Abstract: A hygrometer of the electrical impedance type includes a sensing element (1) having a porous moisture adsorbing layer (3), the electrical properties of which vary with the quantity of water vapour adsorbed on it sandwich between a substrate (2) and a porous conducting electrode (4). The conducting electrode (4) is divided into a number of separate measuring zones (6A, 6 and 6C) by gaps (5). The hygrometer also includes a redundancy system circuit (7) which is connected to each of the separate measuring zones (6A, 6B and 6C) and which only provides a moisture indication when the output from at least two of the zones (6A, 6B and 6C) are in agreement. This arrangement enables failures of the sensing element (1) to be detected and distinguished from a false moisture indication.

Abstract: A dewpoint meter has a sensor comprising a mirror (1) situated in a chamber (2) through which gas, the dewpoint of which is to be measured is passed through an inlet port (3) and an outlet port (4). The mirror (1) is cooled by a Peltier effect heat pump (11) and the formation of condensation on the mirror as it is cooled is detected by a detector comprising a light emitting diode (7) and a photodiode (8). The temperature at which condensation occurs, which is the dewpoint of the gas, is measured by a thermometer. Instead of transferring the heat from the heat pump (11) directly to a heat sink (13) as is usual, a heat pipe (12) is interposed between the heat pump (11) and the heat sink (13). This avoids the necessity for having the heat sink closely adjacent the mirror and thus avoids errors caused by heat from the heat sink affecting the temperature of the mirror.