General working fluid, as new oil, contains moisture in the amount of 50-80 ppm (0.005-0.008 percent). But, the ratio increases with moisturized air injected by actuators and air breathers. Moisture generates rust on the inner walls of hydraulic equipment, and enhances the deterioration of lubricants and working fluids. Measurement of moisture in working fluid is conducted by the Carl Fisher Method, which uses characteristics of a reagent that reacts only with a set amount of water, with 10 ppm detection range.
The acceptable amount of dust and moisture in working fluid depends on the equipment. Refer to Tables 14.6 and 14.7. The code of JIS B 9933 (ISO 4406) in Table 14.6 indicates a contamination level by pre-set class numbers that correspond to sizes and numbers of particles (for example, the number of particles of 5 ^m or greater and the number of particles of 15 ^m or greater). Recently, this rating system is beginning to be widely accepted.
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