The Anonymous Widower

How Much Water Vapour Is In A Cubic Metre Of Air at A Given Temperature And Relative Humidity?

I needed to know this as if I knew the temperature and relative humidity in my bedroom, when I went to bed and got up, I could work out how much water vapour had transferred to or from the air during my sleep.

One of my friends at school is an expert on these sort of calculations for industrial clients.

He came round on Friday night and we discussed it through, but I don’t think I got more than a basic grasp.

The reason is that he works from charts, whereas all my working life, I’ve started with proven formulae and worked everything out from first principles. But then I trained as a Control Engineer.

A problem I had with the psychometric charts he uses, is that they are all in a measurement system, that is totally foreign to me – Imperial. This is because most of the publishers are across the pond and they still use units, I last used in my early teens. When I went to work at ICI in the late sixties, the company had metricated in 1955 or so.

At least after our meeting and discussion, I now know what I’m searching for.

I finally found this web page, which gives a table of saturated vapour density for water in air. Although, it’s an American web site, at least it gives this in gm/cu. m.

The web page gives the SVP in gm/cu. m. at various temperatures

  • 0°C – 4.85
  • 10°C – 9.4
  • 15°C – 12.83
  • 20°C – 17.3
  • 25°C – 23
  • 30°C – 30.4
  • 37°C – 44
  • 40°C – 51.1

As an illustration, suppose you have a temperature of 25°C and a relative humidity of 50%. I measure it on my Maplin meter.

Maplin Hygro-Thermometer

Maplin Hygro-Thermometer

At that temperature a cubic metre of water can hold 23 grams of water. But as the relative humidity is 50%, it is actually only holding 11.5 grams of water. As my bedroom is about five metres square and two and a half metres high, that means the room contains over 719 grams of water.

Now look at 30°C and the same relative humidity of 50%.

The same calculation gives 950 grams of water in the room.

So if with the central heating, the electric blanket and the fact that each person probably is equivalent to a one bar electric fire, your bedroom, about the same size as mine, goes from say 25°C to 30°C, the air will need another 230 grams of water to be in equilibrium, or in layman’s terms, happy with how it relates to everything.

So from where does the air get this water it needs?

You!

No wonder a lot of people go to bed with a night bucket, so they can replenish the fluid they’ve lost to the air.

August 31, 2014 - Posted by | World | ,

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