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Ground Looping in Ionic Measurements

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Section 7. Installation 7.5.4 Ground Looping in Ionic Measurements When measuring soil-moisture with a resistance block, or water conductivity with a resistance cell, the potential exists for a ground loop error. In the case of an ionic soil matric potential (soil moisture) sensor, a ground loop arises because soil and water provide an alternate path for the excitation to return to CR1000 ground. This example is modeled in the diagram, figure Model of a Ground Loop with a Resistive Sensor (p. 92). With Rg in the resistor network, the signal measured from the sensor will be: , where o Vx is the excitation voltage o Rf is a fixed resistor o Rs is the sensor resistance o Rg is the resistance between the excited electrode and CR1000 earth ground. RsRf/Rg is the source of error due to the ground loop. When Rg is large, the error is negligible. Note that the geometry of the electrodes has a great effect on the magnitude of this error. The Delmhorst gypsum block used in the Campbell Scientific 227 probe has two concentric cylindrical electrodes. The center electrode is used for excitation; because it is encircled by the ground electrode, the path for a ground loop through the soil is greatly reduced. Moisture blocks which consist of two parallel plate electrodes are particularly susceptible to ground loop problems. Similar considerations apply to the geometry of the electrodes in water conductivity sensors. The ground electrode of the conductivity or soil moisture probe and the CR1000 earth ground form a galvanic cell, with the water/soil solution acting as the electrolyte. If current is allowed to flow, the resulting oxidation or reduction will soon damage the electrode, just as if dc excitation was used to make the measurement. Campbell Scientific resistive soil probes and conductivity probes are built with series capacitors to block this dc current. In addition to preventing sensor deterioration, the capacitors block any dc component from affecting the measurement. 91

Section	Page
Revision and Copyright Information	1
Warranty	3
Assistance	5
Table of Contents	7
Section 1. Introduction	27
1.1 HELLO	27
1.2 Typography	27
Section 2. Cautionary Statements	29
Section 3. Initial Inspection	31
Section 4. Quickstart Tutorial	33
4.1 Primer – CR1000 Data-Acquisition	33
4.1.1 Components of a Data-Acquisition System	33
4.1.1.1 Sensors	33
4.1.1.2 Datalogger	33
4.1.1.3 Data Retrieval	33
4.1.2 CR1000 Module and Power Supply	34
4.1.2.1 Wiring Panel	34
4.1.2.2 Power Supply	35
4.1.2.3 Backup Battery	35
4.1.3 Sensors	36
4.1.3.1 Analog Sensors	36
4.1.3.2 Bridge Sensors	37
4.1.3.2.1 Voltage Excitation	37
4.1.3.3 Pulse Sensors	38
4.1.3.3.1 Pulses Measured	39
4.1.3.3.2 Pulse-Input Channels	39
4.1.3.3.3 Pulse Sensor Wiring	39
4.1.3.4 RS-232 Sensors	40
4.1.4 Digital I/O Ports	41
4.1.5 SDM Channels	42
4.1.6 Input Expansion Modules	42
4.2 Hands-On: Measuring a Thermocouple	42
4.2.1 What You Will Need	43
4.2.2 Hardware Setup	43
4.2.2.1 External Power Supply	43
4.2.3 PC200W Software Setup	44
4.2.4 Write Program with Short Cut	46
4.2.4.1 Procedure: (Short Cut Steps 1 to 6)	46
4.2.4.2 Procedure: (Short Cut Steps 7 to 9)	47
4.2.4.3 Procedure: (Short Cut Steps 10 to 11)	48
4.2.4.4 Procedure: (Short Cut Steps 12 to 16)	49
4.2.4.5 Procedure: (Short Cut Step 17 to 18)	50
4.2.5 Send Program and Collect Data	51
4.2.5.1 Procedure: (PC200W Step 1)	51
4.2.5.2 Procedure: (PC200W Steps 2 to 4)	51
4.2.5.3 Procedure: (PC200W Step 5)	52
4.2.5.4 Procedure: (PC200W Step 6)	53
4.2.5.5 Procedure: (PC200W Steps 7 to 9)	53
4.2.5.6 Procedure: (PC200W Steps 10 to 11)	54
4.2.5.7 Procedure: (PC200W Steps 12 to 13)	55
Section 5. System Overview	57
5.1 CR1000 Datalogger	58
5.1.1 Clock	59
5.1.2 Sensor Support	59
5.1.3 CR1000 Wiring Panel	60
5.1.3.1 Measurement Inputs	60
5.1.3.2 Voltage Outputs	61
5.1.3.3 Grounding Terminals	62
5.1.3.4 Power Terminals	62
5.1.3.4.1 Power In	62
5.1.3.4.2 Power Out	62
5.1.3.5 Communications Ports	63
5.1.4 CR1000KD Keyboard Display	63
5.1.5 Power Requirements	64
5.1.6 Programming	65
5.1.6.1 Operating System and Settings	65
5.1.6.2 User Programming	66
5.1.7 Memory and Final Data Storage	66
