warmth in. Constant low heat stops large changes of temperature and air holds any moisture instead of dumping water content on metal objects. Not been a fortune in outlay but worth it to avoid spoiling tools, machinery, work in progress projects. OK, so some heat escapes when door opened to get in or out but warmer air takes its moisture with it. I have little rust problem. Some tools and materials kept in the house. I use plastic boxes to store small tools and pieces of steel with VPI paper. Tinplate tins and boxes only used to store brass, copper ali, stainless. I have acquired a GHT bending rolls which is very shiny, wonder if it's made from stainless? It's stored in a small metal case with plenty of VPI paper, just in case….

[Howard LewisParticipant @howardlewis46836]

The temperature determines how much moisture the air can hold. If the temperature drops, and the air is saturated with moisture, condensation wall occur.

Even with electric heating, if a mammal is present, the air exhaled will contain moisture.

Moist air is heavier than dry (Church walls are often dehumidified by inserting unglazed ceramic tubes, with the open end, outside,slanting downwards, to get rid of the heavier moist air)

In our case, ventilation, even a little helps..

Insulation is essential. At best it conserves heat, at worst it reduces the rate of temperature change, and the risk of condensation.

My old shop was unheated and unventilated, and in winter, the oil on the Myford would be grey as it emulsified with the moisture in the air.

My present shop in East Anglia, UK, is relatively small, but is well insulated (50mm of glassfibre between 19mm exterior cladding, and 12mm interior, on all walls, ditto for the roof ) There are two 25mm diameter fixed vents at floor level and a 6" fan at high level (With a rainproof hood outside ) The intake fan is very rarely used., since the door is often kept open.

In times of frost, a 60 watt tubular heater, under a steel bench, is switched on. After a couple of days, the bench no longer feels cold.

Rust is not a problem, VERY rare..

If I am concerned about any measuring equipment, I warm a small tube of Lanolin (To make it runny and easier to spread ) in hot water and coat the metal with it.

Howard

[SillyOldDufferModerator @sillyoldduffer]

Posted by Nick Chase 1 on 13/04/2022 19:18:29:

as i understand it , relative humidity is related to air temperature

when its warm the air can hold a lot of water vapour, so 90% humidity might hold a cup full in a sq meter of air

when its cold the air cant hold so much so then 90% humidity might only mean a thimble full in a sq meter

so you could try opening the doors in the morning when the air outside is cold and only contains a little bit of moisture and then you replace out the warmer air inside and the extra water it holds with the dryer cold air

that way you will gradually dehumidify the workshop for free?

Can be done, though not as simple as just measuring the temperature. You need to ventilate whenever the Relative Humidity inside is higher than the Relative Humidity outside. Quite a good Arduino project using two DHT22 sensors, a relay module and an extractor fan!

In many workshops the humidity is almost always higher inside than out, making permanently on ventilation a good idea. Works because workshops contain wet processes like sweaty people breathing, drinking tea, splashing suds and playing with naked flames – blowlamps etc. Not automatically true though, I tend to pop in and out of my well-insulated part-of-the-house workshop rather than spend several hours in it at a time and I rarely use flames or suds. Quite different from Mr Keen brazing boilers all day in a basic wooden shed.

Dave

[Dave HalfordParticipant @davehalford22513]

Posted by DMB on 13/04/2022 14:28:50:

When bulb supply runs out, may well get small heater. They're made for pianos, would you believe? About (15W?) Aquarium s, Vivarium s, plants as in greenhouses, soil heaters. Just need to look around to gain knowledge of different designs and wattages.

Don't try an Aquarium heater, they burn out if used in air.

[duncan webster 1Participant @duncanwebster1]

Posted by Howard Lewis on 14/04/2022 10:16:50:

……

Moist air is heavier than dry……

Howard

No it isn't, air has a molecular weight of just over 28 (mostly nitrogen), water vapour has a mecular weight of 18, so at the same temperature and pressure, moist air is lighter

[CalumParticipant @calumgalleitch87969]

I had damp problems in my little workshop (an old stone farm building) and I seem to have mitigated them pretty effectively with an electric fan heater attached to a plug-in thermostat. It is set to run when the temperature drops to 6 degrees and stop running when it rises to 7. The fan heater itself is adjusted as low as it can go and I think runs at a few hundred watts when it is on, which as far as I can tell isn't much, especially now the temperatures here have risen a bit. The workshop feels dry to the touch now, bits of paper left in it don't feel damp and greasy.

[SillyOldDufferModerator @sillyoldduffer]

Posted by duncan webster on 14/04/2022 12:19:35:

Posted by Howard Lewis on 14/04/2022 10:16:50:

……

Moist air is heavier than dry……

Howard

No it isn't, air has a molecular weight of just over 28 (mostly nitrogen), water vapour has a mecular weight of 18, so at the same temperature and pressure, moist air is lighter

I got this the wrong way round in a magazine article and have been in sackcloth and ashes ever since. Figures and graphs available from The Engineering ToolBox.

