Hamilton Electronics EPHUM011 manual 200

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

What happens to the pH ARC Sensor’s calibration function upon product calibration (CP6)?

A product calibration adds an offset to the linear calibration function defined by the standard calibration at CP1 and CP2.

			Product calibration of ARC pH sensors
	200				Standard Calibration function
					Product Calibration function
	150
[mV]	100
	50
potential		CP1		CP6
	0
	5	6	7	8	9	10
Electrical
	-50
	-100
	-150				CP2
	-200
				pH

Figure 2.7.3.2.1: Effect of the product calibration CP6 on an existing standard calibration function defined by CP1 and CP2.

The operator starts with a Standard Calibration with calibration points CP1 and CP2:

CP1: pH value of calibration standard: 6	electrical potential: 64.2 mV
CP2: pH value of calibration standard: 9.21	electrical potential: -125.2 mV

The sensor internally calculates the calibration function, using the calibration points CP1 and CP2. The resulting calibration function, compensated to the standard temperature 25°C, is shown as a straight line. The calibration function is described by two parameters: the offset at pH 7 and the slope.

Some weeks later, the operator believes that the Standard Calibration function is not correct anymore. As the process is running and he is not able to perform a standard calibration under defined conditions in the lab, he decides to perform a product calibration CP6, in other words adjusting the standard calibration function to the process conditions:

CP6: pH value of product: 7.2 electrial potential: 5 mV

The sensor internally adds an offset to the calibration curve. The slope remains unchanged.

Another special feature of this calibration point is to switch off and back on again a product calibration. These functions are called “restore standard calibration”and “restore product calibration”.

Note:

The sensor’s internal criteria for a successful product calibration are:

the sensor is currently in an environment corresponding to the pH ARC Sensors measurement range.

the manually assigned pH value does not deviate more than 2 pH units from the value measured prior the product calibration

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