Biomedical Engineering Reference
In-Depth Information
FIGURE 2-63
Flow measurement
using a variable-area
flowmeter
(rotameter).
Variable-area meters, often called rotameters, consist of a tapered tube and a float,
as shown in Figure 2-63. Although classified as differential pressure units, they are, in
reality, constant differential pressure devices. Flanged-end fittings provide an easy means
for installing them in pipes. When there is no liquid flow, the float rests freely at the bottom
of the tube. As liquid enters the bottom of the tube, the float begins to rise. The position
of the float varies directly with the flow rate. Its exact position is at the point where the
differential pressure between the upper and lower surfaces balances the weight of the float.
Because the flow rate can be read directly on a scale mounted next to the tube, no
secondary flow-reading devices are necessary. However, if desired, a sensor can be used
to measure the float's level and transmit a flow signal.
2.4.13.6 Turbine Flowmeters
Many medical appliances still use the basic turbine or vane to measure flow because if
properly installed and calibrated they provide the highest accuracies attainable for any
currently available flowmeter for both liquids and gases. The unit consists of a multiple-
bladed rotor mounted with a pipe perpendicular to the liquid flow. The rotor spins as the
liquid passes through the blades, as shown in Figure 2-64. The rotational speed is a direct
function of flow rate and can be sensed by magnetic pickup, photoelectric cell, or gears.
Alternatively, a tachometer can be used. The number of revolutions or electrical pulses
counted for a given period of time is directly proportional to flow volume. Turbine meters,
FIGURE 2-64
Turbine flowmeter.
