humidistatic dehumidifiers

[charlie-ia]

thats a good start, thankyou. we're only able to get over for a couple of days the first time, so plenty to keep us going!

 

[Aidan]

does anyone know the phone number for the sorcerers apprentice?

(probably copyright disney)

I think this is it; I had it bookmarked on another hard-drive.

http://www.buildingpres.co.uk/

I trust he's not mickey mouse.

 

[charlie-ia]

Very interesting. will read more of that, cheers!

 

[Nemesis]

Another couple of useful sites:

http://www.handr.co.uk/literature/woodworm.htm

http://www.ridoutassociates.co.uk/article01a.htm

 

[Aidan]

Aiden talks about a lot of tech & options that I dont see as useful for your case.

Oh dear; NT's being rude about me. I must have upset him again.


1) The thing about water vapour is that it can't be seen, smelt or felt and people are fairly insensitive to it; most have difficulty in distinguishing between say 35 or 55% Rh, but can tell 18 from 20 degC. You can feel a draught of air, you can't detect a movement of water vapour in still air. So, you're pretty much reliant upon Rh measuring instruments.

A reliable human hair Rh meter is expensive (£40 ish, Barometer World, can't recall the makers); budget electronic meters and humidistats are inaccurate when new and can acquire huge errors in use. I've seen 25% errors, but how would you know? A sling hygrometer/psychrometer is the only cheap & reliable way of checking the accuracy of an Rh meter (see http://en.wikipedia.org/wiki/Psychrometrics).

2) A refrigeration dehumidifier works by chilling the air and so condensing out most of the water vapour. The heat from the hot condensor coil (hot black pipey grid thing on the back of your fridge) and the compressor motor (hot black electric can thingy under fridge) is dissipated into the room and this heating serves to further lower the Rh.

An absorption/dessicant dehumidifier uses an electric heating element to regenerate the media by evaporating the absorbed moisture. It needs a lot of heat, and the heat & water vapour must be discharged through a duct to outside. That heat is lost, so they're less efficient.

Refrigerant devices generally don't like low temperatures because it makes the oil too viscous (so freezers usually don't work in an unheated garage in winter) and also the condensed moisture freezes on the evaporator coil. In these situations, heating is usually more effective at lowering the Rh than a dehumidifier .

3) Dehum lowers the partial pressure of the water vapour in the room air, and the water vapour in the outside atmosphere will then move from high vapour pressure to low vapour pressure. IIRC, Dalton's law covers it, but I never had cause to apply it.

The most impressive demonstration of this I saw concerned an absorption dryer on a compressed air system. The pipes were ABS plastic. The dryer dried the air to a dew point of -40degC & so lowered the VP in the downstream pipes. Water vapour from the atmosphere was then permeating through the plastic pipe walls and making the air too wet, even though the compressed air was at 60 or 80 psi. The downstream pipe was changed to copper, problem solved.

A dehumidifier can accumulate an impressive volume of water and people assume it has all come out of their damp building structure. Some will have, but most of it will have come from the outside air, through gaps in the structure or in make-up air. It will also pass through the structural elements (walls, ceiling) that aren't saturated. Heating would lower the Rh, but I don't think it affects the VP (& can't be bothered to check) and so won't attract vapour from outside.


I think you'd be better off in just eliminating the source(s) of water entering the building and allowing it to dry naturally. I'd only use a dehumidifier on summer nights.

Right on all points aiui. My attempts to use refrigeration below 10C have been almost totally ineffective.


NT

Heat.

 

[NT]

Aiden talks about a lot of tech & options that I dont see as useful for your case.

Oh dear; NT's being rude about me. I must have upset him again.

I havent been rude about you at all. I dont see any need for that sort of carry on.


NT

 

[Aidan]

I havent been rude about you at all. I dont see any need for that sort of carry on.
NT

Aiden?
Talks a lot about stuff of no use?
Not rude?

I don't see any need for that sort of carry on.

 

[NT]

Aiden?
Talks a lot about stuff of no use?

thats not what was said. I see theres no point in replying any further.


NT

 

[biffvernon]

Well I thought it was very interesting, Aiden. How do the electronic humidity meters work?

 

[Aidan]

How do the electronic humidity meters work?

Haven't got the foggiest.
:lol: :lol:

 

[charlie-ia]

you two naughties behave! (now im being rude too!)
the thing about this site i like is you can find detail if you want it, and realy basic stuff if you dont. the range of opinions is very important.
carry on everybody!

 

[NT]

How do the electronic humidity meters work?

