What
is the Coriolis Principle?
To some of us the Coriolis Principle is an exact science,
but to most of us it is still a black art. Well, imagine
a fluid flowing (at velocity V) in a rotating elastic
tube as shown below. The fluid will deflect the tube.
Further, consider a Mass M moving from
the center to the edge of a rotating plate.
This Mass M will take path B as shown below
If the mass M is guided by Wall A (i.e.
the tube), a Coriolis Force will be exerted on the wall
as shown below.
CORIOLIS FORCE : Fc = -2 M V W
Now, consider the interior of the RotaMASS sensor as
shown below
The tube walls guide the process fluid
as it flows through the U-Tube pathway. With no fluid
inside the tubes the Driver excites the tubes apart
at a nominal 150Hz as shown below.
No Flow:
Parallel Deflection |
Mass Flow:
Coriolis Twist |
Now imagine fluid of Mass M flowing through and out
of the RotaMASS tubes. As the fluid flows down the first
half of the U-Tubes it will tend to deflect the tubes
in towards each other. Conversely, when the fluid flows
up the second half of the U-Tubes it will tend to deflect
the tubes out away from each other. This Coriolis Twist
action is shown above.
Now consider the diagram below. The baseline deflection
of the tubes from the Driver is shown by the blue trend
and the Coriolis Twist from the Pickup Coil is designated
by the red trend.
Now the temperature of these tubes dramatically
affects their flexibility. So temperature measurement
is very critical as follows;
The Mass flow equation for the RotaMASS
can be described as follows;
Where,
M
Ac
Ae
Ac/Ae
Sk
Sk(20°C)
fv
Skt
|
= Mass flow rate
= Amplitude of coriolis oscillation
= Amplitude of excitation oscillation
= Phase Angle
= Sensor constant (calibration constant)
= Sk(20°C) (1+Skt x (T-20°C)) temperature
correction
= Sensor constant at 20°C
= Excitation frequency
= Temperature correction coefficient (material
constant) |
The Density equation for the RotaMASS can be described
as follows;
p
fI(20)
fv(20)
KD
fv(20)
FKT
|
= Density
= Exciting frequency of the empty tubes at 20°C
= Exciting frequency of the filled tubes at 20°C
= Density calibration constant
= fv / (1+FKT (T - 20 °C)) temperature correction
of the actual frequency
= Temperature correction coefficient, depending
on material and size
|
For further information about the Coriolis
Principle and RotaMASS
please consult your local Yokogawa Australia Sales Engineer
If you are interested in knowing more
about how a flow sensor works
or interesting in buying flow sensors or are simply
interested to
know more about flow sensing, please log on to
www.flowmeterdirectory.com
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