Does hDrop need to be calibrated?

hDrop says no. Three sessions here all showed overestimation, with the one controlled test returning 37 per cent. A nude weigh-in before and after a session with zero fluid intake gives a provisional personal divisor and makes the fluid-loss figure meaningful.

Why does hDrop overestimate fluid loss indoors?

The conductivity electrodes need liquid sweat. Outdoors, sweat evaporates and skin stays drier. In a low-airflow gym, sweat pools and keeps the electrodes saturated, likely causing the algorithm to read a higher local rate than actual whole-body loss warrants. Body-mass change captures evaporated fluid too, which is why it returns a lower total.

How do you calculate sweat loss from body weight?

Weigh naked before the session. Exercise with no fluid intake. Weigh naked immediately after. The kilogram difference approximates litres lost. If you did drink, add that volume to the weight loss figure.

Is the hDrop sodium data more reliable than the fluid loss data?

Based on these sessions, yes. The electrode measures ion concentration directly at the skin, a more direct problem than scaling a local sweat rate to whole-body fluid loss. Sodium is not subject to the same pooling and evaporation effects.

What is a normal sweat sodium concentration?

Most endurance athletes fall between 20 and 80 mmol/L, with the majority between 30 and 60 mmol/L. The 57 to 65 mmol/L recorded at Dartmoor sits at the higher end of moderate and points to a sodium-dense electrolyte strategy for long hot sessions.