Calibration - pH and ORP
29
Instruction Manual
Calibration - pH and ORP
LIQ_MAN_6081-P
February 2014
Section 10: Calibration - pH and ORP
10.1
Introduction
For pH sensors, two-point buffer calibration is standard. Both automatic calibration and manual
calibration are available. Auto calibration avoids common pitfalls and reduces errors. Its use
is recommended. In auto calibration the 6081 calculates the actual pH of the buffer from the
nominal value entered by the user and does not accept calibration data until readings are stable.
In manual calibration the user enters buffer values and judges when readings are stable. The pH
reading can also be standardized, that is, forced to match the reading from a referee instrument.
Finally, if the user knows the electrode slope (at 25°C), he can enter it directly.
The ORP calibration is a single-point calibration against an ORP standard.
A new pH sensor must be calibrated before use. Regular recalibration is also necessary.
A pH measurement cell (pH sensor and the solution to be measured) can be pictured as a battery
with an extremely high internal resistance. The voltage of the battery depends on the pH of
the solution. The pH meter, which is basically a voltmeter with a very high input impedance,
measures the cell voltage and calculates pH using a conversion factor. The actual value of the
voltage-to-pH conversion factor depends on the sensitivity of the pH sensing element (and the
temperature). The sensing element is a thin, glass membrane at the end of the sensor. As
the glass membrane ages, the sensitivity drops. Regular recalibration corrects for the loss of
sensitivity. pH calibration standards, also called buffers, are readily available.
In automatic calibration the transmitter recognizes the buffer and uses temperature-corrected
pH values in the calibration. The table below lists the standard buffers the controller recognizes.
The controller also recognizes several technical buffers: Merck, Ingold, and DIN 19267.
Temperature-pH data stored in the controller are valid between at least 0 and 60 °C.
pH at 25 °(nominal pH)
Standard(s)
1.68
NIST, DIN 19266, JSI 8802, BSI (see note 1)
3.56
NIST, BSI
3.78
NIST
4.01
NIST, DIN 19266, JSI 8802, BSI
6.86
NIST, DIN 19266, JSI 8802, BSI
7.00
(see note 2)
7.41
NIST
9.18
NIST, DIN 19266, JSI 8802, BSI
10.01
NIST, JSI 8802, BSI
12.45
NIST, DIN 19266
NOTE
NIST is National Institute of Standards, DIN is Deutsche Institute für Normung, JSI is Japan Standards Insti
-
tute, and BSI is British Standards Institute.
NOTE
A damaged or degraded o-ring may compromise the NEMA 4X/IP66 rating of the unit even when the rear
cover is correctly installed. Please take care to protect the o-ring when removing and replacing the rear
cover of the transmitter.
During automatic calibration, the transmitter also measures noise and drift and does not accept
calibration data until readings are stable. Calibration data will be accepted as soon as the pH
reading is constant to within the factory-set limits of 0.02 pH units for 10 seconds. The stability
settings can be changed. In manual calibration, the user judges when pH readings are stable. He
also has to look up the pH of the buffer at the temperature it is being used and enter the value
in the transmitter. Once the transmitter completes the calibration, it calculates the calibration
slope and offset. The slope is reported as the slope at 25 °C. Figure 10-1 defines the terms. The
