LCR7000 & RHEOSIGHTS P/N: 974194 |
Rev: A
|
ECO: 52468
www.dynisco.com
From lab to production,
providing a window into the process
-67-
Advantages of a pressure transducer
a) Frictional effects of plunger tip are removed.
b) The barrel Pressure drop from plunger tip to the pressure sensor is removed.
c) Some improvements in precision can be gained.
Disadvantages of a pressure transducer
a) Pressure reservoir requires flow into it for the pressure sensor to stabilize.
b) Increased thermal losses from pressure probe.
c) The pressure reservoir needs cleaning for materials that degrade.
d) The device itself is inherently less accurate than a load cell.
e) Reduced dynamic range.
g) Must be removed when hot to prevent damage to the device. (Polymer may
adhere to the diaphragm and cause it to fail if removed cold.)
1.
Fully developed, steady-state, laminar flow is achieved.
2.
There are no radial or circumferential velocity components
3.
There are no body forces (inertial effects etc.).
4.
Viscosity is independent of pressure
5.
Isothermal conditions are achieved.
The Weissenberg-Rabinowitsch correction accounts for the fact that most polymeric
materials, when flowing through a capillary, behave like a Newtonian fluid (shear stress is
directly proportional to shear rate) only at very low shear rates. For a Newtonian material,
with a constant flow rate, the velocity of the melt stream is at a maximum in the center of
the die. The velocity then falls to zero at the capillary wall. The change in this parabolic
velocity profile, as we move from the center of the die to the wall, represents the shear rate
for that position in the die. At high flow rates, polymeric materials show a much larger
change in velocity near the wall of the die compared to Newtonian materials. Thus, at
higher shear rates, the calculated apparent shear rate (equation shown above) is an
underestimate of the true shear rate. This results in an over estimation of the viscosity.
10.
Why Use a Pressure Transducer?
11.
Common Assumption
12.
Weissenberg-Rabinowitsch Correction
