By David W. Spitzer, P.E.

The relationship between users and suppliers is often an adversarial one, which is based on the inherent characteristics of each party. The user has money that the supplier wants, and the supplier offers equipment and services that the user wants. The user wants to pay as little as possible for the supplier ”s equipment and services, while the supplier wants the user to believe that their equipment and services are nothing but the best.

The technical issues involved in this relationship are a bit more complex, at least as they pertain to the user-supplier relationship in the fluid handling industry. The specifications associated with flowmeters can be used to illustrate this point.

In simplistic terms, users want flowmeter specifications so they can determine how the flowmeter will perform under operating conditions. The supplier has little choice but to supply the requested specifications. However, the amount of information disclosed and its detail may be controlled to put the supplier”s equipment in a favorable light. To avoid being manipulated by such an error of omission, the user should have already done his/her homework prior to discussions with the supplier.

For example, consider the specifications associated with Coriolis mass flowmeters. Many suppliers will tell users that Coriolis mass flowmeters are extremely accurate and exhibit a wide turndown. When asked for the accuracy specification, most vendors will respond with a statement of between 0.1 to 0.5 percent of rate. This response (especially if it is 0.1 percent of rate) will tend to put the supplier”s flowmeter in a favorable light because it is a percent of rate accuracy.

Turndown specifications may be quoted as 100-to-1 or higher, which is significantly superior to the turndown associated with most flowmeter technologies. The implication is that the 0.1 to 0.5 percent rate accuracy is applicable over the entire 100-to-1 range of flows. This also tends to put the supplier”s flowmeter in a favorable light.

The conversation about performance can end here if the user does not know that the accuracy of a Coriolis mass flowmeter is this percentage of rate plus its zero stability. When prompted, suppliers often shed little light on zero stability by stating that it is small and has a relatively small effect on performance. This may be valid for high flowrates, yet far from valid at lower flowrates where the zero stability dominates the accuracy specification.

For example, Coriolis mass flowmeters with accuracies of 0.1 percent to 0.5 percent of rate sized for 0-100 kg/min had accuracy statements of between 0.11 and 1.7 percent of rate at 10 percent of flow (10 kg/min) and approximately 1 percent to 16 percent of rate at 1 percent of flow (1 kg/min). About half of these Coriolis mass flowmeters have accuracies in the range of 3 percent to 5 percent of rate at 1 kg/min, even thought their rate accuracy is specified as between 0.1 percent and 0.2 percent of rate.

The point here is that the user should investigate technology prior to speaking with suppliers. On the other hand, suppliers should be knowledgeable of the complete specifications for their equipment.

Many instruments are subject to errors of omission with regard to their performance specifications — even common instruments that you may have been purchasing for years. In order to effectively communicate, both the user and supplier should be at the top of their games.

About the Author
David W. Spitzer, P.E., is a regular contributor to Flow Control . He has more than 25 years of experience in specifying, building, installing, start-up, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.

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