Abstract: In a fluid mixing device calibration and complementary gases are supplied to constant flow devices (10,12). The flows through devices (10,12) are adjusted to be equal. The two gas flows are connected to switching valves (14,16), which allow the gas flows to be either vented or fed to the frequency multiplier (18). The two valves (14,16) are coupled to switch simultaneously but in opposite positions. Thus, the flow rate into the frequency multiplier (18) is constant but may be switched to comprise either calibration or complementary gas. The frequency multiplier divides the input stream into a number of equal streams which, after different time delays have been introduced, are recombined to produce a single stream of reduced concentration ripple and periodicity. The output from the frequency multiplier (18) fed to a series of chambers (20-24) where each chamber produces an attenuation of the concentration ripple and a time response to a change in concentration.

Abstract: A light radiation absorption gas detector for detecting the presence of a specified gas, comprising a light source, a region containing a gas to be detected through which light from the source is passed, an interference filter having a substantially flat face and a light radiation absorption peak at a wavelength which facilitates detection of the specified gas, a shaft to which the filter is by means of arms secured for rotation so that a perpendicular to the flat face of the filter is at an angle to the axis of the rotation and light detector means responsive to light from the source which has passed through the region and the filter, for detecting predetermined light amplitude variations, wherein light from the source is arranged to be projected towards the filter at an angle offset from the axis of shaft rotation whereby a filter pass band which includes the said wavelength is scanned consequent upon rotation of the filter, thereby to produce amplitude variations of light in the presence within the region

Abstract: Monitoring apparatus for producing an output in response to a predetermined condition comprising bistable means (4) having a set input, a reset input and an output; trigger means (2, 6) for applying a first train of pulses (.phi..sub.1) to one of the inputs of the bistable means and for applying a second train of pulses of the same frequency as and different phase to the first train of pulses to the other input of the bistable means in response to a predetermined condition: and an output device (8) responsive to oscillation of the bistable means between its stable states.