Zero Torque Feature - Fisher Scientific Power Overhead Stirrer Manuel D'instructions

Zero Torque Feature

Theory
Torque (turning force) is a measure of the force on an object such as a shaft or
a propeller. For example, pushing or pulling the handle of a wrench connected
to a nut or bolt produces a torque (turning force) that loosens or tightens the
nut or bolt. The units used to quantify torque are Newton-meters, Newton-
centimeters, or foot pounds.
In Mixing Applications the magnitude of torque depends on three quantities:
First, the speed in revolutions per minute; second, the diameter of the propeller; and
third, the viscosity of the material being mixed. Additionally, a small amount of
torque is required to move the mixing impeller. In small bench top applications
this additional or overhead torque is negligible and is generally independent of
the force required to mix material.
Zero Torque Feature
The zero torque feature allows the operator to null out the additional over-
head torque of the stirring apparatus similar to a tare feature on a scale. The
feature can also be used during the mixing process and allows the operator to
see change in torque caused by the change in material viscosity.
Zero Torque Operating Instructions
Run the stirrer at low speed for a ½ hour before zeroing torque. As the motor
warms up the torque can change slightly. Setting the torque when the stirrer is
warm will increase the accuracy.
1. Follow installation instructions. Before attaching the mixing impeller, press the
power button, ramp to the desired speed and press the start/pause button to start
the stirrer. Speed in rpm is displayed.
2. Press the display button. The stirrer displays actual torque. Note this cannot be
done unless the stirrer is running.
3. Press the Zero Torque button. The display now reads zero and the N-cm LED
will fl ash to indicate that the torque has been set to zero. Pause mixing. The
RPM LED will fl ash to indicate the stirrer has been paused.
4. Place the impeller into the chuck and then into the mixing vessel and press the
start/pause button to start mixing. The torque of the material being mixed will
now be displayed.
Follow all safety instructions when mixing.
Note:
At any time while mixing the torque display can be set to zero. Viscosity changes
can then be monitored as solutions change during the mixing process. A negative
number indicates a reduction in viscosity and a positive number indicates an
increase in viscosity.
14
14