DAVID W SPITZER'S E-ZINE (August 2023)

The frequency of the pulses is measured, and the following equation expresses their relationship to flow rate.

Phase shift ultrasonic flowmeters measure the phase shift between two continuous cyclic ultrasonic signals, one traveling upstream, and the other traveling downstream. The following equation describes the relationship between phase angle and flow rate when the frequency of the cyclic ultrasonic signal is constant.

The following equation describes the relationship between the phase angle and flow rate when the frequency of the cyclic ultrasonic signal is varied to control the phase angle difference.

Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters.

Analog signal transmission utilizes a continuous signal such as 4 to 20 milliamps or 0 to 1000 Hertz to represent 0 to 100 percent of full scale (although some transmitters will produce a signal slightly in excess of full scale). The maximum signal is limited to the transmitter full scale, so the transmitter full scale is typically selected to ensure that the maximum measurement (perhaps under extreme conditions) does not exceed the full scale represented by 20 milliamps or 1000 Hertz. As a result, the transmitter tends to operate relatively low in its operating range where its accuracy is typically degraded. In addition, totalizing continuous signals such as 4 to 20 milliamps usually requires a signal conversion that further degrades accuracy.

Process measurements in excess of the full scale will measure as if the process measurement were at full scale, so flows rates above full scale flow will totalize less than the actual flow. Further, process control system would operate as if the flow were at full scale even though the actual flow could be two or three times higher. This inaccurate information could cause an unsafe or abnormal process condition. It should be noted that some transmitters are capable of producing signals above full scale (typically approximately 10 percent) to mitigate some of these issues.

On the other hand, digital signal transmission utilizes a digital number to represent the measurement. In doing so, the transmitter can often be calibrated at a lower full scale that better represents its typical operation. Calibrating at a lower full scale can result in a measurement that is more accurate. Occasional measurements above full scale are not limited to a maximum signal such as 20 milliamps or 1000 Hertz so the process measurement will be transmitted with reasonable accuracy. In addition, the process control system (and operator) will have better information with which to make decisions.

Note that while the use of digital communications can reduce the calibration range of the transmitter, a different transmitter with a lower upper range limit should not necessarily be selected. This is because the process measurements are limited to the upper range limit of the transmitter so the actual measurements should not exceed this limit.

This article originally appeared in P. I. Process Instrumentation magazine. 

1. Coriolis and vortex shedding
2. Coriolis, vortex shedding, and ultrasonic (wetted sensor)
3. Coriolis, vortex shedding, and target
4. Positive displacement and variable area
5. None of the above

The statement of this problem can be interpreted and strictly parsed to allow all of these flowmeters to be classified as flowmeters with moving parts. For example, the tubes in a Coriolis mass flowmeter vibrate. Therefore, the vibrating tube could be considered to be a moving part and the flowmeter could be considered to have a wetted moving part. Forces generated by the vortices in a vortex shedder cause the bluff body to move (micro-inches) so this flowmeter could similarly be classified as a flowmeter with wetted moving parts. Similarly, wetted ultrasonic sensors also vibrate and the obstruction in a target flowmeter will move slightly.

That said, common usage of the term "moving parts" refers to actual parts of the flowmeter that physically move. In this sense, none of the parts in the Coriolis, vortex shedding, ultrasonic (wetted sensor), or target flowmeters have parts that move... although they may vibrate. Therefore, Answers A, B and C are not correct in this context.

Positive displacement flowmeters have physical parts such as gears, rotors, and other mechanisms that physically move. The floats in variable area flowmeters similarly move to indicate flow. Answer D is correct.

Additional Complicating Factors

There are variants within each technology that often limit generalizations. Note that in this case, ultrasonic flowmeters were limited to ultrasonic flowmeters with wetted sensors. Ultrasonic flowmeters with non-wetted sensors fall into an entirely different category with other flowmeters with no wetted parts.

This article originally appeared in P. I. Process Instrumentation magazine.