A sensing system and method suitable for use in humidity sensing instruments such as radiosondes to determine the true relative humidity of the atmosphere contemporaneously with other parameters at the same altitude as the radiosonde rises or descends. The indicated relative humidity of the air contiguous with a humidity sensing element and the temperature of the humidity sensor and the surrounding air temperature, remote from the sensor, are measured. The true relative humidity of the surrounding air is determined by the product of the indicated relative humidity and the ratio of the saturated vapor pressure at the sensor temperature to that at the surrounding air temperature.

A method for determining the relative humidity of gaseous materials, preferably for determining the relative humidity of air in rooms, such as greenhouses, whereby, using psychrometric moisture measurement, the temperature of the stream of gas, the relative humidity of which is to be measured, is determined by means of temperature sensors. A gas stream acting upon a moistenable wet temperature sensor is cooled up to a cooling limit (wet-bulb point) and the wet bulb point temperature is determined. The relative humidity is ascertained using the measured temperature of the gas and the wet-bulb temperature. In order to be able to carry out a reliable determination of the relative humidity over longer periods of time even in a high humidity range, the stream of gas, the humidity of which is to be measured, is initially cooled to at least its dew point temperature in order to moisten the wet temperature sensor directly, the dew point temperature is determined and the wet temperature sensor is moistened to a determinable extent with condensate condensing from the gas stream. After this, for determining the wet-bulb point temperature, the stream of gas is cooled adiabatically with evaporation of the condensate condensed on the wet temperature sensor.

The present invention is a humidity sensor which can be used in a radiosonde. The humidity sensor includes a heat sink attached to the radiosonde, a Peltier cooler attached to the heat sink, a carbon element with a thermistor attached therein or thereon, and a control means. The resistance of the carbon element is adjusted to a predetermined level to maintain a relative humidity of about 33, in particular. The control means monitors this resistance and adjusts the Peltier cooler accordingly. The thermistor responding to the temperature of the carbon element outputs a resistance indicative of the temperature on the sensor surface.

For detecting the occurrence or the imminence of a change of phase within a fluid, a predetermined quantity of heat is generated within thermally conducting means in contact with the fluid. The temperature of said means is measured and compared with a reference temperature so selected as to correspond to the apparition of a second phase within said fluid. In particular, effective or imminent apparition of gas bubbles can be detected in a liquid wherein the temperature of the thermally conducting means has a reference temperature close to the temperature of the liquid.

An apparatus for sensing dew and frost, comprising a sensing portion which includes a substrate (1) having less permittivity than ice and includes a pair of electrodes (2, 3) arranged to touch the substrate (1) and further includes a resistive film (4) on the substrate (1) covering the pair of electrodes (2, 3), the impedance between the pair of electrodes (2, 3) changing in three states, or dry, bedewed and frosted states.