A versatile automatic gas-sampling system

Commercially available equipment for automatic gas-sampling is both expensive and bulky. Air infiltration into refrigerated areas should ideally be measured with low-cost, portable, automatic gas-samplers, capable ofoperating in a wide range of ambient temperatures. This paper describes such a sampler. Although the need for portability limits the size ofthe equipment and therefore the volume of sampled gas, the consequent reduction in accuracy was not significant in this application. Depending on which tracer gas is used, its concentration in the sampled gas can be determined by infra-red spectroscopy or gas chromatography. The portable gas-sampler was used to determine the infiltration of warm air into a number of single-storey coldrooms with volumes from 4000 to 38 000m and various types of door protection l-l]. Infiltration was determined by measuring the rate of decay of the tracer gas SF6 (sulphur hexafluoride). If the tracer gas becomes well mixed with the air in the room, the rate at which its concentration, c, decreases with time can be related to the rate ofair change, r (i.e. the infiltration rate divided by the volume of the room) by: