The precision that we may expect from a weighing system is the highest value obtained between

the two following calculations (a) and (b):

 

(a) Limit by minimum division of the load cell (related to repeatability):

emin(rep) = vmin *  N (a)

Being,

emin(rep) =minimum error that can be obtained by the minimum division of the cell

vmin = the minimum load cell verification interval

N = number of load cells

 

(b) Limit by range of use of the cell (related to linearity):

emin(lin) = Max / nlc (b)

being,

emin(lin) =minimum error that can be obtained by range of use of cell

nlc = number of load cell verification intervals

Result: emin = the highest of emin(rep) o emin(lin)

 

Recommendation:

The precision is the error. The resolution or division of “display” is the fraction that is displayed. In

certified scales, the resolution or division of display should not be finer than the error of the

instrument itself. In certain industrial environments, an increased resolution is used, twice as fine

than the real error or the precision of the system.

Examples:

Ej. 1) Normal scale

Data of the weighing system:

Product Max= 600 kg

Dead Load DL = 120 kg

Total Load = 720 kg

Supports N = 3

Data of the Cells:

3 units Model 350 i 500 kg

Emax = 500 kg

nlc= 3000 divisions

vmin = Emax / Y = 500 / 10.000 = 0,05 kg

Calculations:

(a) Limit by minimum division of cell (related to repeatability):

emin(rep) = vmin * √ N = 0,05 * √ 3 = 0,0865 kg

(b) Limit by range of use of cell (related to the linearity):

emin(lin) = Max / nlc = 600 / 3000 = 0,2 kg

Result:

emin = 0,2 kg

For these scales we shall choose a display resolution of d = 0,2 kg

Ej. 2) Scale with quite a lot of dead load in respect to the weight of the product

Data of the Weighing System:

Product Max= 400 kg

Dead Load DL = 320 kg

Total Load = 720 kg

Supports N = 3

Data of the Cells:

Model 350 i 500 kg

Emax = 500 kg

nlc= 3000 divisions

vmin = Emax / Y = 500 / 10.000 = 0,05 kg

Calculations:

(a) Limit by minimum division of cell (related to the repeatability):

emin(rep) = vmin * √ N = 0,05 * √ 3 = 0,0865 kg

(b) Limit by range of use of cell (related to linearity):

emin(lin) = Max / nlc = 400 / 3000 = 0,133 kg

Result:

emin = 0,133 kg

For this scale we shall choose a resolution of display of d = 0,2 kg in an environment of certified

scales for commercial transactions or also the smaller d = 0,1 kg for an environment of industrial

control.

 

Ej. 3) Scales with a great amount of dead load and very little product weight

Data of the Weighing System:

Product Max= 220 kg

Dead Load DL = 500 kg

Total Load = 720 kg

Supports N = 3

Data of the Cells:

Model 350 and 500 kg

Emax = 500 kg

nlc= 3000 divisions

vmin = Emax / Y = 500 / 10.000 = 0,05 kg

Calculations:

(a) Limit by minimum division of cell (related to the repeatability):

emin(rep) = vmin * √ N = 0,05 * √ 3 = 0,086 kg

(b) Limit by range of use of cell (related to the linearity):

emin(lin) = Max / nlc = 220 / 3000 = 0,073 kg

Result:

emin = 0,086 kg

For these scales we shall choose a display resolution of d = 0,1 kg.