E-Zine December 2005
Differential pressure flow transmitters are used in conjunction with various
primary flow elements to measure the amount of fluid transferred in pipes. In general, the literature
can leave one with the impression that most of the flow measurement errors result from the primary flow
element. Which device introduces more error --- a Venturi flow element with an accuracy of 0.50% or a
differential pressure flow transmitter with an accuracy of 0.10%? If you answered the question, your
answer is likely incorrect.
For starters, you may have noticed that the question itself is incomplete. The
percentage associated with the accuracy of the Venturi is a percentage of the actual flow rate while
making the assumption that the Venturi is hydraulically operated in the properly manner. The percentage
associated with the differential pressure flow transmitter can be a combination of a percentage of set
span (typically full scale), calibrated span, and upper range limit. Therefore, these percentages are
percentages of different quantities, so they cannot be directly compared. In other words, 0.10% is not
necessarily better than 0.50%.
Because the differential pressure flow transmitter error is a combination of fixed
percentages of fixed values, the transmitter error can be calculated and is also fixed. This transmitter
error can be expressed as a percentage of the flow rate that is not fixed, but rather increases as
the flow rate decreases. Note that the flow error calculation is complicated by the nonlinear
relationship between flow rate and the differential pressure produced by the Venturi, so the
transmitter error expressed as a percentage of flow rate is different at each flow rate.
Therefore, at a given flow rate, the calculated transmitter error can be
compared with the Venturi error. In general, the error associated with the Venturi tends
to dominate high in the flow range while the transmitter error tends to dominate lower in the range.
Further, the contribution of the transmitter can be surprising large. For example, according to The Consumer Guide to Differential Pressure Flow Transmitters, the flow error associated with the best
differential pressure transmitter with a 0-250 mbar (0-100 inch WC) span is tabulated below that
illustrates the increasing domination of the transmitter error as flow decreases.
|50 % of full scale flow
||0.23 % of flow rate
|30 % of full scale flow
||0.64 % of flow rate
|10 % of full scale flow
||5.90 % of flow rate
Part 2 will discuss transmitter accuracy in more detail.
Click here to go to Part 2