Endress+Hauser Liquiline M CM42 - Sensor Calibration Parameters

Calibration and adjustment CM42-MAA101EBE00
6 Endress+Hauser
The Nernst equation, which is used to calculate the pH value, is a typical linear relationship:
pH = -lg(a
H+
), a
H+
... activity of the hydrogen ions
U
i
... raw measured value in mV
U
0
... zero point (=voltage at pH 7)
R ... universal gas constant (8.3143 J/molK)
T ... temperature [K]
F ... Faraday constant (26.803 Ah/mol)
The slope of the Nernst equation (-2.303RT/F) is known as the Nernst factor and has the
value -59.16 mV/pH at 25 °C.
2.2.2 Delta slope
The device determines the difference in the slope between the calibration that is currently valid
and the last calibration. Depending on the sensor type, this difference is an indicator of the
condition of the sensor. The smaller the slope, the less sensitive the measurement, and the
accuracy deteriorates particularly in the low measuring range.
Depending on the operating conditions, users can define limit values that represent the still
tolerable absolute values of the slope and/or slope differentials. If the limit values are exceeded,
maintenance must be performed on the sensor at the very least. The sensor must be replaced if
the insensitivity problems persist after maintenance has been carried out.
a0013593
Fig. 1: Linear function
aZero point
bSlope
U=U
i0
-
2.303 RT
F
pH