We need to measure the flow of a liquid through a pipe at work. This seems easy enough on the surface. I’m thinking…just “click” off the liquid in some measurement as it goes by…right? Well, it could be…maybe. In reality, it turns out that it may not be all that simplistic. There are other things to consider. What kind of meter should you use? There are many. What exactly are you trying to measure? What’s the density or viscosity of the liquid? Are these variable during the process? Are you actually trying to ‘regulate’ flow or ‘meter’ flow or both?
During the consideration of this task of measuring the flow of a liquid, we would need to define ‘flow’. The straight-forward definition is… the flow of material past a given location. When we dig into it further and look at actual formulas for measuring flow, it starts getting more complex.
For instance, look at this formula for ‘Flow Rate’ (FRV). This is a volume per unit time or ‘volumetric’…
FRV = V x A, where A equals the cross-sectional area of the inlet pipe and V is the average velocity of the liquid being measured. Now consider this formula in practice. We have liquid flowing through a pipe that has 2 square meters of cross-sectional area, traveling at an average velocity of 2 meters per second. So, I have a flow rate of 4 cubic meters per second.
Next, look at this formula for ‘Flow Rate’ (FRM). But this time, we will deal with ‘mass per unit time’ or a ‘mass metric’…
FRM = FD x FRV, where FD is the fluid density and FRV is our volumetric product from the first example. So, if 4 cubic meters per second of fluid is flowing through a pipe, with a density of 2 kilograms per cubic meter, your FRM is 8 kilograms per second.
Do you want to measure flow in terms of a ‘mass’ or ‘volume’? It depends on your application. In some instances the process may have a limit on the ‘volume per unit time’ measurement it can accept. Some examples might be…filling a tank, painting widgets, etc. Other processes may have a limit on the ‘mass per unit time’ it can accept…like mixing chemicals or measuring chemicals for sale.
You can see that a person can purchase a meter that measures flow in terms of volume or mass. I found that you can even buy meters that ‘infer’ volume and mass using a similar formula to ~ ‘1/2 FD x V x V’. These meters are commonly referred to as ‘head flow meters’. But, further reading indicates that this ‘inference’ approach is problematic and can introduce errors when calculating ‘mass’ flow.
Careful consideration of what you really intend to measure about ‘flow’ should lead you in the right direction when purchasing a meter. Do you want to measure velocity, mass, volume or inferential values of each?
Wade Stankich
Information Technology Manager
