Charge and Current Measurements, Leakage Measurements | Fluke 35040

Theory of Operation	2
Basic Operating Equations

Charge and Current Measurements

The instrument’s electrometer charge and current measurements are factory calibrated to a United States National Institute for Standards and Technology (NIST) traceable standard. Governing regulatory agencies or best of practice protocols may require that the instrument be calibrated by an independent calibration agency. The calibration agency may install the correction factors (QScaleFactor, IScaleFactor) to make the instrument read out directly as calibrated. These factors are viewable by the operator but may not be changed or turned off. In almost all cases, the scaling factors will be nearly unity.

Q= QElectrometer [C] × QScaleFactor [C/C]

⎛ Q(i )− Q(i − 1)⎞
I = ⎜		⎟	[C/s]	× IScaleFactor [A/A]

⎝⎜ T (i )−T (i − 1) ⎠⎟

T = time of electrometer conversion [s]

Leakage Measurements

Leakage measurements (QOffset , ILeakage and TOffset) are only applicable when automatic leakage compensation is enabled.

The leakage current is defined as the current present between exposures. The displayed leakage current or dose rate value (3-1/2 digit) is updated at a rate of approximately one time per second and held during an exposure. Extrapolated charge due to leakage current is nulled from the measurement. In order to do this, a short-term history of the leakage current is necessary.

When an exposure is not in progress and the absolute value of the current is less than the start threshold, pressing the RESET/MEASURE button sets the leakage current ILeakage to zero and starts the acquisition of current measurements for determining ILeakage. To get the “best” determination of ILeakage takes time, at least 10 seconds. The ion chamber, cables, and instrument should not be disturbed while making the leakage measurement.

QOffset = Q(TOffset )[C]

n

∑I [A]

ILeakage = 1

n

DoseRateLeakage = ILeakage [A] × ICCF [Units / C] × ADCF × RateMultiplier [s]

TOffset = T [s]

2-5

Image 19

Fluke 35040 manual Charge and Current Measurements, Leakage Measurements, = time of electrometer conversion s

Contents

Therapy Dosimeter 35040 Fluke Biomedical Table of Contents Addendum Features of Firmware Version 27JAN00 or Later Troubleshooting and ServiceSample Customization Reports Iii Blank Introduction FeaturesIntroduction Safety Symbols And Terms Before You Start Safety PrecautionsOperators Manual Manual Addenda SpecificationsFactory Calibration Error Source ± %rdg Maximum Error Current Measurements Full Scale Range Display Resolution Noise ± countsFull Scale Range Display Resolution Timed Charge Measurements Effective Exposure Time Measurements SpecificationsGeneral Measurements High Performance Power Supply and Battery Charger Unpacking and Inspection 35040 Functional Description Hardware Block Diagram of the Model 35040 Therapy Dosimeter Serial Communications Port Power SupplyFront Panel MicroController Multiplexer and A/D Converter Basic Operating EquationsElectrometer Bias Supply QAccumprevious Quantity Description Units CommentsQExposure QAccum Leakage Measurements Charge and Current Measurements= time of electrometer conversion s Timed Charge and Dose Exposure Charge and Dose Exposure Current and Rate Setting ThresholdsPredicted Timed Charge and Dose Accumulated Charge and Dose DoseRate = I a × Iccf Units/C × Adcf × RateMultiplier s Exposure Time ⎝ P Air Density Correction FactorMm Hg = inch Hg x Units Display Minimum Maximum Standard Resolution Error Introduction SetupSetup Front Panel Familiarization Rear Panel Familiarization Powering The System Ion Chamber Connector Power-Up Self-Test and Display Messages General Considerations Basic Measurement TechniquesMaking Non-Timed Exposure Measurements Timer Use Timer Setup3Making Timed Exposure Measurements Using Other Ion Chambers Battery Care Front Panel Controls PowerBattery Care Operation Instructions Powering Up the Model 35040 Therapy Dosimeter Using the Test Function Button Powering Down the Model 35040 Therapy DosimeterOperation Instructions QScaleFactor=1.0000 IScaleFactor=1.0000 5Automatic Leakage Compensation 12. The Test Function Screen Showing the Timer Settings Calibrated by Keithley RMD Nov 15 17. The Test Function Screen Showing the Timer Settings 6Timer Setup Using the AIR Density Button Using the Detector Select Button Using the Bias Select Button Using the Units Select Button Set 300.0 V 100% Central Electrde Pos IC Bias 300.0 V CEP Overranging Conditions Using the RESET/MEASURE Button Overrange Conditions Reset ErrorCondition Values Overranged Annunciator Condition Reset Error With Automatic Leakage Compensation EnabledAnnunciators Annunciators Condition Overranged Measurement Measurement Indicators and ConditionsIndicator Measurement Condition ? left of value Starting the Customization Software Installing/Uninstalling Customization SoftwareCustomization Connection to the Computer Menu Items and Controls DownloadingCOM Port Drop Down List Close or File Close Ion ChambersFile Save AS and File Print Edit Special Items OK and Cancel 36. The Special Items Menu Calibration Temperature has focus Download and Cancel Measurement Displays Display Line Operators Manual Available Display Lines Changing a Line on a Display Using the APPEND, DELETE, and Insert ButtonsDisplay Line Group Selection Selecting an Item and Placing It 38. The Timer Intervals Form Timer Intervals 39. The Bias Settings Form Bias Settings Examples Deleting a Display Changing the Pressure UnitsCustomizing a Measurement Display Appending a Display Errors Troubleshooting Troubleshooting and Service Symptom LoBias annunciator displayed Symptom Display does not light up when Power is pressedSymptom ??Bias annunciator displayed Symptom HiBias annunciator displayed Do not make Symptom Internal Reset Error displayedSymptom Configuration Error displayed Bit Description Action Measurements. Return Symptom Current Overrange DisplayedSymptom Dashes displayed instead of digits Symptom Bias voltage measurement changes by one digit Example 299.9 to Display Will Not Light Hours? Is the battery voltage greater than 5.9 volts? Low Battery Go to Bias ErrorLine Model 35040 Therapy Dosimeter Fuse Replacement Line Fuse Replacement Cleaning The Instrument Interface Connector CleaningReplaceable Parts Information Replaceable Parts Information Warranty InformationReplaceable Parts List Recalibration, Repair Information & Technical Assistance Blank Sample Documents Sample Customization Reports 35040 Appendix Sample Customization Reports 35040 Appendix Sample Customization Reports Blank Overview Introduction New FeaturesTimed Measurements Specifications Update New Features B Setup Timer Use Procedure Timer Setup ProcedureMaking Timed Exposure Measurements Setup B RESET/MEASURE Automatic Leakage Compensation ADC ALC Operation Instructions B Start NA pA Stop Thres Current Decimal Places Timer Setup Table B-2. Measurement Indicators and Conditions Annunciators, Indicators, and Overrange ConditionsTable B-1. Annunciators Condition Overranged Measurement1 Table B-3. Overrange Conditions Theory of Operation Theory Of Operation Configuration Error Service Required Error Number Measurement Menu Main MenuMeasurement Displays Figure B-9 shows the Measurement Displays dialog box Table B-4. Available Display Lines Figure B-10. The Timer Intervals dialog box Page Fluke Biomedical