The development of a technique for determining the oil content of medical compressed air

Abstract

The maximum permissible concentration of oil droplet contamination in medical compressed air is 0.5 mg/m3. Because of the increasing use of compressed air in hospitals, a rapid and straightforward test is required which will enable a hospital engineer to establish whether the air supply is free from oil. A jet of air is impacted onto the surface of a porous membrane held in a device designed to ensure an impaction efficiency of at least 50 per cent for droplets greater than 1 micrometre in diameter. The collected oil spreads uniformly from the centre of the membrane and causes a visible change in its opacity which may be measured under green light using a transmission densitometer. This change in opacity, defined as the logarithm of the ratio of the incident-light intensity, was found to be a function of the mass of oil deposited on the membrane and was independent of the type of oil used. From the point of view of the hospital engineer it is necessary only to sample 3 m3 of free compressed air with the instrument and see whether a visible grease spot is obtained. If a stain is apparent then the concentration of oil in the compressed air exceeds 0.5 mg/m3, indicating that servicing of the appropriate filters is necessary.