What is a Rotameter and how does it work?

31st May 2020

A rotameter is a flow meter device that measures the volumetric flow rate of a gas or fluid. It consists of a tapered tube with a moving internal float. Sometimes known as mechanical flow meters, gravity flowmeters or variable area flowmeters. The gravity term relates to the fact the rotameter needs to be installed vertically, as the flow rate, is read from the float position in the tapered tube, determined by the balance of forces underneath the float from gravity.

The metering tube starts with the smaller taper at the bottom and expands out to the top, with the scale graduations on the tube. With no flow, the float sits at the bottom of the tube, as the fluid flow increases the float begins to rise until an equilibrium is maintained with the viscous forces and gravity.

Calibration: Rotameter calibration is undertaken for a given gas or fluid at a given set of conditions. These conditions are normally written on the side of the flowmeter, along with flow range and measurement units. When using rotameters it is always recommended to correct the flow tube readings to any changes in flow conditions. Manufacturers often detail the required corrections for their meters, but this does not give a true reflection across different manufactures. Calibration of air flow rotameters can be completed to ISO 17025, UKAS accredited and standard traceable to national standards within the YCL Laboratory.

What can effect the accuracy of a rotameter: As rotameters are only designed for a single flow condition, many situations can give rise to additional errors in measurement flow when using a rotameter, such changes in temperature, pressure, humidity, viscosity, gas or fluid types, vibration, installation misalignment, dirt, float damage, pulsations, instability with float and unknown reference conditions. Normal accuracy of a rotameter varies between 1 10% of FSD and often they only have a limited turndown ratio of 10:1.

Nevertheless, rotameters are simple, low cost devices that can provide very good flow measurement solutions for a great deal of applications.