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MODEL 3081 pH/ORP SECTION 7.0
CALIBRATION OF pH MEASUREMENTS
7.7 MAKING THE TRANSMITTER READING MATCH A SECOND pH
METER (STANDARDIZATION).
7.7.1 Purpose
1. This section describes how to make the transmitter reading match the reading from a
second pH meter. The measurement made with the second meter is called the standard
pH (pH
std
). The process of making the two readings agree is called standardization.
2. This section also describes how to enter an independently determined slope into the
transmitter.
7.7.2 What Happens During Standardization?
1. The user enters the pH reading from a second meter into the transmitter. The transmitter
changes the displayed pH to the new value.
2. The transmitter converts the difference between the pH readings, ∆pH, into a voltage dif-
ference. The voltage difference, ∆V, is calculated from the equation:
∆V = [0.1984 (t + 273.14)] ∆pH
where t is the temperature in °C. The voltage difference, called the reference offset, is then
added to subsequent pH cell voltage measurements before the voltage is converted to pH.
See Sections 13.5 through 13.7 for details on how the pH meter converts voltage into pH
readings.
3. Before the transmitter accepts the offset, it compares the offset with the value (rOFFSt)
programmed into the transmitter in Section 8.4, Diagnostic Parameters. If the difference
exceeds rOFFSt, the transmitter will not accept the data and will not update the display to
the corrected pH.