Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 Manual de usuario descargar pdf (Pagina 189)

<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

500

- 200

ITP Zero

Figure 16.2 Calibration parameters

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.

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