# Measure visocity of water by poises

1. Dec 10, 2005

### MathematicalPhysicist

i read that you measure visocity of water by poises (sounds like poison ), how does it equate with the quantities?
i.e, like [N]=[kg*m^2/sec]

2. Dec 10, 2005

### FredGarvin

For most applications, the standard is in centipoise.

1 cp = .001 Pa*sec or 1.45 x10^-7 Lbf*sec/ft^2 for us non SI'ers.

BTW...a newton is $$\frac{kg*m}{sec^2}$$

3. Dec 14, 2005

### MathematicalPhysicist

Pa*sec is pascal times seconds, right?
so visocity of liquids is actually the atmospheric presure against time, or in other words how do you explain to a layperson what visocity of liquids is?

4. Dec 14, 2005

### FredGarvin

In the most basic sense: Viscosity = resistance to flow. If you want to go a bit farther, viscosity is a measure of the ability of a fluid to resist shear stress.

You have to be careful in interpreting the units for viscosity. The pressure is not atmospheric pressure. It is shear stress. The mathematical relation for viscosity is:

$$\tau = \mu \frac{du}{dy}$$

Where:
$$\tau$$ = shear stress
$$\mu$$ = viscosity
$$\frac{du}{dy}$$ = velocity distribution

5. Dec 14, 2005

### Staff: Mentor

http://en.wikipedia.org/wiki/Viscosity
Wikipedia

Viscosity is commonly perceived as "thickness", or resistance to pouring, however it really describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Adapted from Wikipedia.

Solids exhibit viscosity when subject to very high compressive or tensile stresses, e.g. extrusion or other forms of cold or hot working.

The pressure is not necessarily atmospheric pressure, but applied pressure, and this more a case that the term is in units of pressure. This expression refers to kinematic viscosity $\nu$ vs dynamic viscosity $\mu$, which have the following relationship

$\nu$ = $\mu$/$\rho$