Relative Humidity. How to get it right?

While browsing through Lowyat.Net, I came across the post below by 'chiongguo' which I feel all my readers should read. It is an explanation which makes sense and has not been published before.

"I think misting system doesn't work well and often resulted in mould. Even with a hygrostat the control is very bad.

I am now designing my first BH and had considered a number of systems and misting system definitely, in my considered opinion, to be the worst approach to use. Let me explain a bit about the physics of relative humidity and from this we can get some understanding of the various systems that can be designed.

Humidity means basically the amount of water vapour in the air. At any one temperature the air can hold a certain maximum of such water vapour. As an eg. at 30 degC the air can hold AT MOST 30 g/m^3. If there is only 15 g/m^3 of water vapour than the relative humidity is 50%.

If this air is in an enclosed box and inside this box is a pool of water. The water in the enclosed box will evaporate until it reaches 30 g/m^3. The relative humidity now will be 100%.

If you now poke a hole in the box the relative humdity will start to drop until the amount of water vapour evaporated is equal to those lost through the hole. So in order to ensure that you get 80% relative humidity we could make the hole smaller or increase the rate of evaporation. Rate of evaporation can be achieved by increasing the surface area of the water exposed to the air.

Blowing water into the air will increase the relative humidity and often to saturation (100%) and even with a hygrostat it will always over compensate due to what engineers called transport lag. As the water vapour blown into the air is quite large droplets it will not remain in the air for long. If you want water vapour to remain much longer an ultrasonic misting/humidifier has to be used. This can be quite expensive.

I read in a indon book that what they do in indonesia was to lay on the floor jude cloth that is wet by pools of water. But as bird pooh will drop onto the jude, over time it can be a stinking mess that is almost impossible to clean. Finally I came up with a design, just completed the installation yesterday, where a small channel of water is placed along the wall (about 4 feet off the ground). I use the rain drainage channel to hold the water. And over the sides I will drape cheap dish washing cloth. The water will soak the cloth and the water will evaporate from the surface of the cloth. The water in the channel is controlled by a float valve.

I had tested the system in my kitchen and it worked. I recorded an evaporative rate of 200g of water over a 12 hour period(morning till evening) for a 2 sq.ft of cloth area.

We can control the surface area to control the relative humidity. Once this is established the relative humidity will remain quite stable over a temperature range. It is all natural and everything is self-adjusting. Environmentally friendly just like in natural caves.

Air entering into the BH is often quite dry and to ensure that the fluctuation is not too drastic I bought a water tank and placed it at the roving area."

I fully agreed with 'chiongguo' on his explanation on relative humidity and ways to control them. That is why some BHs have water tubs and pools inside the nesting area. Having water flowing down the wall and water fountains will increase the surface area of water which in turn will increase the rate of evaporation and help the BHs to achieve the desired relative humidity. The rate of evaporation depends on the size of the entrance hole. A bigger entrance hole means that the rate of evaporation is higher and surface area of water has to be increased through more water features mentioned earlier and vice versa. .

'chiongguo' had further discussed on the use of ventilation holes. This is what he had said which I fully agreed with him.

"I notice and had seen a number of designs of ventilation holes and the reason for ventilation holes was for heat ventilation. If you have more holes, then your relative humidity will drop. I think Harry often suggested closing those holes and he was actually right but I read in other places a lot of these consultants were insulting his advice. Actually what he adviced was correct.

If you are using these holes for getting rid of heat from WITHIN the BH than the strategy will not work. The solution will cause more problems than solve.

Heat MUST BE PREVENTED from reaching the BH at all cost - this means insulation.

Now if you have a double wall than the ventilation holes will reduce the heat from reaching into the BH and turbulence and lost of relative humidity will not be a problem.

A very strange design was to have a long pipe from the inside wall to the outside wall and the person explained to me it was to get rid of the heat inside. More often than not the heat outside is higher and it will flow inwards rather than outwards.

For maintaining temperature below 31 degC , HEAT MUST BE PREVENTED FROM ENTERING THE BH.

Air-conditioning etc. are a waste and in the long term expensive."

This is the reason why I am not using ventilation holes in my BH design. Look out for how I will control the temperature and relative humidity. It will be slightly different from the norm.