What is a Rotameter or Variable Area Flowmeter ?
In the venturi meter or orifice meter (variable head meters), the area of constriction / area of the flow is constant and the pressure drop across the meter varies with flow rate / discharge, while in variable area meter, the pressure drop across the meter is constant and flow rate is a function of the area of constriction / area of flow. Any changes within flow rate can be measured in terms of change in area of flow.
The most important area meter is the rotameter or variable area meter.
Working Principle of Rotameter/ Variable Area Meter
Rotameter or Variable Area Meter operates on the principle that there is a different constriction or orifice area (flow area) for each flow rate and the pressure drop on other side of meter is constant. In other words, the pressure drop within the meter is constant and the area through which the fluid flows varies with the flow rate. These area is related, through a proper calibration, to the flow rate of rotameter or variable area meter.
Construction of Rotameter/ Variable Area Meter
Rotameter or Variable Area Meter consists of a glass tube mounted vertically with large end up. This tube contains a freely moving float. The diameter of float is smaller than diameter of tapered tube of rotameter. The density of a float material is higher than liquid. A perforated type plate is provided at the both ends of tapered tube for arresting the float within the tube. A guide is provided for the float so that it always remains at the centre within the tube along the axis of the tube. The float is usually constructed out of corrosion resistance materials such as stainless steel (SS – 304 or SS – 316), aluminium, monel, bronze, nickel and plastics. The material of float decides the flow range of rotameter.
A nearly linear flow scale is etched/marked on the glass tube or it is mounted close to the tube so that the position of the float can be marked and the flow rate is then obtained from a direct scale or a calibration curve of the flow rate can also be provided over the tapered tube or near the tube. The float is the indicating element and reading edge of float is taken at the largest cross-section of the float. The either ends of the meter may be flanged or screwed. Rotameters do not require straight runs of pipe before and after the point of installation.
For low pressure and temperature applications, glass tubes are used but for opaque liquids, high temperature and pressure applications or for other conditions where glass is impracticable (not suitable), metal tubes are used. When a metal tube is used (the float in this case is invisible) or when a liquid is very dark or dirty, an external indicator is required (for the meter reading).
Working of Rotameter/ Variable Area Meter
In the rotameter or variable area meter as the flow varies, the float falls or rises, thus changing the area of annular space between the wall and float of the tube. The area available for flow is the annular space or annulus between the float and the wall of the tube.
When no fluid flows through the meter, the rotameter float rests at the bottom of the tappered tube. But as the fluid begins to flow from the lower side of tube, then the float is rises until the differential pressure balance the weight of float and the fluid flows through the meter through the annular space. As the flow rate increases, the float rises further in the tube, thus, increasing the flow area keeping a differential pressure across it constant. On the other hand, as the flow rate decreases, the float falls in the tube, thus decreasing the flow area with constant pressure drop across it. At a given flow rate, float stabilises at a certain fixed position in the tube and at steady state it is recorded as the rotameter reading on the scale provided.
The variation of the flow area with flow rate can be measured in terms of change within the float position. A rotating motion of float helps to keep it steady.
Rotameters are widely used in the chemical industry for the measurement of flow rates of compressible and incompressible fluids. They provide a direct reading of the flow rate.
Advantages of Rotameter
- Relatively low cost
- Minimum pressure drop
- Simple in Design
- It can measure flow rates of corrosive fluids
- It can measure both Liquid or Gas Flows
- No external power needed
Disadvantages of Rotameter
- It can not be used where there is fast changes occur within measurements
- Difficult to handle the glass type
- It should be mounted only vertically
Calibration of a Rotameter
The calibration chart for a given rotameter prepared over its entire range is a relationship between the rotameter reading (i.e., height of float, float position) and the volumetric flow rate. A typical calibration chart is shown in Fig. The calibration chart as well as a proper correction for the viscosity and density of a fluid when the chart of one fluid is to be used for the other fluid may usually be supplied by a manufacturer.
For calibrating a given rotameter, the flow of the fluid (liquid e.g., water) through the meter is started by slightly opening a valve at the inlet to the meter. Time is allowed to attain a steady state and for this valve opening, the float position is noted and the liquid is collected in a measuring tank over a known period of time. The volumetric flow rate is obtained from the volume collected and time noted. This procedure is repeated for several valve positions to cover the entire range of the meter and the calibration chart is prepared.
On the calibration chart, we should provide information such as : density, temperature and name of the fluid handled.
Volumetric flow rate = Volume of liquid collected/time
