Last month we looked at equilibrium moisture content (EMC) and equilibrium relative humidity (ERH) and how useful these are to us in understanding just what is going on when we measure subfloor moisture levels with moisture meters and hygrometers.
I did promise you I would introduce the delights of the psychrometric chart. Now don’t go getting all cold and clammy over this one, you don’t need to use one in your day to day use of hygrometers and moisture meters, but if you want to gain a little more understanding of moisture within the built environment, a working understanding of psychrometry, as it’s known, can be quite illuminating.
You will have gathered by now that there are relationships between relative humidity, air temperature and dew point temperature. From these we can calculate the actual amount of water in the air expressed either as grams of water vapour per kilogram of air if you are European and in grains per pound of air if you are American.
Now these figures have very little relevance to the flooring professional, but they are very important to a drying technician charged with the responsibility of drying out a flood damaged property. He or she will need to know how much water by weight is being
removed by a dehumidifier as this will enable calculations to be made on how many dehumidifiers are required and an estimate to be made as to how long the job will take.
To make these calculations you could crunch the numbers in complex formulae or, as is common practice, use a psychrometric chart. This is far easier to use in practice than it would seem at first sight.
Some are convinced that a psychrometric chart is the product of a spider on acid, so crowded does it look. But once you know what you are looking at it is a great help in giving the figures the drying professional needs. To properly evaluate temperature, relative humidity and the humidity ratio (weight of
water per weight of air – grains per pound or grams per kilogram) dr ying technicians must use one of these char ts.
Other tools can be used to calculate humidity ratio and, indeed, the Protimeter MMS will make this calculation for you. If you know the relative humidity and the air temperature, plotting these two values on a psychrometric chart will enable you to read off the humidity ratio.
By doing this for the air outside and the air inside one can calculate if outdoor air will have the capacity to dry the building and by how much. Remember it is always cheaper to use ambient air than a dehumidifier to condition that air but you need to know.
So this month we are a little bit away from the coal face as we know it, but I hope this has given you a flavour of what the dr ying industr y does before you go in and refit that floor in a water damaged building.
If you have an MMS, simply comparing the grams per kilogram figure outside to that inside will tell you whether simply opening doors and windows will help dry things out, or not. C F J
I pgrant@tesco.net I www.moisturemeasurementservices.co.uk I M: 07885 221675

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