The Poise (P): CGS Unit of Dynamic Viscosity
The poise (symbol P) measures dynamic viscosity in the centimetre–gram–second (CGS) system, preserving a century of rheological data while modern industry primarily expresses viscosity in pascal-seconds.
Pair this article with the dynamic viscosity explainer, the Reynolds number guide, and the Reynolds number calculator to keep laboratory viscosity measurements and computational fluid dynamics (CFD) inputs harmonised.
Definition, Symbol, and Conversion
Dynamic viscosity quantifies internal friction within a fluid. In CGS notation, one poise equals the viscosity that produces a shear stress of one dyne per square centimetre when adjacent fluid layers move with a relative velocity of one centimetre per second separated by one centimetre: 1 P = 1 dyn·s·cm⁻². Converting to SI gives 1 P = 0.1 Pa·s, while 1 centipoise (cP) equals 1 millipascal-second (mPa·s). These relationships appear frequently in rheometer outputs, fluid property tables, and process simulation software, making dimensional awareness indispensable.
The poise inherits base dimensions of M·L⁻¹·T⁻¹ in CGS form, mirroring the SI expression. Because many laboratory instruments report centipoise, engineers often store viscosity as µ = (value in cP) × 10⁻³ Pa·s to maintain SI compatibility. When documenting experiments, specify both the numeric value and the unit to prevent ambiguity—"µ = 12.4 mPa·s (12.4 cP)" communicates more clearly than presenting a bare number.
Shear-Rate Dependence
For Newtonian fluids, viscosity remains constant with shear rate. However, many industrial fluids—polymer melts, drilling muds, blood, and food products—are non-Newtonian. When citing poise-based data, include the shear-rate at which the measurement was taken. Instruments typically record shear rate in s⁻¹; pairing this with viscosity ensures analysts converting to SI do not apply Newtonian assumptions inadvertently.
Historical Trajectory of the Poise
Jean Léonard Marie Poiseuille’s 1840s work on capillary flow established the empirical law linking volumetric flow rate to pressure drop, viscosity, and tube geometry. By the late 19th century, French and German physicists formalised viscosity measurements using capillary viscometers, adopting the CGS unit "poise" in honour of Poiseuille. The unit gained global recognition at the 1900 Paris International Congress of Physics, which codified the CGS system for scientific communication.
Throughout the early 20th century, chemical, petroleum, and biomedical laboratories standardised viscosity measurements in poise and centipoise. The automotive industry’s Society of Automotive Engineers (SAE) viscosity grades for lubricating oils originally referenced centipoise values at defined temperatures. Even after the General Conference on Weights and Measures promoted the SI pascal-second in 1946, existing standards—ASTM D445 for kinematic viscosity and ISO 3104 for petroleum products—retained centistokes and centipoise references to ease transition.
Understanding the poise’s historical dominance helps engineers interpret legacy documents, from refinery operating manuals to medical device validation reports. Converting these archives demands careful notation because early documents sometimes abbreviate centipoise as simply "c" or omit the prefix entirely.
Key Concepts for CGS–SI Interoperability
Temperature Dependence
Viscosity is highly sensitive to temperature. When reporting values in poise, include the measurement temperature with at least a tenth-of-a-degree resolution. A 10 °C increase can halve the viscosity of many oils, shifting a 1.0 P measurement to 0.5 P. Conversions to SI should preserve this context—express the value as µ = 0.5 Pa·s at 40 °C (5 P) to allow accurate interpolation in process models.
Density Linkages
Converting between dynamic and kinematic viscosity requires the fluid density. In CGS, kinematic viscosity uses the stokes (St), with 1 St = 1 cm²·s⁻¹. The relation ν = µ/ρ holds regardless of unit system, but ensuring ρ is in g·cm⁻³ when µ is in poise keeps conversions straightforward. Once in SI, express ν in m²·s⁻¹ to integrate with momentum and energy equations.
Documentation and Significant Figures
Laboratories frequently report centipoise to three significant figures. When converting to Pa·s, match the significant figure count and propagate uncertainties. A 2.135 cP measurement becomes 2.135 mPa·s; rounding to 2.14 mPa·s is acceptable if the uncertainty justifies it. Always record the instrument class—Ubbelohde viscometer, rotational rheometer, or oscillatory shear device—to support reproducibility.
Applications and Case Studies
Chemical engineers rely on centipoise inputs when calibrating process simulators for crude oil blending. Heavy crudes can exceed 5 000 cP (5 Pa·s), necessitating heating systems, drag-reducing additives, or dilution with lighter fractions. Recording both poise and pascal-second values aids cross-disciplinary teams who manage pipeline hydraulics, pump sizing, and energy accounting.
Biomedical engineers studying blood rheology often publish centipoise values at multiple shear rates to capture shear-thinning behaviour. Translating these results into SI allows integration with computational haemodynamics codes that expect Pa·s. The same attention to unit integrity benefits pharmaceutical formulation, where polymer-thickened syrups must meet regulatory specifications for filling machinery and patient acceptability.
Food scientists evaluating sauces, dairy products, or chocolate also continue to discuss viscosity in centipoise because industry benchmarks, such as the Bostwick consistometer scale, evolved alongside CGS units. When these datasets feed into energy audits or equipment selection tasks documented in SI, rigorous conversion avoids misestimates of mixer power or heat-transfer coefficients.
Worked Example: Centipoise to Pascal-Second
Imagine a process safety report citing a solvent viscosity of 2.8 cP at 25 °C and density 0.92 g·cm⁻³. Converting to SI begins by recognising that 2.8 cP equals 2.8 mPa·s, or 2.8 × 10⁻³ Pa·s. Dividing by the density expressed in kg·m⁻³ (920 kg·m⁻³) yields the kinematic viscosity: ν = 3.043 × 10⁻⁶ m²·s⁻¹. These values feed directly into the Reynolds number calculator for pump and piping assessments, keeping dimensionless parameters internally consistent.
If the same report needs to reference the original CGS unit for archival compatibility, present the result as µ = 0.028 P, ν = 0.0304 St. Including both forms prevents confusion when cross-referencing older documents or supplier datasheets.
Standards and Quality Assurance
Calibration laboratories align viscosity measurements with ISO/IEC 17025 by using standard reference materials (SRMs) traceable to national metrology institutes. SRM certificates frequently list values in both mPa·s and cP, demonstrating the enduring relevance of the poise. Quality manuals should state the preferred reporting unit and describe the conversion method to avoid transcription errors.
ASTM and ISO procedures emphasise temperature control, sample conditioning, and instrument calibration intervals. When reporting in poise, include the uncertainty budget components—temperature stability, timing accuracy, and meniscus detection—for capillary methods, or torque transducer calibration and rotational speed for rheometers. Transparent reporting preserves comparability across laboratories even when unit preferences differ.
Why Retain the Poise?
Retaining the poise acknowledges the extensive legacy data sets that underpin design correlations, empirical coefficients, and regulatory limits. Engineers often revisit decades-old reports to diagnose equipment failures or validate new products; understanding poise-based notation ensures these references remain usable.
Furthermore, communicating with stakeholders across disciplines becomes easier when you can interpret both SI and CGS forms. Suppliers may provide viscosity curves in centipoise, while corporate standards demand SI. Demonstrating fluency in both languages reduces the risk of mis-specified pumps, heat exchangers, or quality-control thresholds.
Ultimately, respecting the poise is about safeguarding traceability. By documenting conversions carefully and anchoring each dataset to physical conditions, you create a transparent path from historic literature to modern digital engineering environments.