Calibration - Ph; Introduction; Calibration Slope And Offset - Emerson Rosemount Analytical HART 5081-A-HT Instruction Manual

MODEL 5081-A

13.1 INTRODUCTION

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 extreme-
ly 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 con-
version factor. The value of the voltage-to-pH conversion factor depends on the sensitivity of the pH sensing ele-
ment (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 stan-
dards, also called buffers, are readily available.
Two-point calibration is standard. Both automatic calibration and manual calibration are available. Auto calibration
avoids common pitfalls and reduces errors. Its use is recommended.
In automatic calibration the transmitter recognizes the buffer and uses temperature-corrected pH values in the cal-
ibration. The table below lists the standard buffers the controller recognizes. The transmitter also recognizes sev-
eral 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°C
(nominal pH)
1.68 NIST, DIN 19266, JSI 8802, BSI (see note 1)
3.56
3.78
4.01
6.86
7.00
7.41
9.18
10.01
12.45
During automatic calibration, the controller 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. See Section 7.10.
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 trans-
mitter.
Once the transmitter completes the calibration, it calculates
the calibration slope and offset. The slope is reported as the
slope at 25°C. Figure 13-1 defines the terms.
The transmitter can also be standardized. Standardization is
the process of forcing the transmitter reading to match the
reading from a second pH instrument. Standardization is
sometimes called a one-point calibration.
68
SECTION 13.0
CALIBRATION — pH
Standard(s)
NIST, BSI
NIST
NIST, DIN 19266, JSI 8802, BSI
NIST, DIN 19266, JSI 8802, BSI
(see note 2)
NIST
NIST, DIN 19266, JSI 8802, BSI
NIST, JSI 8802, BSI
NIST, DIN 19266
Note 1: NIST is National Institute of Standards,
DIN is Deutsche Institute für Normung, JSI is
Japan Standards Institute, and BSI is British
Standards Institute.
Note 2: pH 7 buffer is not a standard buffer. It is
a popular commercial buffer in the United
States.
FIGURE 13-1. Calibration Slope and Offset
SECTION 13.0

CALIBRATION - pH