In this type of orifice meter, there is a fixed aperture and flow is indicated by a drop in differential pressure. In area meter, there is a variable orifice and the pressure drop is relatively constant. Thus, in the area meter, flow is indicated as a function of the area of the annular opening through which the fluid must pass. This area is generally readout as the position of a float or obstruction in the orifice.
The effective annular area in area meter is nearly proportional to height of the float, plummet or piston, in the body and relationship between the height of float and flow rate is approximately linear one with linear flow curve as well as scale graduations.
Types of Variable Area Flow Meters
Area meters are of two general types :
- Rotameters and
- Piston type meter.
In this meter, a weighted float or plummet contained in an upright tapered tube, is lifted to the position of equilibrium between the downward force of the plummet and the upward force of the fluid in addition to the buoyancy effect of the fluid flowing past the float through the annular orifice. The flow rate can be read by observing the position of the float.
Piston Type Meter
In this meter, a piston is accurately fitted inside a sleeve and is lifted by fluid pressure until sufficient post area in the sleeve is uncovered to permit the passage of the flow. The flow is indicated by the position of the piston.
Fig. shows the types of Variable area flow meter (a) Rotameter and (b) Piston Type meter.
The flow rate (volume) through a variable area meter is essentially proportional to the area and, as a result, most of these meters have essentially equal-scale increments. A typical indicating rotameters scale is non linear by about 5%.
An important characteristic of the variable area meter is that the pressure loss across the float is a constant. The overall differential across the meter will increase at higher flow rates because of friction losses through the fittings.
The most common accuracy is ±2% of full scale reading. This increases considerably with individual calibration and scale length. Repeatability is excellent.
Variable area flow meters are the most commonly used means for measuring low-flow rates. Full scale capacities range from 0.5 cm3/min of water and 30 std cm3/min of air in the smallest units to over 1200 litres/min of water and 1700 m3/h of air in 8 cm height meters.
5. Minimum Piping Requirement
An area meter usually can be installed without regard to the fittings or lengths of straight pipe proceedings or following the meter.
6. Corrosive or Difficult to handle liquid
These can often be handled successfully in an area meter. They include such materials as oil, tar, refrigerants, sulphuric acid, black liquor, beverages, aqua regia and molten sulphur.
In general, if the nature of the fluid does not permit the use of a conventional differential pressure type meter because the fluid is dirty, viscous or corrosive, certain area meters have an advantage over other types of meters.
7. Pressure Drop
By placing very light floats in over sized meters, flow rates can be handled with a combination of very low pressure loss (often 2.5 cm of water column or less) and 10 : 1 flow range.