Most rotometers are made of glass with markings on the outside so that flow readings can be taken visually, similarly to flow indicators; however, metal rotometers are also available.
Rotometers are advantageous due to the simplicity of the device and the constant pressure drop they offer. Also, rotometers do not require straight pipe runs for installation so installation can be performed almost anywhere.
The selection of the appropriate rotometer for a particular application requires attention to a few key details. These include which gas will be used, the required flow rate, the number of streams to be measured and whether a metering valve is included.
Rotometers are favorable for their simplicity, versatility, accuracy, and ease of use. For all these reasons, rotometers are the flow meter of choice for many industries.
Rotometers consist of a float that moves vertically through a slightly narrowing tube. As fluid enters the bottom of the rotometer, the float is forced upward until the force is balanced by gravitational forces.
Like peak flow meters, graduations are marked on the tube so that the level of the float can be aligned with a gradient to provide an accurate, fast and easy-to-understand flow rate reading.
Rotometer tubes are read directly in standard cubic feet per hour of air. Rather than using a constant restriction area and a variable pressure differential, rotometers use a variable restriction and a constant pressure differential to measure flow.
Rotometers may be designed to measure the flow rate of single or multiple streams. Single tube rotometers are usually mounted directly on the piping and they are used with non-corrosive materials.
Single tube rotometers are used if there are special conditions for the measurement or when a basic flowmeter will suffice. Multiple tube rotometers are mounted to a tripod base plate with feet to level the assembly. They may also be bench or wall mounted.
This kind of rotometer takes measurements for flow rates of more than one stream of gas or liquid. Other variations are also available.