Specifications

METTLER TOLEDO Weigh Module Systems Handbook
(12/99)
5-4
Torsional Loading
Torsional loading occurs when a side force twists a load cell (see Figure 5-6). It
can be caused by structural deflection, system dynamics, thermal movement, or
mounting hardware misalignment. Torsional loading will reduce a system’s
accuracy and repeatability. To avoid this problem, always follow proper structural
support and installation guidelines, and use weigh modules that compensate for
tank movement.
Figure 5-6: Torsional Loading Applied to a Load Cell
Tank and Vessel
Design
The accuracy of a tank scale can be affected by the design of the tank itself. A
new tank should be designed so that it will not deflect significantly under the
weight of its contents and will not be subject to pressure imbalances when it is
filled or emptied. If you are converting an existing tank to a scale, you might need
to modify the tank to meet these requirements.
Structural Integrity
A tank, like its support structure, can deflect under the weight of its contents. This
is a special concern if the tank has a large diameter or if the legs are long and
tend to bow (see Figure 5-10a). Flexmount weigh modules are designed to
compensate for minor tank deflection. But serious tank deflection (more than 0.5
degree from level) will cause linearity errors and inaccurate weighments. The
design engineer is responsible for making sure that tank deflection is within
specification. Excessive deflection can be corrected by bracing the tank’s legs or
connecting them together (see Figure 5-10b).
Pressure Imbalances
When a material flows rapidly into or out of an unvented tank, it can create a
pressure imbalance. If a tank is being filled, the air pressure inside the tank would
be greater than the pressure of the air surrounding the tank. For example, suppose
500 cubic feet of liquid from a pressurized pipeline is added to a tank. The liquid
would displace 500 cubic feet of air inside the tank. Unless that 500 cubic feet of