Abstract: A psychrometer or pick-up for measuring the content of water in a gas flow comprises a first sensor or probe for measuring the dry temperature of the gas and is arranged directly in the flow of gas, a second sensor or probe for measuring the wet temperature of the gas and is surrounded by a sheath of a material capable of absorbing a volatile liquid, the second sensor or probe with its sheath also being arranged in the flow of gas, a pair of screens mounted on opposite sides of the second sensor and the sheath for protecting the sheath against thermal radiation, and a wetting device, preferably formed by a dosing pump connected to a nozzle, for injecting successive doses of a predetermined amount of the volatile liquid directly into the interior of the sheath and onto the screens to prevent excessive and insufficient moistening of the sheath and the screens.