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DAVID W SPITZER'S E-ZINE (April 2021)

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Differential Pressure Flowmeter (Part 2 of 4) by David W Spitzer

The upstream and downstream pressures associated with a differential pressure primary flow element are available at the taps of the element. Both of these taps are piped to ports on the differential pressure flow transmitter that measures the pressure drop. The differential pressure flow transmitter is a device that converts the differential pressure across its ports into an analog signal. When the differential pressure flow transmitter has an integral square root function, its output signal can be linear with flow rate.

Some (multivariable) differential pressure flow transmitters can make multiple measurements, such as differential pressure, pressure and/or temperature, from which the flow rate can be calculated. These transmitters are outside the scope of this report.

In general, wetted parts (such as diaphragms) that are in contact with the fluid provide movement or force that is related to the differential pressure across the transmitter pressure ports. Differential pressure flow transmitters have been designed using many technologies including, capacitance, differential transformer, force balance, piezoelectric, potentiometer, strain gage, and vibrating wire.

The quality of the differential pressure flow transmitter signal can be described by its performance specifications. Therefore, given performance specifications, the (internal) sensing technology and details of (internal) operation are not overly important considerations when considering the purchase of differential pressure flow transmitters (although some sensing technologies may have superior reputations).

Due to the nature of the nonlinear relationship between flow and differential pressure, relatively small differential pressure changes can result in relatively large flow changes at low flow rates. To reduce the noise associated with the flow signal at these flows, some differential pressure transmitters force their output signal to zero flow when the signal falls below a certain (preset) differential pressure. Some transmitters with integral square root extraction use a linear relationship between flow and differential pressure below a certain (preset) flow rate. Other algorithms may be available to stabilize the output signal at low flow rates.

Excerpted from The Consumer Guide to Differential Pressure Flow Transmitters

Hone Your Credibility by David W Spitzer

A previous Application Corner column alluded to how you may be an expert in your plant or organization. Large organizations - and even a few select plants - can afford to have a flow measurement expert. However, operating plants and organizations of all sizes need this expertise, albeit in lesser amounts on projects of great relative importance. The expectations of management and a path to becoming the flow measurement expert at your plant or organization are quite similar to how lawyers approach hiring an expert witness.

Plaintiffs and defendants expect their legal experts to know everything about their field of expertise. It should come as no surprise that your management has similar expectations. Even the best experts can have difficulty meeting this high standard. However, there are certain prerequisites that clients seek before hiring an expert, depending upon the case at hand.

Almost everyone hiring an expert will investigate their credibility. After all, the expert will likely be asked to testify in court, and it is important for the expert to be able to articulate complex issues in simple terms to a jury of people that have little or no technical background. A sense of this ability can be obtained in a telephone interview, but the expert's background and experience can be even more important. In particular, writing books and articles as well as having decades of relevant experience and teaching the subject at hand are fundamental for the testimony to get the proper attention and be believed by a judge or jury.

On the other hand, coincidence can play a role in being hired as an expert. In one such case, I was hired virtually on the spot after the lawyer discovered that I worked on a case in which the lawyer was briefly involved at another firm. We knew the same people and it was a natural fit. Providing a curriculum vitae was a little more than a formality for presentation to the client.

Finding meaningful work is not easy and typically requires years of education and experience. The same is true in your plant or organization. What have you done to promote your background and experience?

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

Level Control of Process Tank Manipulating Outflow with a Control Valve by David W Spitzer

It is desired to control the level in a process tank where the flow out of the tank is measured and manipulated with a control valve. Which of the following control strategies could be used?

ABOUT SPITZER AND BOYES, LLC

In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 500 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.

Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.