what is the difference between venturi and orifice meters?
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Answer Posted / karthik
1. The orifice plate can easily be changed to accomodate
widely different flow rates, whereas the throat diameter of
a venturi is fixed, so that its range of flow rates is
circumscribed by the practical limits of Dp.
2. The orifice meter has a large permanent loss of
pressure because of the presence of eddies on the downstream
side of the orifice-plate; the shape of the venturi meter
prevents the formation os these eddies and greatly reduces
the permanent loss.
3. The orifice is cheap and easy to install. The venturi
meter is expensive, as it must be carefully proportioned and
fabricated. A home made orifice is often entirely
satisfactory, whereas a venturi meter is practically always
purchased from an instrument dealer.
4. On the other hand, the head lost in the orifice for
the same conditions as in the venturi is many times greater.
The power lost is proportionally greater, and, when an
orifice is inserted in a line carrying fluid continuously
over long periods of time, the cost of the power may be out
of all proportion to the saving in first cost. Orifices are
therefore best used for testing purposes or other cases
where the power lost is not a factor, as in steam lines.
5. However, in spite of considerations of power loss,
orifices are widely used, partly because of their greater
flexibility, because installing a new orifice plate with a
different opening is a simpler matter. The venturi meter can
not be so altered. Venturi meters are used only for
permanent installations.
6. It should be noted that for a given pipe diameter and
a given diameter of orifice opening or venturi throat, the
reading of the venturi meter for a given velocity is to the
reading of the orifice as (0.61/0.98)2, or 1:2.58.(i.e.
orifice meter will show higher manometer reading for a given
velocity than venturi meter).
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