A 2-wire electronic sensor changes R depending on RH. They were standard equipment on VCRs, as bringing a VCR into a warm from from the cold would cause condensation on the spinning heads - play a tape then and there'd be a terrible mess.

Perhaps one could make humidistats out of old VCRs, I've not tried it and dont know the characteristics of the sensors, but its likely to be a simple task, opamp transistor & relay.


NT


Edit: trimmed quote

 

[Flyfisher]

I've not tried it and dont know the characteristics of the sensors,

I've not tried it either, but I doubt that a sensor designed for a very specific purpose (i.e. VCR or camcorder protection) would be a good starting point if you wanted to detect a wide range of RH for a dehumidifier-type application.

 

[biffvernon]

Ta. So just how reliable and accurate are the sub £10 electronic humidity meters? I happen to have one in my conservatory. The numbers vary from over 70% first thing in the morning when the temperature is very low, to less than 40% later in the day when the sun has warmed the place excessively.

 

[Aidan]

Ta. So just how reliable and accurate are the sub £10 electronic humidity meters? I happen to have one in my conservatory. The numbers vary from over 70% first thing in the morning when the temperature is very low, to less than 40% later in the day when the sun has warmed the place excessively.

I'll have rummage for my sling hygrometer and do some measurements in my conservatory, if it shows up. I might see what the humidistat on the bathroom fan actually works at; after installation it gradually crept up to an 80% setting to get it to come on. Whizzing a couple of mercury-in-glass thermometers around is not approved by the H&S brigade these days.

Your numbers sound like they could be right, the change in Rh would be mostly due to the change in temperature, the moisture content of the air would be roughly constant. It's not very important on these devices, but if you're relying on a humidity sensor to control a humidifier or dehumidifier, then a sensor failure can be embarrassing. A run-away humidifier is not a pretty sight .

There are also capacitative Rh transducers, wet&dry bulb devices and chilled mirror dew-point sensors (very expensive & accurate). There was a build-a-humidity meter project when I was in college X years ago and human hair was the only measuring device used. Still most accurate, the length changes quickly with Rh.

 

[biffvernon]

So you shouldn't rush to the barbers just because it's a damp day.

Yes, I've no reason to doubt the osscilations in my conservatory. Actually what I'm more interested in is the humidity in the room in which I store timber destined for internal doors and where I have one of these which I bought when it was even cheaper. I have it on a time switch that only allows it to switch itself on a cheap rate electricity time. Through the winter it removed about a gallon of water a week.

 

[NT]

I've not tried it and dont know the characteristics of the sensors,

I've not tried it either, but I doubt that a sensor designed for a very specific purpose (i.e. VCR or camcorder protection) would be a good starting point if you wanted to detect a wide range of RH for a dehumidifier-type application.

On what basis do you say that? Not that dehumidifiers need wide range sensing.


NT

 

[Aidan]

You could check the calibration over a saturated salt solution, details here;
http://www.padfield.org/tim/cfys/satslt/satsalt.php

It's messy and seems a lot of effort for a £10 meter.


There's a lot of other humidity control related articles listed in the index on that site, the one about the shrinkage/reaction of wood to humidity may be of interest.

 

[Flyfisher]

I've not tried it and dont know the characteristics of the sensors,

I've not tried it either, but I doubt that a sensor designed for a very specific purpose (i.e. VCR or camcorder protection) would be a good starting point if you wanted to detect a wide range of RH for a dehumidifier-type application.

On what basis do you say that? Not that dehumidifiers need wide range sensing.

On the basis that a humidistatic dehumidifier allows a degree of control over the RH level, thereby implying it can sense a range of RH, whereas a camcorder has no such control. Or, more accurately, a camcorder is really only concerned with detecting condensation, which can cause the tape to stick to the spinning heads with unfavourable results. The actual level of RH, per se, is not a problem for camcorders. You can use them in the desert or the rainforest with equal ease.

 

[Aidan]

I think it's more to do with the temperature of the metal and the indoor dewpoint. You could go from cold outside at 50% Rh to warm indoors at 50% Rh. Although you'd get no change in Rh, condensation would form on the head, a hefty block of metal, until it had warmed up above the indoor dewpoint temperature. It would take another while for the condensation to evaporate.

I don't know how they did measure Rh on VCRs; I suspect the Rh sensor might have ben mounted in a similar mass of metal so that it caused local cooling of the air & so depressed the adjacent Rh. A cheapo new Rh sensor costs about £3.50 from Farnell.