What is a rotameter?

A rotameter is a floating flow meter with a variable cross-section for measuring the flow rate of liquids and gases. This device has a very wide range of applications in various industries, engineering, laboratories or the energy sector.

Depending on the material of manufacture, we distinguish between:

• glass tube rotameters

• plastic rotameters

• metal rotameters

Is a rotameter a flow meter?

Yes, a rotameter is a commonly used variable area flow meter that measures the volumetric flow rate of liquids and gases. There is no difference between a rotameter and a flow meter, and the terms are used interchangeably. The flow measurement technique is implemented by a free-moving float that is balanced in a tapered tube.

Rotameter – construction

A rotameter has the shape of a vertical, tapering glass tube. A linear scale is drawn on the wall of the glass tube, described in units of flow rate. Inside this flowmeter in the tube is a float with a weight selected so that it sinks in the non-moving medium. Float is pushed up by the drag force of the flow and pulled down by gravity. The rotameter has process connections on both sides.

How does a rotameter work? Working principle of rotameter

A fluid flows through that variable area flow meters, which causes changes in the position of the float placed inside the device. The float floats until it reaches the so-called force equilibrium point, i.e. a state in which all the forces acting on it are balanced:

1. the force of gravity of the float ↓

2. frictional force of the flowing medium on the lateral surface of the float ↑

3. buoyancy force

The result can be read off the scale in the case of mechanical rotameters. Nowadays, however, it is more common to find those generating an electrical signal that can interact with the rest of the automation system.

An alternative to the rotameter – the ultrasonic flow meter

An alternative to the rotameter that is definitely worth considering is the ultrasonic flow meter. Both devices are used to measure the flow rate of liquids, however, the ultrasonic flow meter has several additional advantages that give it an advantage over the rotameter.

Ultrasonic flow meter – operating principle

Ultrasonic flowmeters are modern flowmeters whose operating principle is based on the passage of an ultrasonic wave through a liquid or gas stream.

What is difference between rotameter and flow meter?

This wave is emitted alternately between two measuring sensors mounted on the pipeline. The difference in signal transit time is proportional to the total liquid flow rate. The ultrasonic method therefore measures the difference in elapsed time between the sending of the signal from the first to the second sensor.

Ultrasonic flow meter – advantages

The advantages that ultrasonic flowmeters have over rotameters include:

• thanks to ultrasonic technology and the lack of moving parts, no wear and tear and no maintenance is required

• high measuring accuracy

• lighter construction

• fully electronic

• better for large pipelines – the larger the pipeline, the better the signal propagation

• unlike rotameters, ultrasonic flowmeters can be used to measure highly viscous liquids. In a rotameter, the viscous fluid settles on the float, forming a layer on the float which significantly distorts the measurement results.

• the ultrasonic flow meter is also suitable for measuring waste water.

The use of a rotameter in some solutions may be supported by its low price, making it a more economical solution.

Ultrasonic flowmeters vs. rotameters – summary

In most applications, a rotameter is worth replacing with a cost-effective, electronic and lightweight ultrasonic flowmeter.

Some of the most modern ultrasonic flowmeters are our flowTRANS US series products. We have recently extended our range with a new product - the JUMO flowTRANS US W02 ultrasonic flow meter equipped with a display and Bluetooth interface. Check it out!