User Manual

<16. COMMISSIONING OF SENCOM pH/ORP>
16-6
IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
l 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ow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dened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
l 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denedinpHormV.
TheunitofSlope(sensitivity)ismV/pHor%(withthetheoreticalvalueas100%).
l 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.