poises (P) to pounds per foot-second (lb/(ft-s)) conversion

What is the Poise?

The poise is the unit of dynamic viscosity in the centimetre-gram-second (CGS) system, measuring a fluid's resistance to shear flow. It remains common in fields such as chemistry, food science, and petroleum engineering.

Definition

One poise is the viscosity of a fluid in which a shear stress of one dyne per square centimetre produces a velocity gradient of one reciprocal second.

$$1\ \text{P} = 0.1\ \text{Pa·s}$$

In SI terms, 1 poise = 0.1 Pa·s = 1 g·cm⁻¹·s⁻¹. The everyday subunit is the centipoise (cP), equal to 0.01 poise and to 1 millipascal-second, which is why water's viscosity is famously about 1 cP.

Origin and History

The poise is named after the French physician and physiologist Jean Léonard Marie Poiseuille (1797–1869), who studied the flow of liquids through narrow tubes, work later formalized as the Hagen–Poiseuille law. The unit was adopted within the CGS system in his honour.

Law and Notable Facts

The poise is not an SI unit, but it is accepted for continued use alongside SI in many industries. Its centipoise subunit is so entrenched that viscosity tables and specifications worldwide still quote values in cP even where the pascal-second is the formal standard.

Real-World Examples and Conversions

• Water at 20 °C: about 0.01 poise (1 centipoise).
• Air at room temperature: about 0.00018 poise.
• Glycerol at 20 °C: roughly 15 poise.
• 1 poise equals 0.1 Pa·s and 100 centipoise.

What is the pound per foot-second?

The pound per foot-second (lb/(ft·s)) is a unit of dynamic (absolute) viscosity in the English absolute (mass-based) system, where the pound is treated as a unit of mass (pound-mass). It is the FPS analogue of the poise, which is the gram per centimetre-second.

Definition

One pound per foot-second is the viscosity of mass one pound-mass flowing through one foot per second:

$$1\ \text{lb/(ft·s)} = 1.48816\ \text{Pa·s}$$

The exact value comes from $1\ \text{lb} = 0.45359237\ \text{kg}$ and $1\ \text{ft} = 0.3048\ \text{m}$, giving $1\ \text{lb/(ft·s)} = 0.45359237 / 0.3048 = 1.4881639\ \text{Pa·s}$. This is the pound-mass form and must not be confused with the gravitational pound-force-second per square foot, which is about 32.174 times larger.

Origin and History

This unit arises from the English absolute (or "poundal") system of mechanics, in which mass is the base quantity and force is the derived poundal. It appears in older engineering and chemical-engineering literature, particularly British texts, as a direct mass-based counterpart to the CGS poise.

Law and Notable Facts

It is not an SI unit. A useful relationship: $1\ \text{lb/(ft·s)} = 1{,}488.16\ \text{cP}$, and the difference between this mass-based unit and the force-based lbf·s/ft² is exactly the standard gravitational conversion factor $g_c = 32.17405$ (the ratio of pound-force to poundal).

Real-World Examples and Conversions

Water at 20 °C ($0.001002\ \text{Pa·s}$) is about $6.73\times10^{-4}\ \text{lb/(ft·s)}$. A fluid of $1\ \text{lb/(ft·s)} = 1.48816\ \text{Pa·s} = 1{,}488\ \text{cP}$ is roughly as viscous as light honey. Motor oil around $0.2\ \text{Pa·s}$ is about $0.134\ \text{lb/(ft·s)}$.