How It Works
Bulk Modulus Definition
Bulk modulus (B or K) measures a fluid's resistance to compression:
B = -V * (dP/dV) = dP / (dV/V)
- B = Bulk modulus (bar or MPa)
- dP = Pressure change
- dV/V = Volume strain (relative compression)
Typical Values
- Mineral oil: 14,000-17,000 bar
- Water-glycol: 20,000-25,000 bar
- Phosphate ester: 25,000-30,000 bar
- Steel: ~1,600,000 bar
- Air at 100 bar: ~140 bar (isothermal)
Effect of Entrained Air
Even small amounts of air dramatically reduce effective bulk modulus:
1/Be = 1/Bf + X/(P + P0)
- 1% air at 100 bar reduces Be by ~50%
- Air effect decreases at higher pressures
- New systems often have 5-10% entrained air
System Natural Frequency
Bulk modulus affects hydraulic system dynamics:
fn = (1/2pi) * sqrt(Be*A^2 / (V*M))
Lower effective bulk modulus = lower natural frequency = slower response
Mechanical Compliance
Hose expansion and cylinder stretch add to system compliance:
1/Be_total = 1/Be_fluid + 1/Be_hose + 1/Be_cylinder
Bulk Modulus Calculator
Calculate effective bulk modulus considering fluid type, entrained air, and mechanical compliance.
Typical: 1-5% (new system may have 5-10%)
Mechanical Compliance (Optional)
Effective Bulk Modulus
12,500 bar
78% of base fluid
Modulus Components
Base Fluid Modulus
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Temperature Corrected
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Pressure Corrected
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With Air Entrainment
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Hose Equivalent Modulus
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Final Effective Modulus
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System Effects
Fluid Compressibility
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Volume Change at Pressure
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Hydraulic Capacitance
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Analysis
Typical Bulk Modulus Values
| Fluid | Modulus (bar) | Notes |
|---|---|---|
| Mineral Oil | 14,000-17,000 | Most common |
| Water-Glycol | 20,000-25,000 | Fire resistant |
| Phosphate Ester | 25,000-30,000 | Aircraft |
| Vegetable Oil | 15,000-18,000 | Biodegradable |
| Water | 22,000 | Reference |