Emerson Process Management 3081 pH/ORP instruction manual Buffer Calibration Parameters

MODEL 3081 pH/ORP	SECTION 8.0
	PROGRAMMING FOR pH MEASUREMENTS

8.7 BUFFER CALIBRATION PARAMETERS

8.7.1 Purpose

This section describes how to do the following:

1.activate or deactivate auto calibration,

2.identify which buffers will be used during auto calibration,

3.set the stability criteria for auto calibration.

8.7.2 Definitions

1.AUTO CALIBRATION. In auto calibration, screen prompts direct the user through a two point buffer calibration. The transmitter recognizes the buffers and uses temperature-corrected values in the calibration. The transmitter does not accept data until programmed stability limits have been met. If auto calibration is deactivated, the user must perform a manual calibration. In manual calibration, the user judges when readings are stable and manually enters buffer val- ues. The use of auto calibration is strongly recommended.

2.BUFFERS. Buffers are aqueous solutions to which exactly known pH values have been assigned. Assigning a pH value to a buffer involves certain fundamental assumptions. Slightly different assumptions lead to slightly different pH scales. Over the years, various national standards organizations have developed different scales. The Model 3081 pH/ORP transmitter recognizes the common standard scales as well as common commercial buffers. Commercial buffers, which are sometimes called technical buffers, are traceable to standard buffers, but the accuracy of commer- cial buffers is generally less than standard buffers. Tables 8-2 and 8-3 list the buffers the Model 3081 pH/ORP trans- mitter recognizes and the temperature range over which the buffer pH is defined.

TABLE 8-2. pH values of standard buffer solutions and the temperature range over which pH values are defined

	NIST			DIN 19266			JIS 8802			BSI
pH		temp (°C)	pH		temp (°C)	pH		temp (°C)	pH	temp (°C)
1.68		5 - 95	1.68		5 - 95	1.68		0 - 95	1.68	0 - 60
3.56		25 - 95							3.56	25 - 60
3.78		0 - 95
4.01		0 -95	4.01		0 - 95	4.01		0 - 95	4.01	0 - 60
6.86		0 -95	6.86		0 - 95	6.86		0 - 95	6.86	0 - 60
7.00						see note
7.41		0 - 50
9.18		0 - 95	9.18		0 - 95	9.18		0 - 95	9.18	0 - 60
10.01		0 - 50				10.01		0 - 50	10.01	0 - 50
12.45		0 - 60	12.45		0 - 60

NOTE: pH 7 buffer is not a standard buffer. Because it is a popular commercial buffer in the United States, it is included with the standard buffers. The pH of the buffer is defined between 0 and 95°C.

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