A colorimetric patch test is a widely used procedure for monitoring hydraulic fluids and aviation fuels. In particular, it is used adjacent to aircraft or machinery to enable an immediate decision to be made on whether to change the fluid. Aviation fuel is the most critical because of the number of transfers the fuel will go through before it reaches its final destination. A patch test (ASTM D3830) is performed at each point of transfer.
The typical color of a contaminant in any given system remains fairly constant. The greater the discoloration of a filter, the greater the degree of contamination. Increasing the sample volume may increase the sensitivity of the procedure. The Patch Test is generally applicable only to gross levels of contamination.