The name rotameter was chosen in the past because at one time the float was designed with slots to produce a rotational movement for the purpose of centering and stabilizing the float.
The float is free to move up or down within the tapered tube. The tube is tapered to provide linear relationship between the flow rate and the float position within the tube. The fluid enters at the bottom and passes upward around the float and out at the top. The float rises and falls depending on the flow rate, giving a variable area of the annular passage. The upward hydraulic forces that act on the float are in balance with its weight less the buoyant force; this will maintain the float position in equilibrium. The scale shows flow rate in percent.
The rotameter sizing is also based on Bernoulli’s theorem. Rotameters may be used to measure gas or liquids and even some light slurry. Therefore, the sizing where very low flows are measured, capacities are usually given in cc/minute of air or water: this rotameter equation used for the head meter also applies to the rotameter :
The same fundamental equation used for the head meter also applies to the rotameter :
Q =KA√( 2gh)
In this equation the differential pressure (h) is replaced by the factor that causes it to remain constant. This factor denotes the volume and area of the float as well as the density of the float and the fluid.
The equation will be:
Q = kAm √ ((2g Vf/Af) * (Df / (D-1))
Where:
Q = Flow Rate
K = Taper Constant
Am = Annular area measured at indicating edge
G = gravity
Df = Float Density
D = Fluid Density
The rotameter may be subjected to error due to changes in the fluid density small variations in fluid viscosity will not affect measurements if the float is designed to be insensitive to them. This will make Df/D = 2. Therefore (Df/(D-1)) = 1.
Since √1=1, the effect of density is eliminated. The float also must be designed to ignore small changes in the viscosity of the fluid. The rotameter available with a tapered glass tube or metal tube. The choice of tube depends on the application. Where toxic or flammable fluids are to be measured, a metal tube is selected for the rotameter. However in this case the float is not visible and a different indication technique is required. The linear movement of the float must be converted into a rotating movement for local indication or signal transmission. The metering float is extended with a permanent magnet on top of it. Glass tubes are limited up to use for low pressure say 20 Kg/cm2.
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