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.

Flow	Flow Error
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.

The vapor pressure of a liquid is the pressure at which that liquid will boil. Operating under vacuum conditions --- below atmospheric pressure --- can cause ordinary liquids such as water to boil at room temperature. Processes often use this phenomenon to an advantage when drying heat-sensitive products by removing, say, water under a vacuum at room temperature instead of potentially degrading the product by heating it to over 100 degC under atmospheric conditions.

Systems operating under a vacuum are more likely than a pressurized system to inadvertently operate below the vapor pressure of the liquid. This is because liquids in many process vessels in vacuum processes operate at or near their vapor pressure such that a small pressure reduction, such as piping losses, or a small temperature increase can cause the liquid to boil.

The installation of every instrument in vacuum service should be analyzed in detail to determine if a problem could occur. For example, measuring the level of a tank operating below atmospheric pressure using a level probe installed in an external stilling well might seem normal. However, ambient conditions may make the stilling well slightly warmer than the liquid in the tank --- operating at the vapor pressure of the liquid --- and cause the liquid in the stilling well to boil, adversely affecting the level measurement.

Read more on this topic next month.

This article originally appeared in Flow Control magazine at www.flowcontrolnetwork.com.

1.  0 diameters upstream and 0 diameters downstream

2.  5 diameters upstream and 3 diameters downstream

3. 10 diameters upstream and 5 diameters downstream

4. 40 diameters upstream and 5 diameters downstream

   Commentary

   Responses from students in my Industrial Flow Measurement seminar (and many others) would indicate that the rule of thumb for installation is 10 diameters of straight run upstream of the flowmeter and 5 diameters of straight run downstream of the flowmeter so it would seem that Answer C would be correct. 

   However, there is no valid rule of thumb for installation because every flowmeter has straight-run requirements that result from multiple considerations, including the flowmeter technology, flowmeter design and the geometry of the piping in which the flowmeter is installed.

   All of the answers can apply to one or more flow technologies and/or flowmeter designs.

   Additional Complicating Factors

   The ramifications associated with not having one rule of thumb are many. Consider that many flowmeters are installed in locations where limited space does not allow for the required amount of straight run. In such instances, flowmeter performance would be degraded so another technology and/or flowmeter with shorter straight-run requirements should be applied.

   This article originally appeared in Flow Control magazine at www.flowcontrolnetwork.com.