It is fairly easy to calculate average evaporation rates, providing you have some data specific for the locality (such as the average temperature, the average relative humidity and the mean dew point) using thisformula:
lh = ((Tv + (Ta x As2)) x Ea x (Sh – Hr))/Ec
where:
lh = litres per hour of evaporation
Tv = mean temperature of vat (°C)
Ta = mean temperature of atmosphere (°C)
As = Air speed (metres per second)
Ea = evaporation area (m2)
Sh = saturation humidity ratio (kg of water per kg of air)
Hr = humidity ratio (kg of water per kg of air)
Ec = Evaporation constant
Sh – Hr provides us with the evaporation potential and is calculated this way:
Sh = (( Dp / 100) x RH ) / Tv
Hr = ((sh / 100) x RH )
where:
Dp = Dew Point (°C)
RH = Relative Humidity (%RH)
and Ec is the evaporation constant, in this case it is the evaporation constant of water as a percentage of a saturated brine solution, because the liquid to evaporate in the vat is brine. To make a saturated solution of brine (as described in the Geoponika above), requires 357.6g of salt per litre of water. 1 litre of water = 1000g, therefore saturated brine =1.357.6kg
therefore:
Ec = 1000/(1357.6/100) = 73.7
Evaporation constants will only work as a rough guide and only for temperatures below 50°C and additionally, since the workshops were by the sea, the atmospheric humidity would also have a salt content, so the evaporation constant is not perfect, although probably accurate enough for these generalised calculations.
If the Cotta vats are used as an example and we assume the following:
Tv = 40°C – the incubation temperature of a yoghurt maker (based on the Wunderlich method)
Ta = 17.19°C – Meteorological average for area
As = 6.3m/s – Meteorological average for area
Dp = 12.9 – Meteorological average for area
RH = 79 – Meteorological average for area
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