Corneometry

A capacitance probe rests on the skin for a moment and returns a number. Water has a far higher dielectric constant than the other material in the stratum corneum, so the capacitance the probe sees tracks how much water the surface layer holds. This is usually where a press release's hydration figure comes from.

Two things are worth knowing about that figure. It is in arbitrary units rather than a percentage of anything, so a rise is a rise on that instrument's scale and does not mean the skin now contains that much more water. And it reads the surface. A humectant film or a film-forming polymer sitting on top of the skin holds water right where the probe is looking, which lifts the reading whether or not anything underneath has changed.

The measurement is also sensitive to the room. Temperature, humidity, recent washing, occlusion by clothing and how long the volunteer sat still before the probe touched them all move it, which is why careful testing acclimatises people in a controlled room before anything is recorded.

It is not the same instrument as the one used for transepidermal water loss. That measures evaporation, which is a barrier property; this measures water content. The two often move together, and they can also disagree — a surface film can raise one while the barrier under it is unchanged.