# Coriolis Force

S

#### Salai Kuberan E S, Manager/C&I

Here is an excerpt from Micromotion Coriolis flow meter O&M...

" Vibrating motion of the flow tube combined with momentum of Fluid flow thro
tubes induces a Coriolis force that causes each flow tube to twist in proportion
to the rate of mass flow thro the tube during each vibrational cycle.
"Since one leg of flow tube lags behind other leg during twist, Delta time b/w the left & right pickoff signals is multiplied by the flow calibration factor to determine the mass flow rate"

I have some questions :

- Anybody Pl elaborate on context in which the term "Vibration cycle" is used
here.
- What is the role of amount of twist in measuring flow ?
- What does delta time signal indicate ?
- A general description of Coriolis force - sans jargon - is also most welcome

Regards & TIA

Salai

W

E

#### Eidsor.David

All responses based on description of operation from IEH (3rd edition) Process Measurement and Analysis - (EIC- B. Liptak)

> I have some questions :
>
> - Anybody Pl elaborate on context in which the term
> "Vibration cycle" is used here

The driver oscillates the flow tube in a sinusoidal manner.

> - What is the role of amount of twist in measuring flow ?

The Coriolis force causes the "twist" in the flow tube.
This force is related directly to mass flow rate, but direct measurement is
not practical.

> - What does delta time signal indicate ?

The mass flow rate can be determined either by measuring the time difference
between the two dectors (one on each side of the U shaped flow tube), or by measuring the phase difference and the frequency of oscillation.

> - A general description of Coriolis force - sans jargon - is
> also most welcome

The fluid moving toward the flow tube driver opposes the oscillation of the
tube, while the fluid moving away from the flow tube driver assists the oscillation of the tube. These opposing forces cause the tube to distort. When fluid flows through the tube, there is a slight time or phase difference between
the two sides of the tube, resulting from these forces. The mass flow rate is determined by placing sensors on the flow tube, one on either side of the driver, and evaluating the difference between the two signals.

Hope this helps,
David Eidsor

M

#### Mike Johnson

What about the effects of temperature on the tube itself.
Is not the "stiffness" of the tube is dependent on the temperature of the tube,
thus, effecting the tubes ability to bend? Or is such temperature
compensation not a practical or even a consideration in mass flow measurement
using a flow tube.
Also, there is probably a calibration factor that is multiplied by the time
difference in order to generate a mass flow rate.

Mike Johnson

D

#### David W. Spitzer

Mike,

The metalurgical properties of the tubes vary with temperature. Manufacturers take care of this internally, and many offer a process temperature output (which is really the tube wall temperature).

The above said, these flowmeters use the properties of mass (F=ma in the tangential plane) to measure mass, so classical pressure and temperature compensation is not necessary.

Hope this helps.

Regards,

David W Spitzer, PE
Copperhill and Pointer, Inc.
http://www.icu.com/spitzer845.623.1830 (phone/fax)

M

#### Mike Johnson

I thought that the measurement of mass flow rate was made indirectly through the measurement of a difference in the phase of the tube oscillation when there is a fluid in motion within the tube as compared to when the fluid is not in motion. The actual mass is calculated using the calibration factor. Hence, does not the ability to measure mass through a mass flowmeter that uses this phase shift measurement technique is
dependent on the nature of the tubule oscillation?
Secondly, is the nature of the oscillation temperature dependent? In other words, if a fluid with a constant flow rate of w1 Ibs/hr flows
into a coriolis type flowmeter inwhich the tube is at temperature t1 degF reads the flow as w2 then, if the tubes temperature changes to t2 inwhich t2>>t1 or t1>>t2 inwhich the flowmeter reads w3. Will w2=w3, which should equal to w1.

Mike Johnson

D

#### David W. Spitzer

Mike,

See my comments IN CAPS in the text.

David W Spitzer
845.623.1830

In a message dated 8/2/01 2:14:12 PM Eastern Daylight Time, [email protected]
writes:

<< ---------- Forwarded message ----------
From: [email protected]

I thought that the measurement of mass flow rate was made indirectly
through the measurement of a difference in the phase of the tube
oscillation when there is a fluid in motion within the tube as compared to
when the fluid is not in motion. The actual mass is calculated using the
calibration factor. Hence, does not the ability to measure mass through a
mass flowmeter that uses this phase shift measurement technique is
dependent on the nature of the tubule oscillation?
YES, THIS IS A DESIGN ISSUE

Secondly, is the nature of the oscillation temperature dependent?
In other words, if a fluid with a constant flow rate of w1 Ibs/hr flows
into a coriolis type flowmeter inwhich the tube is at temperature t1 degF
reads the flow as w2 then, if the tubes temperature changes to t2 inwhich
t2>>t1 or t1>>t2 inwhich the flowmeter reads w3. Will w2=w3, which should
equal to w1.
THE PROPERTIES OF THE METAL CHANGE WITH TEMPERATURE. THE EQUIPMENT IS DESIGNED TO TAKE CARE OF THIS --- TRANSPARENT TO THE USER

J

#### Jon Watson

I always found the descriptions and diagrams presented by the mass flow meter companies to be extremely confusing.
The best way to understand is by observation.
Try an experiment with a garden water hose. Make a large loop that you can comfortably swing with one hand. Now swing the loop back and forth and get someone to turn the flow on and off. You should see quite clearly that the loop twists.
The degree of twist is a function of the flow rate.
What frequency should you swing the loop at? well in a measuring instrument the optimum frequency is the resonant frequency. This is the natural oscilation frequency where the least energy is required to sustain the oscilation. This frequency happens to be a function of the mass of the system. Since the mass of the hose is fixed this frequency will change with the density of the fluid it contains. It will also change due to the change in "stiffness" of the tube with temperature, the "Youngs Modulus". many coriolis mass flow meters exploit the fact that they employ the resonant frequency to also indicate the density of the fluid.
Some are used for gas flow.

J

#### Jon Watson

I have a PS to my previous comment....
The experiment with a garden hose is such that one cannot help wondering about the sort of circumstances in which one might have been watering the garden, washing the car etc when one might casually have observed this affect, wondered about it and had one of those "Eurika" moments which would mean that you or I could be living in comfortable retirment instead of admiring (envying)the success of Micromotion and all the other mass meter companies!
Ah well, let's hope the next such opportunity is not too far away!