Corneometry is a widely practiced method for the measurement of skin hydration.
It uses a capacitive sensor to measure the relative permittivity of upper skin layers.
Because these depend on hydration of skin, the measured value is a measure for skin hydration.
[1] The name corneometry is derived from the German trademark Corneometer.
[2] In 1979 the first commercial instrument for measuring skin hydration was sold under this name.