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from the Density Measurement department...
Coriolis density measurement
Sensor technologies. topic
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Posted by Kirill St. on 3 July, 2012 - 8:53 am
How do you think the coriolis flow meter measures the density of a fluid?

I understand that the density of a fluid effects on a mass and on the natural frequency of a coriolis. So do the control system of the coriolis always change the frequency of the motor or there is something else more simple to do?

thanks


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Posted by JMW on 4 July, 2012 - 7:09 am
You can get a good understanding of the resonant frequency method for density here

http://www.viscoanalyser.co.uk/Digital%20Density%20DG.pdf
Coriolis meters (please don't ever allow anyone to say "Coriolis Density") measure mass flow.

The earliest coriolis meters (before modern electronics) would vibrate at mains frequency. They could not measure density.
However, in the search for more efficient operation it was recognised that they operate more efficiently if they vibrate at resonant frequency (plus they would operate at a much higher frequency which made them less vulnerable to pipe noise and cross-talk between meters).

Drive circuits using phase lock loop enable them to do just that.

Let's keep it simple and see how a density meter does it.
When first energised, a voltage controlled oscillator drives the vibrating element (tube, fork or whatever). It starts at a low frequency and progresses to higher frequencies. As the element is driven, the pick off coils, which detect the motion of the tube, feedback a signal into the amplifier. At resonant frequency the signal strength increases significantly and it is then greater than the original driving signal. This feedback signal takes over running the VCO.

Off course, the drive circuits are a great deal more sophisticated now, but that is the essence of it.

Once mass meters start to run at resonant frequency, they can measure density because the resonant frequency varies with the mass of the system. That is, the mass of the sensor and the mass of the fluid. The mass of the fluid changes with density.

Of course, it also changes with temperature, pressure, velocity of sound, flow rate, viscosity and a few other parameters but these can all be compensated or isolating the effect of fluid density on the vibrating element.

The density effect is quite independent of the mass effect.

Resonant frequency vibrating element sensors are very useful sensors. In the same way that you can obtain the two different and independent measurements of density and mass, you can also obtain viscosity. There are several ways to do this. One approach is to measure the change in amplitude at resonant frequency which changes according to the fluid damping or viscosity of the fluid. Another is to operate not at resonant frequency but alternating between the half power points (3dB frequencies which are 90degrees out of phase so you add in a phase shifting amplifier).

In theory you should be able to get mass, density and viscosity from the same sensor using the half power point method (superior to the amplitude method or the decay time method used by some other sensors) but you have to remember that performance depends on the sensor itself.

You have to decide whether to optimise the design for density, mass flow or viscosity.

Coriolis manufacturers have different views on multi-functionality. E&H include viscosity. Micromotion do not. There is an obvious link between mass flow and density, it leads to volume, but not between viscosity and mass flow.

Also, be wary of accepting the data sheet technical information at other than strictly within the terms performance is declared. AT factory calibration conditions some mass meters can appear to match dedicated density meter performance but once away from these conditions dedicated density meters will outperform them, for example, due to their better temperature and pressure effects.


Posted by Kirill St. on 8 July, 2012 - 5:39 am
Thanks a lot for the answer.

Another question is how the temperature influence on this kind of measurement?

Is there another parameters which influence on the measurement of the density?


Posted by JMW on 9 July, 2012 - 6:22 pm
Temperature effects:
firstly, it affects the Young's modulus which causes a frequency shift. This is why materials are important and for fiscal applications density meters use NiSpan C. (again, beware the comparison of performance at reference/factory conditions - it is away from those conditions you need to see what happens).
Then it causes a change in the enclosed volume which means more fluid mass which means a correction for that.

Then, dependent on the technology, expansion of the sensor between rigid pipework expanding at a different rate can also affect the measurement.

Other things that affect density: didn't I list them? Velocity of sound, viscosity, flow rate, pressure - they can all have an effect. A good idea is to download a manual from one of the web sites and see what corrections they include and to runs some calculation son the magnitude of the effects on different types.
You might also visit TUV NEL website and look for high viscosity fluids and Mass meters.
http://www.tuvnel.com/tuvnel/investigation_into_performance_of_cori olis_and_ultrasonic_meter/


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Posted by JMW on 4 July, 2012 - 7:13 am
PS: Not all resonant frequency devices can measure density.

This is particularly apparent with vibrating element viscometers.
Some use torsional twisting to determine the viscosity from the damping effect on the resonant frequency signal but because these sensors do not displace the fluid, they are unaffected by changes in fluid density.


Posted by john on 10 January, 2013 - 6:41 pm
Just a quick question guys on Coriolis flow meters.

When the meter is reading zero flow, what would be the frequency?


Posted by JMW on 1 February, 2013 - 8:27 am
The frequency of operation of a coriolis flowmeter depends on the density of the fluid it contains. It is not (significantly) affected by flow.

i.e. the fluid does not have to flow for its density to be measured.


Posted by Avinash Bhargav on 7 April, 2014 - 2:44 am
Dear Mr. JMW thanks for all the inputs. I have a query regarding the practical applications of such instrumentation. In our industry we make NPK fertilizers, the process involves a thick liquor flow (total solids max. 45%). How effective is a coriolis flow meter be for such measurements? we can get 5% suspended solids at times. I feel a magnetic flow meter will be much better but I cant measure density at the same time. Kindly advice.


Posted by Walt Boyes on 7 April, 2014 - 12:16 pm
If you can get it to flow, you can pump it through a Coriolis Mass Flow meter. This is the kind of flow application that used to be done with a magnetic flow meter and a gamma densitometer in series on the pipe. I am concerned about your 45% total solids number versus your 5% suspended solids. I am assuming that you mean that you have 45% total dissolved and suspended solids, yes??

How long the Coriolis meter will last depends on how abrasive or corrosive your liquor is. You might consider a ³straight through² design of Coriolis meter, to reduce the effects of abrasion or corrosion.

Best,

Walt Boyes,
Life Fellow, ISA; Fellow InstMC
Chartered Measurement and Control Technologist
Spitzer and Boyes LLC
3646 Wyoming Street
St Louis MO 63116
waltboyes@spitzerandboyes.com
office: +1-630-639-7090

**Spitzer and Boyes LLC publishes the Industrial Automation and Process
Control INSIDER (www.iainsider.com)**


Posted by JMW on 15 May, 2014 - 8:19 am
If using coriolis then you may be advised to choose one of the straight tube meters.
Or single tube.

The problem with high solids content is that if the flow is low and solids start to settle out they may do so in an unbalanced way leading to more deposition in one tube than the other which then causes lower flow in that tube and more deposition till the tube is blocked.

Coriolis is certainly a solution and with the right choice you will get all the % solids calculations you need and a good result but choose the wrong one and all you will get are problems.

You need to survey the manufacturers, talk to them and make sure you get good and sensible answers to your concerns and good and reliable guarantees. Look for referrals to pre-existing users with similar applications and talk to them.

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