Purpose of the 75 µm wet-sieve test
The test determines the dry mass fraction retained above 75 µm after controlled dispersion and washing. API uses a reference maximum of 4.0%, while OCMA uses 2.5%.
Typical sample preparation
Accurately weigh approximately 10.00 g of bentonite. Disperse it in about 350 mL water using sodium hexametaphosphate and mix under controlled conditions for approximately 30 minutes.
Wet sieving
Transfer the complete suspension to a 75 µm / No. 200 sieve. Wash under controlled pressure, approximately 70 kPa / 10 psi, for about two minutes. Do not mechanically force particles through the sieve.
Drying and calculation
Transfer the retained material to a tared dish and dry to constant mass.
Residue >75 µm (%) = (dry retained mass ÷ original sample mass) × 100
Example
A 10.00 g sample with 0.28 g dry retained residue gives 2.8%. This is within the API reference but outside the OCMA reference.
Use the precise term
The retained fraction may include quartz, feldspar, calcite, mica or incompletely dispersed agglomerates. The technically correct term is therefore residue above 75 µm, not automatically “sand content”.