| Evaporative water loss
EWL = (FeH2O - FiH2O) *STP * FR
with appropriate correction for whatever output units are desired. This ignores any effects of VO2, VCO2, and changed water vapor content on flow rate (in combination, these exchanges usually have a very minor effect on calculated EWL). The equation is simple, but figuring out FiH2O and FeH2O from typical humidity data is not, because of the non-linear relationship between temperature and the water content of a gas.
Water vapor content is usually measured as either percent relative humidity or dew point temperature (in °C). Select the appropriate units for your humidity sensor's output. In either case, values must be converted into water vapor densities and then into fractional concentrations. The equations for computing water vapor density are arithmetically rather nasty (and therefore the speed of conversion isn't as fast for some EWL calculations as for other kinds of gas exchange). If you use % RH, the algorithms need to know the temperature of the humidity sensor. You can either enter this directly or the value can be obtained from a data channel. In the latter case, and for all analyses of dew point data, vapor density calculations must be repeated for each sample point. That slows the rate of conversion considerably, so a 'bar graph' display is shown to indicate progress.
The algorithms used to compute vapor density are derived from Properties of Air, by Tracy, Welch, and Porter (1980; University of Wisconsin; you can find a pdf on the Web via Google Scholar). In turn, these are based on the Smithsonian Meteorological Tables. For those interested the formulae used are as follows:
• Vapor pressure (pw) at temperatures over liquid water (Smithsonian Tables, 1984, after Goff and Gratch, 1946):
Log10 pw = -7.90298 (373.16 T-1)
+ 5.02808 Log10(373.16 / T)
- 1.3816 10^-7 (10^11.344 (1-T / 373.16) -1)
+ 8.1328 10^-3 (10^-3.49149 (373.16 / T-1) -1)
with T in °K and pw in hPa
• Vapor pressure (pi) at temperatures below 0 °C (over ice) (Smithsonian Tables, 1984):
Log10 pi = -9.09718 (273.16/T - 1)
- 3.56654 Log10(273.16/ T)
+ 0.876793 (1 - T/ 273.16)
with T in °K and pi in hPa
The Goff-Gratch equation (for air over liquid water) covers a temperature range of -50 °C to about 100 °C, but is mostly theoretical for very low temperatures. Accuracy is probably ±0.5% or better at temperatures between -20 and 70 °C.
• CAUTION: regardless of the accuracy of the equations, technically it is very difficult to avoid condensation or freezing of water vapor when working at subzero temperatures.