Bocides and Preservatives are essential chemicals for controlling microbial fouling, spoilage and corrosion in industrial fluids and consumer products and for preventing contamination by microbes which threaten health. They progressively lose efficacy if the concentration falls. The Biocide Rapid Test consists of ampules of gel which contain the spores of a thermophilic Bacillus (a safe and harmless bacteria) and an indicator which changes from purple to yellow when growth occurs within the ampule. The sample to be tested is added to the ampule which is then incubated. After 3 hours and 15 mins of incubation at 64° C the growth of the Bacillus is assessed by examining the ampule for a change in colour. If it does not change colour, the sample must have contained sufficient anti-microbial chemical to inhibit the growth of the Bacillus.