Emerson 54e pH/ORP instruction manual Section Theory of Operation, Continuous Sensor Diagnostics

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MODEL 54e pH/ORP

SECTION 6.0

 

THEORY OF OPERATION

SECTION 6.0

THEORY OF OPERATION

6.1 THE pH SENSOR ASSEMBLY

The pH measurement is accomplished by means of a measuring electrode (usually made of glass) which develops a potential directly related to the hydrogen ion concentration (pH) of the solution in which the elec- trode is immersed. A second electrode, called the ref- erence electrode, is necessary to complete the electri- cal circuit and to serve as a constant reference poten- tial against which the potential of the glass electrode can be compared. Together, the two electrodes com-

prise the pH sensor.

designed to avoid blocking of the internal liquid junc- tion. It is essentially a complete electrode within an electrode outer body, using two liquid junctions.

A non-reactive electrolyte (potassium chloride gel) is the filling solution in the outer body, and only this solu- tion is in contact with the process. Clogging of the outer junction is minimized, since neither potassium ions nor chloride ions form insoluble compounds with the majority of materials found in process streams. Contamination of the inner junction is not likely with this configuration.

The Glass Electrode

The glass electrode is a thin-walled bulb of glass con- taining a pH-buffered solution and the elements of a half-cell (typically silver/silver chloride). The glass membrane provides a means of collecting hydrogen ions so the potential can be measured. The hydrogen ions on the surface of the glass membrane are in equi- librium with the hydrogen ions in the solution being measured. As the hydrogen ion concentration in the solution changes, the concentration of hydrogen ions on the electrode changes, creating a change in poten- tial on the glass electrode.

The Reference Cell

The purpose of the reference cell is to maintain a sta- ble reference potential regardless of a change in sam- ple pH. The cell is comprised of the reference elec- trode, reference fill solution and porous reference junc- tion. The reference cell also serves to complete the voltage measuring circuit. Within the reference elec- trode, a silver chloride element is surrounded by a con- centrated potassium chloride solution. The internal ele- ment maintains electrical contact with the pH sensitive glass electrode through the porous liquid junction (a multi-capillary hardwood plug) of the internal reference electrode.

The external wood reference junction and the gelled reference fill solution provide protection for the internal reference electrode to prevent contamination from harsh chemical environments. The internal liquid junc- tion can become blocked if the process contains any material that reacts with the filling solution to form a precipitate. The double-junction reference electrode is

The Temperature Compensation Element

In addition to the glass electrode and reference cell, a third element required in the pH measurement system is the temperature compensation element. Its purpose is to compensate for "apparent" pH changes due to the pH response of a glass electrode increasing with tem- perature. At values around 7 pH, the variation with tem- perature is zero. Thus, no compensation is required at 7 pH. The error caused by varying temperatures is greater at high/low pH values; in other words, as you move away from a neutral pH of 7. This compensation is done automatically by the Model 54e pH/ORP unless specifically disabled.

The Preamp

Because of the high resistance of the glass measuring electrode, a preamplifier must be used to transform the extremely low level, high impedance signal to a low- impedance signal. The Model 54e pH/ORP includes a switchable preamplifier that may be used if the dis- tance to the pH sensor is less than 15 feet. When longer runs of cable are necessary, the preamp can be located in the pH sensor itself or in a remote junction box. Either design allows transmission of the pH signal long distances without suffering degradation or inter- ference from outside voltage sources.

6.2 CONTINUOUS SENSOR DIAGNOSTICS

The Model 54e pH/ORP verifies the integrity of the glass and reference segments of the pH sensor by con- tinuously measuring the impedance between each segment and the solution ground.

