By Jesse Yoder, PhD, Flow Research, Inc.
When they were first made, many Coriolis flowmeters
consisted of a single tube. Before long
it was discovered that Coriolis flowmeters work better with two tubes. A “flow splitter” is used to divide the flow
in two, and it flows through two parallel tubes.
Two fathers of Coriolis flowmeters, Jim Smith and Don Cage,
recognized in one of their original patents the possibility that the tubes
might not split the flow exactly in half.
They make the following comment in their January 1, 1985 patent:
in making fluid mass flow rate measurements with such a parallel path flow
meter will be dependent on both the accuracy with which fluid flow is evenly
divided between the two U-shaped flow tubes, and the equivalence of the angular
velocities with which the two U-shaped flow tubes are sinusoidally driven. For,
if a higher mass flow rate is passing through one U-shaped flow tube than the
other, the U-shaped flow tube conducting the higher mass flow rate will
generate larger Coriolis force effects.
Later in the paragraph they say
that in one example this would have only a one percent effect on uncertainty.
I have seen the splitter
in Coriolis meters and I have always wondered what guarantees that the flow is
evenly split. Yet I am always told that yes, the flow is always split evenly
between the two tubes. Has anyone tested
or proved this?
Here are several factors that could
potentially cause the split to be uneven:
Flow profile. We all know that flow is fastest thru the center of the
pipe. But is this perfectly symmetrical? What if the side of one pipe has some
buildup near the point it feeds into the splitter. Couldn''t this cause the
fastest point to be slightly off center and thus to favor one tube over the
2. What if the
machining on the splitter is slightly off? Couldn''t this
impact the even splitting of the fluid?
3. What if the
machining of the two tubes isn''t exactly identical?
Couldn’t this cause the mass in one tube to be greater than another?
4. Could the
presence of particles or entrained air in the fluid have an
impact on the mass in the two tubes if they are unevenly distributed?
these issues could be taken care of in the calibration of the meter. But I think this is an issue that is worth
addressing for anyone who is interested in the accuracy of Coriolis flowmeters.