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pH ARC Sensors Modbus RTU Programmer’s Manual (EPHUM011)
624300/01 page 32 / 67
2.7 Calibration
2.7.1 Available Calibration Points
In register 5120, the available number of Calibration Points (CP) for Primary Measurement Channel 1
(PMC1) is defined. 8 individual CP are theoretically possible.
Start
register
Number of
registers
Reg1 / Reg2
(bitwise defined)
Modbus
function code
Read
access
Write
access
5120 2 Available number of CP for PMC1
(see figure 2.7.1.2)
3, 4 U/A/S none
Figure 2.7.1.1: Definition of register 5120.
Bit # Hex value Description Definition in ARC Sensors
0 (LSB) 0x01 CP1 Calibration Point 1
1 0x02 CP2 Calibration Point 2
2 0x04 CP3 not available
… … … not available
5 0x20 CP6 Product Calibration
6 0x40 CP7 not available
7 (MSB) 0x80 CP8 not available
Figure 2.7.1.2: Bitwise definition of CP1 to CP8.
Command: Available cali points
Modbus address: 5120 Length: 2 Type: 3
Read
Parameter: Points
Format: Hex
Value:
0x23
Figure 2.7.1.3: Example to read the available CPs. 0x23 = 0x01 (CP1) + 0x02 (CP2) + 0x20 (CP6).
Standard Calibration at 2 calibration points
-200
-150
-100
-50
0
50
100
150
200
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH value
electrical potential glass vs. ref [mV]
CP2
CP1
Figure 2.7.1.4: pH ARC Sensors allow 3 calibration points:
CP1 and CP2 are used for standard calibration (shown in this figure).
The product calibration CP6 is used to adjust the standard calibration function to specific process
conditions (the effect of CP6 is shown in Figure 2.7.3.2.1).