The ASTM D4380 test is used to determine the density of bentonitic slurry in laboratory and field conditions. For freshly mixed slurry, density can also provide an indication of the mix proportions.
What is a Mud Balance?
A mud balance is commonly used for this test. It consists of a slurry cup, graduated beam, fixed counterweight, movable rider, and leveling bubble.
The density is determined by balancing the beam and reading the value directly from the graduated scale.
Reference
ASTM D4380-84 (Reapproved 2001) – Standard Test Method for Density of Bentonitic Slurries.
Apparatus
- Mud balance
- Bentonite slurry sample
- Clean water for calibration
The mud balance should have an accuracy of approximately ±0.01 g/cm³.
Calibration
Before testing, check the mud balance with fresh water.
At 21.1°C (70°F), fresh water should give a reading of 1.00 g/cm³. If it does not, adjust the balance as required.
Test Procedure
- Place the instrument on a reasonably level surface.
- Fill the clean, dry cup completely with bentonite slurry. Fit the cap firmly and allow some slurry to escape through the cap opening to remove trapped air.
- Wipe off excess slurry from the outside.
- Place the beam on its support and move the rider until the leveling bubble is centered.
- Read the density from the graduated scale at the rider.
- Clean and dry the mud balance after the test.
Calculation
The mud balance normally gives the density directly in g/cm³.
Useful conversions are:
- Specific gravity = Density in g/cm³
- Density (lb/ft³) = Density (g/cm³) × 62.43
- Density (lb/gal) = Density (g/cm³) × 8.35
Example
If the mud balance reading is 1.05 g/cm³:
Density = 1.05 × 62.43 = 65.55 lb/ft³
Reporting
Record the bentonite slurry density to the nearest 0.01 g/cm³.
Key Precautions
- Ensure the cup is clean and dry.
- Remove trapped air from the slurry sample.
- Keep the balance properly level.
- Check calibration regularly with fresh water.
- Clean the instrument after every test.
