The sweat loss is as above. This can be done any time, and should be done the same method each time. The rate will change depending on conditions, eg. indoor vs outdoor, cooler vs warmer vs extreme heat waves.
This will help you determine how much fluid you need per hour, in similar conditions.
The sodium concentration is done using a sample of blood taken from the arm, and also a urine sample. This is to help identify the amount of sodium you would lose per hour in the sweat.