ASTM D4898 Test Method for Insoluble Contamination of Hydraulic Fluids by Gravimetric Analysis
9. Procedure
9.1 Select two membrane filters of the pore diameter required. Allow the filters to stabilize to equilibrium with ambient room conditions. Record room temperature and relative humidity.
9.2 Take precautions to minimize apparatus contamination from airborne dust. Use a protective hood or cover.
NOTE 2 - One of the filters mentioned in 9.1 is for a blank determination, which is necessary to provide a correction factor for adsorption of hydraulic fluid by the filter, desorption of filter constituents by the solvent, and variations of moisture content of the filter. This blank should be determined for each sample of a given composition and history. In a series of samples of similar fluids with similar history, the blank need be determined only once. However, with any change of sample volume, a new blank should be determined as the relationship of blank correction to sample volume cannot be extrapolated.
9.3 Using forceps, weigh each filter on a microbalance to the nearest 0.005 mg.
9.4 Place the filters, one above the other, in the filtration apparatus. Ground the clamp to reduce static buildup during filtration.
9.5 Shake the sample container for approximately 1 min.
9.6 Measure the desired sample volume with a graduated cylinder and pour into the filter funnel, or pour the required volume directly into a calibrated filter funnel. Apply a vacuum and filter the entire sample.
9.7 Use a stream of filtered commercial hexane or Refrigerant 113 from a filtered liquid dispenser to rinse the residual sample from the graduated cylinder into the filter funnel and to wash down the inside of the filter funnel. The outside of the filter unit should also be washed at the joint between the filter holder and filter base to remove fluid between the holder and the filter disk. Repeat the washing twice so that a total of approximately 100 mL of commercial hexane or Refrigerant 113 is used for the washing process.
9.8 With the vacuum still applied, remove the funnel, taking care not to disturb the filter. With a fine steady stream, wash any residual hydraulic fluid from the periphery of the filter with filtered commercial hexane. The washing action should be towards the center with care taken not to disturb the deposited contaminates on the filter surface. Continue washing until the total volume of filtered hexane equals 250 mL to 300 mL.
9.9 Dry the filters with aspirator action and release the vacuum.
9.10 Using forceps, remove the filters and place in a covered petri dish. Place the petri dish in an 80 °C oven for 15 min. Other drying methods such as 5 min of continuous aspirator action or vacuum dessication may be substituted for the oven-drying method if such substituted method proves superior to the oven method in achieving a constant mass upon continued drying. The nature of the contaminant will dictate the drying method to be used.
9.11 Allow the filters to equilibrate to ambient room conditions for 5 min in the petri dish with cover ajar. Record the room temperature and relative humidity. When these values differ from ambient conditions recorded as described in 9.1, results will be invalid unless a blank filter is run concurrently (see Note 2). Remove static electricity by placing each membrane on the air ionizer for several seconds.
9.12 Weigh each filter to the nearest 0.005 mg.
9.13 Inspect the same filter (upper membrane) under a microscope at approximately 35x. The presence of large or unusual particles or fibers shall be sufficient cause for requesting another sampling and analysis even though the allowable contamination mass is not exceeded.
9.14 For further examination of the filter, reference may be made to Test Method F314.
10. Calculations
10.1 Calculate the mass of the contaminant as follows:
10.1.1 Subtract the initial mass of the test filter (9.3) from the final mass (9.12) of the test filter (upper membrane).
10.1.2 Subtract the initial mass of the blank filter from the final mass of the blank filter (lower membrane).
10.1.3 Subtract the mass change of the blank filter from the mass change of the test filter.
10.1.4 The contamination mass is expressed as mg/100 mL of fluid sample retained on the specified pore diameter membrane filter.
11. Precision and Bias
11.1 Precision:
11.1.1 Repeatability - Duplicate results by the same operator should be considered suspect if they differ by more than 3.6 % of the mass of contamination, measured as mg/100 mL of the sample.
11.1.2 Reproducibility - Available data do not justify any statement.
11.2 Bias - Since there is no accepted reference material suitable for determining the bias for the procedure for measuring insoluble contamination, no bias can be determined.
