Hamilton Electronics EPHUM011 manual Calibration Procedure

pH ARC Sensors Modbus RTU Programmer’s Manual (EPHUM011)

2.7.3Calibration Procedure

2.7.3.1Calibration at CP1 and CP2 (Standard Calibration)

The ARC Sensor family has a unique calibration routine. When initiating the calibration, the data set of the sensor is automatically traced back within the last 3 minutes and a decision is made immediately if the calibration is successful or not. The operator therefore gets an immediate result. The criteria for a successful calibration are:

the stability of pH value and temperature over the last 3 minutes (see register 5128)

the currently measured pH value fits to one of the calibration standards defined in the selected set of calibration standards

the limits of slope and offset at pH 7 have to be met

			Standard Calibration at 2 calibration points
	200
ref [mV]	150					CP1
	100
vs.
glass	50								CP2
	0
potential
	-50
electrical	-100
	-150
	-200
	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
								pH value

Figure 2.7.3.1.1: Standard Calibration using CP1 and CP2.

CP1 and CP2 define a linear relationship between the electrical potential and the pH value. This linear calibration function is defined by an offset at pH 7 [mV] and a slope [mV/pH]. These two values are stored in register 5448.

Note: the pH value of CP1 can be lower or higher than the pH value of CP2. However, the difference in pH between CP1 and CP2 must always be greater than 1 pH unit.

Notes:

In order to perform a standard calibration at CP1 and CP2, it is necessary to use commercially available calibration standards. The operator is restricted to use those standards that are defined in the six sets of calibration standards (see chapter 2.7.11).

There are two ways of performing a standard calibration:

- standard calibration with automatic recognition of the calibration standard:

in this case, the sensor decides on itself in what calibration standard it is immersed. The criteria to decide on is the electrical potential measured. The sensor checks the list of calibration standards that are available for automatic recognition (see chapter 2.7.11)

- standard calibration with manual selection of the calibration standard:

the operator is selecting the calibration standard in which the sensor is immersed.

Factory calibration is pH 4 at CP1 and pH 7 at CP 2.

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