Methods to determine Coefficient of viscosity by capillary tube method

Methods to determine Coefficient of viscosity by capillary tube method

Capillary Tube Method :

 In this method we use Hagen Poiseuille law to determine the viscosity of the liquid . In this method we measure viscosity by measuring the pressure for a particular length of the capillary tube.

As shown in the below figure 

Let see how this method work. To measure the viscosity of a liquid we will first pour the liquid in a constant head tank (meaning where the head (h)  remains constant and does not change with time). Now we allow the liquid to pass through the capillary tube from the constant head tank .[Note : while doing all this we make sure temperature of the liquid is contant we are measuring viscosity at  a particular temperature]. Then we will measure the amount of liquid collected in measuring tank for a particular time. 

Here h  = Difference of pressure head for length L

Q = Flow rate in m^3

D = capillary tube diameter

L = Tube length for which we know the pressre difference

ρ = Density of the liquid

µ = Coefficient of viscosity

By using  Hagen Poiseuille's Formula we get this equation to calculate viscosity in real life 

$$\mu =\frac{\pi \rho gh{D}^4}{128QL}$$