Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 15)

Determining Microvolts per Graduation 11
7.0 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 the capacity too much, 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 (μV/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 (μV) is one millionth of a volt. The μV 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 μV 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 500lb
and the live load placed on it is 300lb, then 60% of the total
capacity of the cell is live load.
300
____ = .60 or 60%
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 μV/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:
18000mV
___________ = 3.6μV/graduation
5000 grad
If the μV/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 μV/graduation would
be 5.4 μV.
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 μV
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
t h e μ V / g r a d u a t i o n r a t i n g . T h i s i s
because even less of the full scale
output would be live load signal. As an example, if a 1000lb
cell were in the given example instead of a 500lb cell, only
30% of the capacity would be used.
This would give a μV/graduation of
30mV x 30%
------------------------- = 1.8μV/graduation
5000 graduations