What the filter-press test measures
The filter-press test evaluates the filtration behavior of a conditioned bentonite suspension under controlled pressure and temperature. The API-oriented maximum reported filtrate volume is 15.0 mL.
Test condition
The standard low-pressure test uses approximately 700 kPa / 100 psi. The cell, filter paper, gaskets, sample temperature and timing must be controlled.
7.5–30 minute collection interval
The first 7.5 minutes are treated separately. At 7.5 minutes, the initial filtrate and liquid at the outlet are removed. A clean graduated cylinder then collects the filtrate from 7.5 to 30 minutes.
The collected volume is designated Vc.
Reported result
V = 2 × Vc
For example, if Vc = 6.4 mL, the reported filtrate volume is 12.8 mL.
Common errors
- Incorrect or unstable pressure
- Gasket leakage or filter-paper bypass
- Incorrect filter medium
- Temperature variation
- Incomplete sample recombination
- Including the first 7.5-minute filtrate in the reported value
Reduced-area filter presses
A half-area or reduced-area cell can be useful for internal development, but the result should not automatically be converted to a standard API value using an assumed area multiplier. Correlation with the required apparatus should first be validated.