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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.
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.
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
TABLE 8-2. pH values of standard buffer solutions and the temperature range over which pH values are defined
MODEL 3081 pH/ORP SECTION 8.0
PROGRAMMING FOR pH MEASUREMENTS