Load Cell accuracy

[TJT]

Hello,
We are currently using 4 x 200lb rated loadcells, one mounted at each end of 2 weigh bridges for a conveyor belt weightometer. Typically we have about 60 lbs on the weighbridge during normal operation. To calibrate, I hang 110 lbs on the weighbrige and it is usually right on. But the question has come up as to the accuracy of our calibrating method since our operating weight is about 1/2 of our calibrating weight, and our operating rate is about 1/4 the capacity of the loadcells. Is our accuracy in question or is it just a matter of reduced resolution? The excitation voltage is 10VDC & the rated output of the loadcell is 3.0mV/V
Thanks....Tim

 

[itsmoked]

Just remember resolution is just that ! The number of marks on your scale, the maximum graduation or granularity your system provides. It has nothing to do with accuracy. With resolution you measure your 110 pounds to 110.00000 pounds. Doesn't mean the item actually weighs 90.1 pounds! (bad accuracy)

Range is the region over which you can measure. You accuracy may or may not have something to do with your resolution. In systems with digital measuring aspects you may be able to provide more accuracy if you confine the electronics to reading only in the region of interest.

Personally I would take the bull by the horns and investigate your system. Get an accurate scale and some five gallon pails and figure out the minimum weight change your system can show IN THE REGION OF INTERST.

Put 50# on your system then hang a pail you know the exact weight of, on the same point, below the 50# weight. Add water till you see your scale increment. Weigh the pail with water. Re hang. Add more water until the next increment occurs. Weight the pail - the difference between this reading and the last will be of great interest (resolution). As will be what the total weight is (accuracy).

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[IRstuff]

The simplest solution is to calibrate at the nominal load. This eliminates any nonlinearity in the system that might affect the absolute value of your measurements.

TTFN

 

[HEC]

TJT,
You mentioned 4 loadcells to support the conveyor. This is probably the greatest source of error if summing of the load cells is astray. 3points define a plane and will support the load evenly, add the 4 point and you have a surface where the load may not be evenly supported across all 4 cells. This was a big problem for a particular application weighing a batch tank that I struck. The individual load cells were fine but the 4 together was a complete nightmare!

Mark Hutton

 

[randi]

Could this information be of any use to you:

http://glo.mt.com/mt/products/produ...v=V0bmVjX3Ryb3BwdXMicm?v=V0bmVjX3Ryb3BwdXMicm

Mettler is a well known company for weighing systems of any kind.