Poise
Definition and meaning of Poise in chemistry.
Poise is an older scientific unit used to measure the dynamic viscosity of a fluid. It mathematically quantifies exactly how much a moving liquid or gas resists flowing forward. The unit belongs to the older centimeter-gram-second metric system previously favored by early scientists.
In more detail
The poise takes its name from the French physician Jean Léonard Marie Poiseuille. He pioneered the mathematical study of human blood flowing through narrow microscopic capillaries. Before the modern SI system became standard, analytical chemists widely used the poise.
One poise officially equals the resistance of one gram per centimeter per second. In the modern SI system, scientists measure dynamic fluid viscosity in pascal-seconds. One original poise converts exactly to 0.1 pascal-seconds of dynamic viscosity.
While formal scientific publications prefer the modern pascal-second, the poise remains surprisingly common. You will frequently encounter it in older chemistry literature and specialized chemical engineering fields. Industrial chemists still use it when formulating thick liquid products like paints and motor oils.
The most practical derivative of this fundamental unit is the useful centipoise. One centipoise mathematically represents exactly one-hundredth of a standard fluid poise. This smaller metric unit perfectly fits the physical viscosity of many everyday laboratory liquids.
Students often struggle to visualize what different numeric viscosity values actually mean in reality. A high poise value indicates a very thick and slow-moving fluid like cold honey. A low poise value describes a thin and easily poured fluid like liquid hexane. Understanding these historical units helps modern chemists accurately read decades of valuable scientific data.
Key facts
| Field | Physical Chemistry |
|---|---|
| Measured Property | Dynamic viscosity (fluid resistance to flow) |
| Standard Symbol | P (for poise) and cP (for centipoise) |
| CGS Definition | 1 gram per centimeter per second |
| Modern SI Equivalent | 0.1 pascal-second (Pa·s) |
| Convenient Reference | Pure water at 20 degrees Celsius equals approximately 1 centipoise |
Pure liquid water at 20 degrees Celsius possesses a dynamic viscosity of approximately 1.0 centipoise. This exact numeric value equals 0.01 poise or 0.001 pascal-seconds in the modern SI metric system. This incredibly convenient whole number makes pure water an excellent reference point for laboratory measurements. Chemists easily compare the flow rate of new chemical mixtures against this standard watery baseline. For comparison, a typical olive oil measures around 80 centipoise at normal room temperature.
Frequently asked questions
How does a standard poise relate to a centipoise?
A centipoise mathematically represents exactly one-hundredth of a full poise. Chemists strongly prefer the centipoise because it creates convenient whole numbers for common liquids like water.
Why do scientists still use poise instead of modern pascal-seconds?
Many manufacturing industries have used poise for decades to classify heavy oils and thick paints. Keeping the older unit avoids expensive mistakes when engineers read historical data tables.
What does a high numeric poise value physically look like?
A fluid with a high poise value strongly resists flowing or pouring out of a container. Cold honey and thick molasses have much higher poise values than pure water.