Specifications

MODEL 30 pH/DO SECTION 6.0

CALIBRATION

6.4 AUTO CALIBRATION — pH

6.4.1 Purpose

1. New sensors must be calibrated before use. Regular recalibration is also necessary.

2. Use auto calibration instead of manual calibration. Auto calibration avoids common pitfalls and reduces errors.

6.4.2 Definitions

1. AUTO CALIBRATION. The analyzer recognizes the buffers and uses temperature-corrected pH values in the

calibration. The table lists the buffers the Model 30 recognizes.

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.

The Model 30 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 0.02 units for 10 seconds.

The stability settings cannot be changed.

2. SLOPE AND OFFSET. Once the Model 30 success-

fully completes the calibration, it calculates and dis-

plays the calibration slope and offset. The slope is

reported as the slope at 25°C. Figure 6-2 defines the

terms.

6.4.3 Procedure

1. Obtain two buffer solutions. Ideally the buffer pH val-

ues should bracket the range of pH values to be meas-

ured.

2. Remove the sensor from the process liquid. If the

process and buffer temperatures are appreciably dif-

ferent, place the sensor in a container of tap water at

the buffer temperature. Do not start the calibration until

the sensor has reached the buffer temperature. Thirty

minutes is usually adequate.

3. Calibrate the sensor by using the procedure on the following page.

pH at 25°C Standard(s)

(nominal pH)

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

FIGURE 6-2. Calibration Slope and Offset