Emerson PN 51-1055pHC/rev.K PH at 25C Standards Nominal pH, Calibration Slope and Offset

MODEL SOLU COMP II	SECTION 6.0
	CALIBRATION

6.3 AUTO CALIBRATION — pH

6.3.1 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.

6.3.2 Definitions

1.AUTO CALIBRATION. The analyzer recognizes the buffers and uses temperature-corrected pH values in the calibration. The table lists the buffers the Solu Comp II recognizes.

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

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 Solu Comp II also measures noise and drift and does not accept calibration data until readings are sta- ble. Calibration data will be accepted as soon as the pH reading is constant to within 0.02 units for 10 sec- onds. The stability settings cannot be changed.

2.SLOPE AND OFFSET. Once the Solu Comp II suc- cessfully completes the calibration, it calculates and displays the calibration slope and offset. The slope is reported as the slope at 25°C. Figure 6-1 defines the terms.

6.3.3 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.

FIGURE 6-1. Calibration Slope and Offset

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

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