Calibration; Calibration With Calibration Weights - Siemens SIWAREX WP251 Manual

Scale parameters and functions

8.4 Calibration

8.4 Calibration
8.4.1

Calibration with calibration weights

The incoming analog millivolt signal from the load cells is converted to a digital value (digit)
in an analog-to-digital converter. A weight is calculated using this digital value. This weight is
then used by all weighing module functions for messages and for determining the status.
The characteristic curve of the measuring system must be defined before the weight can be
calculated from the digital value. In the simplest case, the characteristic curve is defined by
interpolation points 0 (calibration weight 0 and calibration digit 0) and 1 (calibration weight 1
and calibration digit 1). The first working point (point 0) is defined by the unloaded scale
(empty) with its self-weight. Based on the weight of the scale's own construction, the load
cells return a measuring voltage to the weighing module Following analog-to-digital
conversion of the measuring voltage, the zero point is assigned to the digital value
(calibration digits for the zero point).
If the scale is loaded with a defined calibration weight (e.g. 50% of the measuring range), the
calibration weight will be assigned to the new digital value from the analog-to-digital
converter.
In addition, the characteristic curve can be defined by a third interpolation point, which must
lie above point 1.
Make sure that the difference between two calibration weights is at least 40 000 digits, as the
calibration command may otherwise be rejected. This corresponds to approximately 2% of
the nominal load of all load cells (2% of (number of load cells x nominal load of one single
load cell)).
The calibration procedure involves the following steps:
● Specify the calibration weight and other parameters in data record DR 3.
● Transfer the DR 3 data record to the scale.
● Trigger the "Set Calibration Point 0" when scale is empty.
● Load the scale with the specified calibration weight.
● Trigger the "Set Calibration Point 1" command.
● Transfer data record DR 3 from the scale to SIWATOOL/SIMATIC DB/Modbus-Master
and save the data as a backup, if necessary.
You must follow the correct calibration sequence with increasing calibration weights.
Load cell characteristic value
1 mV/V
2 mV/V
4 mV/V
The diagram below illustrates the relationship between calibration digits and the calibration
weight.
76
Digits (approx.) at nominal load
1 000 000
2 000 000
4 000 000
Manual, 12/2015, A5E37203357A
SIWAREX WP251