The full equation for estimating sweat loss is:
total sweat loss in kilograms =
= (change in body weight in kilograms) + (fluid intake in kilograms) + (trapped sweat in clothing in kilograms) – (urine loss in kilograms) – (respiratory water loss in kilograms) – (metabolic mass loss in kilograms)
change in body weight in kilograms =
= (weight before running in kilograms) – (weight after running in kilograms)
trapped sweat in clothing in kilograms =
= (weight of clothes after running in kilograms) – (weight of clothes before running in kilograms)
respiratory water loss =
= (0.019 * (oxygen uptake in liters per minute) * (44 – (ambient water vapor pressure in mm Hg)) * (run time in minutes))
where:
• Including trapped sweat in clothing strikes me as if this amount was being counted twice.
• The equation for metabolic mass loss was hard to decipher (see page 1346). This only amounted to 0.1 kilograms.
• The units for sweat loss was in kilograms, but the authors also referred to loss in liters per hour. The conversion requires knowing the density of the sweat, with liters = (kilograms sweat) / (density). According to the Geigy Scientific Tables, sweat has a specific gravity from 1.001 to 1.008. To keep things simple I left this isodense with water.
To simplify this for field use, the following equation (formula F-3) can be used:
field estimate total sweat loss in kilograms =
= (change in body weight in kilograms) + (fluid intake in liters) – (urine loss in liters)
Limitations:
• The equation tended to overestimate sweat loss by about 0.2% in warm weather and by about 14.1% in cool weather.
• If the urine was not measured, the equation overestimated sweat loss by 6.4% in warm weather and 31.2% in cold weather.
