Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 - Manual (Page 59)

<4. COMMISSIONING OF pH/ORP>
4-5
IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
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Matrix
Matrix means a temperature compensation which uses the temperature compensation
matrix. The temperature compensation matrix is a table of pH values at various temperatures
corresponding to the pH values at the standard temperature. For details, see Appendix 1.
When the temperature or the precompensated pH value is out of the range of the temperature
compensation matrix, the temperature compensation error (warning) will be issued. This is not a
device error.
In this case, however, the temperature compensation is performed by extrapolation.
NOTE
To display the precompensated pH value, set the Process Temp. Compensation to “None”.
Select Measurement setup → Temp. compensation → Process Temp. Compensation.
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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.
4.2.4 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 4.2 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 4.2 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%).
PH
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