How It Works
ASTM D341 Walther Equation
The relationship between viscosity and temperature for petroleum oils follows the Walther equation standardized in ASTM D341:
log(log(v + 0.7)) = A - B * log(T)
- v = Kinematic viscosity (cSt)
- T = Temperature (Kelvin)
- A, B = Oil-specific constants
Viscosity Index (VI)
VI indicates how much viscosity changes with temperature. Higher VI means more stable viscosity:
- VI 95-100: Conventional mineral oils
- VI 130-150: Multigrade hydraulic oils
- VI 150+: Synthetic hydraulic fluids
Operating Ranges
Recommended viscosity ranges for hydraulic systems:
- Optimum: 16-36 cSt (most efficient operation)
- Acceptable: 10-100 cSt (normal operation)
- Maximum: 750-1000 cSt (cold start limit)
- Minimum: 10 cSt (hot running limit)
Unit Conversions
- 1 cSt = 1 mm2/s
- SUS = 4.6324*cSt + 1.0 + 200/cSt (for cSt > 2)
- cP (centipoise) = cSt * density (g/mL)
Viscosity-Temperature Calculator
Calculate hydraulic oil viscosity at any temperature using ASTM D341. Select an ISO grade or enter custom viscosity data.
Viscosity at Operating Temperature
32.5 cSt
Operating Status
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Viscosity Conversions
Kinematic (cSt / mm2/s)
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Saybolt Universal (SUS)
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Dynamic (cP / mPa.s)
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Viscosity Index (VI)
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VI Classification
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Temperature Points
| Temp (C) | Visc (cSt) | Status |
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