Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 - Manual (Page 189)

<16. COMMISSIONING OF SENCOM pH/ORP>
16-6
IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
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NEN6411
This algorithm takes into account the dissociation of water in strong acid and strong alkaline
solutions. It is particularly useful for pH measurement of boiler feed water.
16.2.4 Calibration settings
The screen flow differs depending on the combination of objects to be measured.
Calibration settings for a pH converter involve slope (sensitivity), zero (aspot), and ITP
(isothermal point). Figure 16.2 shows the pH value against the mV output of the sensor. The
characteristic for pH measurement is an offset also known as aspot [mV] or zero [pH] and a
Slope [%, mV/pH]. For an ideal sensor, the theoretical slope is 59.16 mV/pH at 25ºC. The slope
can be entered in mV/pH or as a percentage of the theoretical slope (100% corresponds to
59.16 mV/pH). ITP represents a pH value where the output of the sensor does not change with
temperature. Note that slope and zero are defined at 25ºC.
ΔpH
ΔmV
ITP (Isothermal point)
Aspot
0 mV
0 °C
10 °C
25 °C
mV
500
0
- 200
14
pH
ITP Zero
0 7
Figure 16.2 Calibration parameters
n pH settings
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Zero and Slope units
Zero is an alternative to Asymmetry Potential. The method of zero (aspot) unit conforms to the
DIN standard for IEC 60746-2 instruments. Zero is defined in pH or mV.
The unit of Slope (sensitivity) is mV/pH or % (with the theoretical value as 100%).
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Limits and timing
Zero High/Low
Zero (aspot) high and low limits. During calibration, it is checked whether the new zero exceeds
these high and low limits. Narrowing the band will prevent bad calibration procedures and
calibration of bad sensors, resulting in higher accuracy. The default values should be adjusted to
suit the application and the “users” criterion.
Slope High/Low
Slope (sensitivity) high and low limits. During calibration, it is checked whether the new slope
exceeds these high and low limits. Narrowing the band will prevent bad calibration procedures
and calibration of bad sensors, resulting in higher accuracy. The default values should be
adjusted to suit the application and the “users” criterion.