YOKOGAWA SA11, Инструкция по эксплуатации, Страница: 23 / 56

IM 12A06S01-00EN-P
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4.1.4 Process compensation
The pH/ORP value can be compensated for specific temperature dependent process
conditions.
• Reference temperature
For the temperature dependent process compensation a reference temperature has to
be set to which the pH/ORP value must be calculated. Normally 25°C is used, so this
temperature is chosen as the default value.
• TC
It is possible to adjust the temperature coefficient (TC) factor directly. This method uses
the linear compensation function. If the temperature coefficient factor of the sample liquid
is known from laboratory experiments or has been previously determined, it can be set.
This TC is a pH variation to 1 °C (ΔpH/ΔT). In combination with the reference temperature
setting, a linear compensation function is obtained, which is suitable for all kinds of
chemical solutions.
• Matrix
This is 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 2.
The compensation is active up to 10°C and/or 1 pH deviation outside the values given in
the matrix table. In this case the temperature compensation is performed by extrapolation
of the matrix values. When temperature or pre-compensated value is outside this
deviation range, no calculated value will be generated.
• 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.
4.1.5 pH settings
pH is the result of the measurement in between the pH glass membrane and the reference
electrode. The characteristics for pH measurement are an offset (ZERO) and a SLOPE.
For an ideal sensor, the theoretical slope is 59.16 mV/pH (at 25ºC). The ZERO can be
entered in pH or mV. The SLOPE can be entered in mV/pH or as a percentage of the
theoretical slope (100% corresponds to 59.16 mV/pH).
ITP (Isothermal Point) represents a pH value where the output of the sensor does not
change with temperature. Note that SLOPE and ZERO are both defined at 25ºC.
The ZERO, SLOPE and ITP can be set for calculation of the pH value. When the SA11 is
connected to a Yokogawa sensor with ID-chip the latest values are automatically updated.
When afterwards these values are set by the HOST system or set by a renewed calibration
of the sensor, these values will be active for calculation and stored in the sensor too.
Figure 4.1: pH characteristic
ΔpH
ΔmV
ITP (Isothermal point)
Aspot
0 mV
0 °C
10 °C
25 °C
mV
500
0
- 200
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pH
ITP Zero
0 7