Here's a problem. Say Duncan and I survive a crash in the Sahara desert and find the plane's cargo is cling film. Being Model Engineers we decide to escape by by building a lighter than air balloon filled with water vapour, Flight of the Phoenix style.

In our undies, we weigh 130kg in total. Assuming the envelope and basket to be weightless and humidity in the Sahara to be zero, how big does the envelope need to be to lift us and how much water is needed to fill it?

And in the blockbuster movie, which Hollywood super-stars will play us? I see myself as George Clooney, my daughter suggested drag star Brendan O'Carroll…

Dave

[Howard LewisParticipant @howardlewis46836]

Duncan, we have had this argument before.

WHY do you think that the solution for a damp church wall is an unglazed ceramic tube with the open end pointing downwards? (I learned this from a book on church maintenance that my late father, a priest, had )

Fog stays close to the ground.

According to you it should float upwards.

Cumulo Nimbus clouds are there because of convection upcurrents, not because of the density of the moist air of which they are constituted.,. Sometimes the upcurrents are so violent that aircraft can be damaged.

Forget molecular weights. Water does not run upwards, does it?

Air consists of roughly 79% nitrogen and 20% oxygen, so a gramme molecule of air will contain 11.06 grammes of Nitrogen and 3.2 grammes of Oxygen. So, a gramme molecule of air weighs 14.8 grammes.

A gramme molecule of water weighs 18 grammes

So water is heavier than air, as we all know. hence, moist air falls.

A wet tumbler dries when it is placed with the open end downwards. Open end up, it dries extremely slowly.

Q E D

Howard

[Tim StevensParticipant @timstevens64731]

I'm sorry, but I have difficulty with an explanation which says 'forget molecular weights' and then quotes molecular weights.

Tim

[DMBParticipant @dmb]

I do know that if one electrical item fails in a series circuit it works like a switch, whole circuit goes dead. Does it matter in my circumstances? No, because I go into the workshop most days so not many hours with heat off on one machine only. It will still be 'enjoying' the benefit of residual warmth trapped under the plastic covers. Even then, there is a certain amount of warmth in the general workshop atmosphere from the other heater, which is a tubular one, via thermostat and timer. Only off for about a third of the day. It's on Myford drip tray at the back and lathe has plastic cover. Escaping heat warms the rest of the workshop. Only 10x 8ft, so not a very large volume to keep warm. Most of the items, bench, drill, two mills, lathe, all covered against falling dew or even rain from roof, should itt spring a leak. I'm now even growing hedging to keep sun off shed walls to try and reduce temperature rises. There is a plan to put a further thick layer of continuous insulation in the roof apex soon, which will hold back heat transmission in from sun, out due to colder outside temps. It will be a blanket of white recycled plastic. Got a lot left on the £17 roll. Used it to cover loft traphatch in the house, surprised at the difference that it made. BnQ earlier this year.

 

Edited By DMB on 14/04/2022 15:51:27

[Howard LewisParticipant @howardlewis46836]

Well spotted Tim

But the logic remains the same!

Howard

[SillyOldDufferModerator @sillyoldduffer]

Posted by Howard Lewis on 14/04/2022 15:33:45:

…

So water is heavier than air, as we all know. hence, moist air falls.

…

We all agree water is heavier than air, but Duncan is talking about water vapour, not the liquid or solid forms. Fog and Clouds aren't vapour, they're droplets of water or ice crystals.

I think ceramic tubes in walls help remove damp by ventilation, by draining liquid water from inside a spongy stone wall and by condensing vapour inside the tube so it drips out. I don't see the density of water vapour relative to dry air matters to the way porous tubes operate.

Sums explaining Duncan's position here.

Dave

[duncan webster 1Participant @duncanwebster1]

Avagadro's law says that the molecular weight in grams (ie 1 mole)of any gas occupies 22.4l at a temp of 0C and 1bar pressure. Clearly water vapour is lighter than air.

I'm not going up in SOD's balloon, because the pressure inside would have to be above atmospheric, or it would collapse, so the temperature is above 100C. Now I know it's hot in the Sahara, but not that hot, so all the vapour would condense, and we'd come crashing to the ground.

[duncan webster 1Participant @duncanwebster1]

I forgot to mention a molecule of oxygen is 2 atoms,so it's molecular weight is 32, not 16, similarly nitrogen is 28, not 14. As stated before, water vapour is 18.

[BazyleParticipant @bazyle]

The tubes in walls thing slope down just so that rain running down the walls doesn't go in. Nothing to do with damp air weight. And it is only foggy every day in horror movies.
However there is a huge difference in climate between different areas of the country. The south east especially is much much drier than the south west and I imagine many other parts of the country. Rainfall data will tell you that but you may not realise that also that has the effect of humidity going along with it.

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