Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 66)

4-3
INSTALLATION & SERVICE TIPS
Determining Microvolts per Graduation
Whether sizing load cells for a mechanical conversion, replacing truck scale cells, or designing a weighing vessel, it’s tempting to pick
a grossly oversized load cell for “overload insurance.” This practice can create a problem that can cost you many hours in troubleshooting
and redesign. If you oversize capacity, you may cut your signal output to a point where your system will not operate as planned.
Determining your application’s required microvolts per graduation (
µ
V/grad) will allow you to properly size a load cell, ensuring adequate
signal and overload protection.
The signal sensitivity of electronic digital weight indicators is specified as a minimum microvolt per graduation value. A microvolt (
µ
V)
is one millionth of a volt. The µ V per graduation value is the amount of scale output signal change required to change the meter display
one graduation. If the scale output signal is below this value, the meter will not perform properly.
The following process will help you determine the
µ
V per graduation rating of your weighing system:
1. Determine full scale output of the load cell (output signal at 100% of capacity).
For example: A cell rated at 3.0mV/V, when supplied with 10V of excitation from a digital weight indicator, will provide 30 mV of full
scale output.
3.0mV/V x 10V = 30mV.
2. Determine how much of the output will be caused by the live load in your application. If the cell has a capacity of 500 lb and the live
load placed on it is 300 lb, then 60% of the total capacity of the cell is live load.
300
500
3. Determine how much signal represents the live load by multiplying full scale load cell output by the actual amount of live load at
full scale.
30mV x .6 = 18 mV
4. Actual
µ
V/graduation rating is determined by dividing the live load signal by the number of graduations the electronic digital weight
indicator is programmed to read. If the indicator is set for 5,000 graduations then:
18mV
5000 grads
If the
µ
V/graduation rating was less than the minimum sensitivity rating on the indicator, the installation will not work. The live load signal
needs to be increased. How can this be done?
■ Increase the excitation level. In #1, if 15V of excitation were used instead of 10V, then 15 x 3.0mV/V = 45mV. By completing the rest
of the formula, the
µ
V/graduation would be 5.4
µ
V.
■ Use a cell with higher full scale output. This works if the original cell was less than 3.0mV/V; generally no standard cells are available
with more than 3.0mV/V output.
■ Counterbalance the dead load off the load cell. This may allow the use of a smaller capacity load cell, thus raising the µ V rating, as
a greater portion of the total output will be live load signal.
If you experience a signal problem, using an oversized load cell will worsen the
µ
V/graduation
rating. This is because even less of the full scale output would be live load signal. As an example, if a 1000
lb cell were in the above illustration instead of a 500 lb cell, only 30% of the capacity would be used. This
would give a
µ
V/graduation of = 1.8
µ
V/graduation.
= .60 or 60%
= 3.6
µ
V/graduation
! Caution
30mV x 30%
5000 graduations