Coriolis meters: how they work
When explained clearly, Engineers will have no intellectual problems understanding the physics behind coriolis mass flow measurement.
Engineers are more often than not, hands-on people who like to touch and feel something, and see it work, for a more intuitive understanding.
Seeing is believing.
The Coriolis Effect
Here is a simple experiment that tells you everything you need to know about a coriolis meter and which will help us understand the approach to multi-functionality.
Do this now, then come back and read some more.
[ol][li]Go and find a garden hose. [/li]
[li]Set the flow running quite fast. [/li]
[li]Now make a generous loop in the hose and hold it loosely in both hands where the hose ends cross. [/li]
[li]Get your assistant to take the bottom of the loop in two hands and raise it till the loop is horizontal. [/li]
[li]Now ask your assistant to let go of the loop cleanly i.e. without imparting any twist, while you continue to hold the other side of the loop. The loop will now swing back toward the vertical. [/li] [/ol]
Does it twist as it goes?
Does the twist accentuate as the loop accelerates?
ThatÆs it.
Hang on, what about frequency?
What about it? Frequency has nothing to do with the coriolis effect.
Well O.K. let us see where frequency enters the picture:
Before we let go of the hose, let us try one more experiment and see what else we can deduce:
[ol][li]Send your assistant to stop the flow through the loop, let the loop hang vertically [/li]
[li]Now start to swing the loop gently backwards and forwards. [/li]
[li]Now get your assistant to turn on the water back on and watch what happens. [/li][/ol]
Does the loop twist?
Adjust the flow rate.
What happens to the twist?
The physics will confirm that it is the mass rate that is important, rather than the volume rate.
Incidentally, you might notice that with no flow, your loop isnÆt quite flat. This is the zero error and this initial twist can vary if the manufacturer is not careful. Early meters required periodic measurement of the zero flow twist to compensate the phase angle measurement.)
We can see that the meter works, and how it works. The physics will explain why. If you want the physics, visit the manufacturers web sites.
This description is one of the most clean and comprehensible: (http://www.automationtechies.com/sitepages/pid1353.php)
Summary:
For a given tube:
[ul square] [li]the amount of twist is a function of the flowrate [/li]
[li]the amount of twist is also a function of the velocity at which the tube moves perpendicular to the flow[/li][/ul]
Frequency has nothing to do with the coriolis effect. The first experiment showed that. The second part of the experiment is a vital part of constructing a practical process device.
Imagine:
[ul square]
[li]If you can imagine being able to rotate this loop about an axis at a constant speed you will comprehend that that is all you need to generate the twist effect. [/li]
[li]Frequency has nothing to do with coriolis mass flow rate measurement,[/li]
[/ul]
A rotating loop isnÆt very practical when we substitute metal tubes for the rubber hose and so, even though we use thin wall tubes and a very flexible geometry, we still have a much stiffer system with a very small amount of twist.
We want a continuous measurement but not to introduce rotating unions nor to take up a large volume of space with a spinning loop.
We decide that instead of imparting a continuous angular momentum, if we can vibrate the tube at a particular frequency, we will develop a varying twist where the degree of twist is a function of where the tube is in the excitation cycle.
Instead of measuring the twist, we now measure the phase difference between two sides of the loop.
Because it is most energy-efficient way to drive the system, it is driven at the resonant frequency.
Frequency has nothing to do with the Coriolis effect.
It just happens to be a convenient way to operate an industrial mass flow meter.
There is no such thing as a ôCoriolis densityö meter.
This term is poor science and engineering despite it being found in an ISA instruments handbook as a collective term for all vibrating element density transducers, whether they measure mass flow or not. A more correct description for vibrating element density meters can be taken from the prior API standard where they are described as ôDigital Density Metersö, a term this author prefers and extends to some vibrating element viscosity meters.
Now see the FAQs on Digital Density meters and Vibrating element Viscosity meters.
