Page 29 - Instrumentation and Measurement

The pressure difference is measured between a point equal to the diameter of the tube upstream of
the orifice and a point equal to half the diameter downstream. The orifice plate has the usual non-
linear relationship between the pressure difference and the volume rate of flow. It is simple, reliable,
produces a greater pressure difference than the venturi tube and is cheaper but less accurate, about ±
1.5%. It also produces a greater pressure drop. Problems of silting and clogging can occur if particles
are present in liquids.

FIGURE 2.24 (A) Dall Tube, (B) Orifice plate.

The rotameter (Figure 2.25) is an example of a variable area flow meter; a constant pressure
difference is maintained between the main flow and that at the constriction by changing the area of
the constriction. The rotameter has a float in a tapered vertical tube with the fluid flow pushing the
float upwards. The fluid has to flow through the constriction which is the gap between the float and
the walls of the tube and so there is a pressure drop at that point. Since the gap between the float and
the tube walls increases as the float moves upwards, the pressure drop decreases. The float moves up
the tube until the fluid pressure is just sufficient to balance the weight of the float. The greater the
flow rate the greater the pressure difference for a particular gap and so the higher up the tube the float
moves. A scale alongside the tube can thus be calibrated to read directly the flow rate corresponding
to a particular height of the float. The rotameter is cheap, reliable, has an accuracy of about ± 1% and
can be used to measure flow rates from about 30x10-6 m3/s to 1 m3/s.

FIGURE 2.25 Rotameter.

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