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Contents Model 54e pH/ORP PH/ORP Hart Analyzer/ControllerElectrical Shock Hazard Essential InstructionsPage Description and Specifications Model 54E PH/ORP Microprocessor AnalyzerSection Title Appendix Title List of TablesTable No Title About This Document Table of Contents List of FiguresSection Description and Specifications General DescriptionDescription of Controls Current Outputs Specifications Physical Specifications GeneralPower Relative Humidity 95%, non condensing AlarmsRecommended Sensors Analyzer Specifications @ 25COrdering Information Code OptionsPart NO. Description Unpacking and Inspection Section InstallationMechanical Installation Locating the ControllerPanel Mounting Alarm Relay Output Wiring Section WiringPower Input Wiring Analog Output Wiring454EPH02 PH Sensor Wiring PH Sensor CompatibilityTions will Result in Controller MALFUNC- Tion 454EPH01 Cable Dressing Final Electrical Check Wiring to Model Introduction Section CalibrationHold Mode Off Adjust temp 25.1 CTemperature Calibration Autocal buffer Buffer 1 -- WaitAutomatic TWO-POINT Calibration Auto buffer cal done Use the arrow keys to select the correct bufferCalibration Notes Manual TWO-POINT Calibration Calibrate pointPt 04.00pH 25C Pt calibration doneSINGLE-POINT pH Calibration Standardize 7.01 pH59.16 mV/pH Temperature Compensation OptionsTemp comp Auto Output trimHold Mode Output trimTrim Outputs PH Settings List Section Software ConfigurationRanges Factory Settings User Settings Diagnostics Section Outline of Menu Levels Setpoint 07.00 pH Alarm High 14.00pHChanging Alarm Setpoints Simulate testsChanging Output Setpoints PID only Setpoint 7.00 pH4mA 0.00 pH 20mA 2.00 pH Setpoint + 06.90 pHMA +00.00 pH OutputOutput MA 0.00 pH 20 mA 14.00 pH Output 1 12.00 mASimulated tests Alarm setpoints Output setpointsTest output Test alarm Testing Outputs and AlarmsTest alarm 1 Open Test alarm 1 OpenMeasure pH Display right AL2Security Caution Display contrast Timeout OnOutput Control Parameters Control ModeDisplay Changing Output ParametersFault 22.00 mA Range 4-20 mAlRange 4-20 mA Hold Disable feature Fault 22.00mAOutput 2 control Output 2 setup Hold feature setupAlarms Alarm 3 controlInterval timer Relay Default None Alarm LowSetpoint 0.01 pH Delay 0 sec Hysteresis 0.01 pH Delay 0 secURV 2.00 pH Relay default NoneSetpoint +07.00 pH Time period 30 secAlarm 4 Setup Alarm FaultFeed limit Disable Timeout 3600 secTimer Disable Recovery 600 secOn time 120 sec Interval Timer Examples Diagnostics Diagnostics OffZero offset 60 mV Stabilize pH 0.01 pH Stabilize time 10 sec DiagnosticsAuto Calibration Setup Autocal StandardStandard Buffers Auto Calibration SetupSection Theory of Operation Continuous Sensor DiagnosticsInterval Timer Typical set pointsSee .8 for instructions on chang- ing these setpoints Time Proportional Control TPC Mode Code Alarm RelaysAnalog Outputs Normal ModeAction Definitions Controller Mode PriorityController Mode Priority Chart Condition DefinitionsMeasurement and Set Point Feedback Control PID Control Mode CombinationsPID Control Code PID ControlProportional Mode Gain Proportional Gain Plus Integral ResetControl Loop Adjustment and Tuning Process Reaction Curve Process Reaction Curve MethodPID Control Section Special Procedures and Features Password ProtectionLevel 1 3 Password Privileges Lock all Configuring SecuritySecurity Custom CurveSolution Temperature Compensation Temp coeff +.000Sensor iso 07.00 pH Operate iso +07.00 pH Troubleshooting is easy as 1, 2, 3… Section TroubleshootingDiagnostic Messages Description of problem Diagnostic MessagesSymptom Action Quick Troubleshooting GuideDiagnostic Variables Theoretical pH vs. Millivolt Values at 25C 77F Ohms Ambient Temperature Troubleshooting GuidelinesTemperature Compensation Circuit Troubleshooting ProcedurePreamplifier Troubleshooting Procedure Preamp in a junction box FigureController preamp check Figure Junction Box Preamplifier Check No Power Troubleshooting GuideCoated glass electrode Clean with soft cloth and clean water Password protected Part Number Description Replacement PartsGeneral Warranty RepairNON-WARRANTY Repair Section Return of MaterialQuick Startup ORP Appendix a ORP ConfigurationItem Choices ORP Settings User Settings Table A-1. ORP Settings ListDiagnostics Section Choices ORP Settings User SettingsSimulated tests Output setpointsORP Calibration Standardize 400 mVSensor or Circuit Board only Return of Materials RequestReturn of Material WarrantySpecifications subject to change without notice
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