Frequently Asked Questions
Q1 What is absolute viscosity and why is it important in engineering?
A1 Absolute viscosity, also called dynamic viscosity, is a fluid property that measures resistance to flow under shear stress. It's typically expressed in mPa·s (millipascal·seconds). In engineering, absolute viscosity is critical for calculations involving fluid flow through pipes, heat transfer between gases and surfaces, and pressure drop estimation. Accurate viscosity values are essential for proper sizing of pumps, compressors, and control valves, making it one of the most fundamental properties in process engineering.
Q2 Why does gas viscosity increase with temperature while liquid viscosity decreases?
A2 The difference stems from molecular behavior at different scales. In liquids, molecular cohesive forces dominate, so increased temperature breaks down these attractive forces, reducing viscosity. In gases, molecules are far apart and move independently. At higher temperatures, gas molecules move faster and collide more frequently, creating greater resistance to flow and thus increasing viscosity. This fundamental difference is why heat-correction formulas for gases and liquids are distinctly different.
Q3 How are viscosity values typically measured and reported?
A3 Viscosity is measured using viscometers or rheometers under controlled conditions. The most common measurement method for gases is the capillary tube method, where the time required for a fixed volume of gas to flow through a narrow tube is measured. Measurements are always reported at specific temperatures and pressures (usually atmospheric). The standard reference temperature for many tables is 20°C (68°F), though values at multiple temperatures are provided for practical calculations. Viscosity values are reported in SI units (mPa·s) or older units (cP or poise).
Q4 Can I interpolate viscosity values for temperatures between those in the table?
A4 Yes, linear interpolation is generally acceptable for moderate temperature ranges between tabulated values. For most gases between 0°C and 600°C, a simple linear interpolation provides accuracy within 1-2%. However, for greater accuracy, especially over wide temperature ranges, use the Sutherland formula or reference specific gas properties from sources like NIST. For very precise calculations, always consult original data sources or use property estimation software.
Q5 How does viscosity affect pressure drop calculations in pipes?
A5 Viscosity is a key parameter in pressure drop calculations through the Darcy-Weisbach equation. The pressure drop is inversely proportional to certain fluid properties and directly proportional to viscosity in laminar flow regimes (low Reynolds numbers). In turbulent flow, viscosity effects are minimal but still present. For accurate pressure drop prediction in industrial applications, always use viscosity values at the actual operating temperature of the fluid.
Q6 What is the difference between absolute viscosity and kinematic viscosity?
A6 Absolute (dynamic) viscosity measures the inherent resistance of a fluid to flow, independent of density, expressed in Pa·s or mPa·s. Kinematic viscosity is the ratio of absolute viscosity to density (ν = μ/ρ), expressed in m²/s or cSt (centistokes). Kinematic viscosity is useful in dimensionless numbers like Reynolds number. For most engineering calculations involving flow rates and pressure drops, absolute viscosity is required; kinematic viscosity is used when accounting for fluid properties independently.
Q7 Which gases have the highest and lowest viscosity?
A7 From the table, Neon has the highest viscosity among the listed gases (0.0644 mPa·s at 600°C), while Hydrogen has the lowest (0.0184 mPa·s at 600°C). These differences are due to molecular size and mass differences. Smaller, lighter molecules like hydrogen have lower viscosity, while heavier noble gases like xenon and neon have higher viscosity. This is why viscosity values vary significantly across different gases.
Q8 How should I handle blank cells in the viscosity table?
A8 Blank cells indicate that reliable viscosity data is not available at those temperatures, either because the gas reaches a phase change (condensation) or because data hasn't been experimentally determined. Do not extrapolate or guess values for missing data. Instead, consult specialized references, NIST databases, or use property estimation software. For critical calculations, contact the chemical supplier or refer to peer-reviewed scientific literature for the specific gas and temperature combination you need.