Venturi Meter Performance as Compared to Flow Tube Performance
There is a very common misunderstanding that a “venturi meter” is a “flow tube” and vice versa. Nothing could be further from the truth; they each have different performance and design characteristics.
Flow Tube Performance
Popularized from about 1960 to 1975, the flow tube has a variety of brand names such as “Dall Flow Tube”, Penn Meter Tube, and Low Loss Flow Tube. It was developed as an improvement over the already established venturi meters that were called Classical long and short form venturi meters made by a number of manufacturers. There were a few claims concerning the “improvement” over the existing venturi meter technology:
- Lower head loss, stated as a percent of differential, than the venturi meter.
- Differential magnification, depending on the beta ratio of the meter, which allowed the user to go down to a lower flow rate while still maintaining the relatively high differential that the Mercury type transmitters required in order to develop a signal.
- Extremely short by comparison to the classical type venturi meter.
The Dall tube was licensed to BIF for North American distribution. The inventor, Dr. Dall, claimed its accuracy was +/-1.0% of actual rate of flow from approximately 100,000 pipe Rd and beyond. BIF developed a very good level of sales, primarily because of the points noted above. However, by about 1965, an increasing number of users were complaining that the accuracy claim of +/-1.0% was not correct. Their claims were supported by laboratory test data which showed that the Dall tube discharge coefficient was a function of flow rate, and that the accuracy was significantly poorer than was originally claimed.
Venturi Meter Performance
The venturi meter has been in service since the late 1800’s and continues to be used around the world for accurate and reliable flow measurement. The head loss is a function of the meter beta ratio, with larger throat sizes having a lower head loss but without any differential magnification. Differential magnification, while attractive at a time when the secondary instruments used Mercury as the motive mechanism for the transmitter, is no longer needed or, for that matter, desired.
Test data for the venturi meter is linear as a function of pipe Reynolds number, which is not the case with the Dall tube. The linear nature of the discharge coefficient for the venturi meter is what provides the reliable and repeatable accuracy that is the cornerstone of accurate flow measurement. While there is a low Reynolds number effect for the venturi type meter, once the linear range of the device is achieved, it remains linear. This results in the same accuracy for all flow rate levels.
In 1966, Dezsoe Halmi*, in his position as Senior Principal Engineer for BIF, began the development of his Universal Venturi Tube (UVT). By 1972, it had replaced the Dall Tube for all intents and purposes.
What to do if you have a Dall tube in service
PFS can provide you with a flow rate vs accuracy analysis for any type of flow tube. Using this as your guide, you may find that the worst-case accuracy condition may be acceptable. This depends on the specific application conditions that the meter is used for. We can help you determine if there are any physical issues that need attention, up to and including a possible rehabilitation (field or factory). Again, this depends on what level of performance you require. If a replacement meter is required, even though the basic venturi meter design lengths are longer than the Dall tube, the venturi meter length can be modified to, in most cases, fit within the available space.
*Dezsoe Halmi retired from BIF in 1980 and formed PFS in 1981