Ultrasonic flowmeter sensors are available with wetted and non-wetted designs. Clamp-on sensors have no wetted parts, and are desirable when the fluid is corrosive, toxic, or when contamination is an issue. Clamp-on sensor installation is often more convenient and less expensive because few piping modifications are typically needed to attach the sensors to the outside of the pipe.

Clamp-on technology cannot be used to directly measure the transit time across the flowing fluid because the ultrasonic energy also takes time to travel through the pipe walls and interfaces. In addition, the angle of refraction of the ultrasonic energy at the inside pipe wall can vary with the speed of sound of the fluid. Some ultrasonic flowmeters measure the speed of sound in the fluid and perform calculations to correct the flow measurement.

Wetted sensors directly interface the sensor to the fluid to provide superior ultrasonic connections and eliminate the effects of the pipe walls and their interfaces on the measurement. Hot-tapped wetted sensor designs allow sensor installation and removal without shutting down flow. Retractable wetted sensor designs allow sensor removal without shutting down flow.

Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters.

For example, turbine flowmeters should be operated above a certain Reynolds number. Failure to do so will cause K-factor shifts that can result in measurement error. Some turbine flowmeters can use flow computer functionality to correct this error by calculating the Reynolds number in conjunction with a theoretical or experimental relationship between Reynolds number and the flowmeter K-factor. However, the correction is not necessarily performed perfectly because there is an uncertainty associated with the parameters used to calculate Reynolds number as well as an uncertainty associated with the K-factor calculation.

There are additional parameters that are typically ignored but should be considered. Process and ambient temperature can have a profound effect on the quality of measurement. For example, a flow transmitter designed to operate in ambient temperatures up to 60ºC may operate properly in a boiler house during the winter when the temperature is 30ºC. However, it can operate sporadically in the summer when the ambient temperature approaches 70ºC in an enclosed space near the steam pipes in the middle of the afternoon on a sunny day.

Ambient conditions can also profoundly affect flowmeter operation. In one such instance, a flowmeter operated properly during the winter but would suddenly measure zero flow in the middle of the afternoon on warm summer days. The cause of the problem was traced to the sun warming the pipe to the extent that the liquid would vaporize and cause the flowmeter to stop operating. This was a major issue because zero flow would cause the reactor to trip. By the time the reactor was restarted a few hours later, the ambient temperature was lower and the flowmeter operated properly (until the next warm day).

Even the electrical power supply can cause problems albeit not often. What if the power supply voltage is outside of its specifications or varies? Flowmeter operation and performance can suffer. Did you ever calibrate a low range differential pressure transmitter outdoors in the sun and then notice a calibration shift a few minutes later when a cloud shaded the transmitter?

In general, we tend to concentrate on how the process affects flowmeter operation in the context of a given flowmeter technology. However, we should not forget that other seemingly trivial factors can play havoc with the measurement.

This article originally appeared in Flow Control magazine. 

1. 20 percent
2. 30 percent
3. 50 percent
4. 100 percent
5. Any of the above

Fully developed flow in the pipe will exhibit a velocity profile that is not flat, that is, the fluid velocity at different locations in the pipe will be different. Therefore, Answer E is not correct.

The velocity profile has the boundary condition that the fluid velocity at the pipe wall is zero. The fluid velocity will increase as the sensor is moved away from the pipe wall. The maximum velocity will occur at the center of the pipe where the effects of the pipe wall are low. Therefore, the location where the fluid velocity is at its maximum is the center of the pipe that is 100 percent of the pipe radius from the pipe wall. Answer D is correct.

Additional Complicating Factors

The effects of a distorted velocity profile can be profound --- even when the sensor is properly located. Locating the sensor without sufficient upstream and downstream straight run can distort the velocity profile and cause the flow measurement to exhibit exorbitant errors. Locating the sensor in a velocity profile that is jetting due to a partially open valve or similar restriction poses particular problems for insertion flowmeters because only one or (at best) a few measurements are used to infer relatively large flow rates. 

This article originally appeared in Flow Control magazine.