APPLICATION NOTE NO. 18-4

SBE 18 pH Sensor Calibration Equation Error
Revised August 1992

Application note in pdf format

The equation used in SEASOFT through 4.007 calculates pH as:

pHold = (pHref + (Vout - B) / M) / (°K * 1.98416e-4)

where...

°K = temperature in degrees Kelvin

This equation ignores the glass electrode potentials and temperature compensation that are used to create a 0 volt output (independent of temperature) from the pH sensor at pH 7.

The new equation (used in SEASOFT V 4.008 and later) has the form:

pH = 7 + (Vout - offset) / (slope * °K * 1.98416e-4)

where...

slope and offset are computed (PHFIT V 2.0) from a least squares fit of Vout and pH in a series of buffer solutions using the measured temperature of the buffer solutions.

The relationship between the two equations (ignoring the difference between M and slope) is:

pH = pHold + (7 - pHvref / (°K * 1.98416e-4)) + ((B - offset) / (slope * °K * 1.98416e-4))

where...

B and pHvref are the calibration coefficients used to compute pHold and slope is the calibration coefficient computed by PHFIT V 2.0

Last modified: 06-Apr-2007

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