5.1.8 Data Retrieval	67
5.1.8.1 Via Telecommunications	67
5.1.8.2 Via Mass-Storage Device	67
5.1.8.3 Via CF Card	68
5.1.8.4 Data File-Formats in CR1000 Memory	68
5.1.8.5 Data Format on Computer	68
5.1.9 Communications	68
5.1.9.1 PakBus	69
5.1.9.2 Modbus	69
5.1.9.3 DNP3 Communication	69
5.1.9.4 Keyboard Display	70
5.1.9.4.1 Custom Menus	70
5.1.10 Security	70
5.1.10.1 Vulnerabilities	71
5.1.10.2 Pass-code Lockout	72
5.1.10.2.1 Security By-Pass	74
5.1.10.3 Passwords	74
5.1.10.3.1 .csipasswd	74
5.1.10.3.2 PakBus Instructions	74
5.1.10.3.3 IS Instructions	74
5.1.10.3.4 Settings	75
5.1.10.4 File Encryption	75
5.1.10.5 Communications Encryption	75
5.1.10.6 Hiding Files	75
5.1.10.7 Signatures	75
5.1.11 Maintenance	76
5.1.11.1 Protection from Water	76
5.1.11.2 Protection from Voltage Transients	76
5.1.11.3 Calibration	76
5.1.11.4 Internal Battery	76
5.2 Datalogger Support Software	77
Section 6. CR1000 Specifications	79
Section 7. Installation	81
7.1 Moisture Protection	81
7.2 Temperature Range	81
7.3 Enclosures	81
7.4 Power Sources	82
7.4.1 CR1000 Power Requirement	83
7.4.2 Calculating Power Consumption	83
7.4.3 Power Supplies	83
7.4.3.1 External Batteries	83
7.4.4 Vehicle Power Connections	83
7.4.5 Powering Sensors and Devices	84
7.4.5.1 Switched Voltage Excitation	85
7.4.5.2 Continuous Regulated (5 Volt)	85
7.4.5.3 Continuous Unregulated (Nominal 12 Volt)	86
7.4.5.4 Switched Unregulated (Nominal 12 Volt)	86
7.5 Grounding	86
7.5.1 ESD Protection	86
7.5.1.1 Lightning Protection	88
7.5.2 Single-Ended Measurement Reference	89
7.5.3 Ground Potential Differences	90
7.5.3.1 Soil Temperature Thermocouple	90
7.5.3.2 External Signal Conditioner	90
7.5.4 Ground Looping in Ionic Measurements	91
7.6 CR1000 Configuration	92
7.6.1 Device Configuration Utility	92
7.6.2 Sending the Operating System	93
7.6.2.1 Sending OS with DevConfig	94
7.6.2.2 Sending OS with Program Send	95
7.6.2.3 Sending OS with External Memory	96
7.6.3 Settings	96
7.6.3.1 Settings via DevConfig	96
7.6.3.1.1 Deployment Tab	98
7.6.3.1.2 Logger Control Tab	102
7.6.3.2 Settings via CRBasic	103
7.6.3.3 Durable Settings	103
7.6.3.3.1 \	104
7.6.3.3.2 Default.cr1 File	106
7.6.3.4 Program Run Priorities	106
7.6.3.5 Network Planner	107
7.6.3.5.1 Overview	107
7.6.3.5.2 Basics	108
7.7 Programming	108
7.7.1 Writing and Editing Programs	109
7.7.1.1 Short Cut Editor and Program Generator	109
7.7.1.2 CRBasic Editor	109
7.7.1.2.1 Inserting Comments into Program	110
7.7.2 Sending Programs	110
7.7.2.1 Preserving Data at Program Send	110
7.7.3 Syntax	112
7.7.3.1 Numerical Formats	112
7.7.3.2 Structure	112
7.7.3.3 Command Line	114
7.7.3.3.1 Multiple Statements on One Line	115
7.7.3.3.2 One Statement on Multiple Lines	115
7.7.3.4 Single-Line Declarations	115
7.7.3.4.1 Variables	115
7.7.3.4.2 Constants	122
7.7.3.4.3 Alias and Unit Declarations	124
7.7.3.5 Declared Sequences	125
7.7.3.5.1 Data Tables	125
7.7.3.5.2 Subroutines	132
7.7.3.5.3 Incidental Sequences	132
7.7.3.6 Execution and Task Priority	132
7.7.3.6.1 Pipeline Mode	133
7.7.3.6.2 Sequential Mode	134
7.7.3.7 Execution Timing	135
7.7.3.7.1 Scan() / NextScan	136
7.7.3.7.2 SlowSequence / EndSequence	137
7.7.3.7.3 SubScan() / NextSubScan	137
7.7.3.7.4 Scan Priorities in Sequential Mode	137
7.7.3.8 Instructions	139
7.7.3.8.1 Measurement and Data-Storage Processing	139
7.7.3.8.2 Argument Types	140
7.7.3.8.3 Names in Arguments	140
7.7.3.8.4 Expressions in Arguments	141
7.7.3.8.5 Arrays of Multipliers and Offsets	141
7.7.3.9 Expressions	142
7.7.3.9.1 Floating-Point Arithmetic	142
7.7.3.9.2 Mathematical Operations	143
7.7.3.9.3 Expressions with Numeric Data Types	143
7.7.3.9.4 Logical Expressions	145
7.7.3.9.5 String Expressions	147
7.7.3.10 Program Access to Data Tables	148
7.7.3.11 System Signatures	150
7.7.4 Tips	150
7.7.4.1 Use of Variable Arrays to Conserve Code Space	150
7.7.4.2 Use of Move() to Conserve Code Space	150
7.8 Programming Resource Library	151
7.8.1 Calibration Using FieldCal() and FieldCalStrain()	151
7.8.1.1 CAL Files	151
7.8.1.2 CRBasic Programming	151
7.8.1.3 Calibration Wizard Overview	152
7.8.1.4 Manual Calibration Overview	152
7.8.1.4.1 Single-Point Calibrations (zero, offset, or zero basis)	152
7.8.1.4.2 Two-point Calibrations (multiplier / gain)	153
7.8.1.5 FieldCal() Demonstration Programs	153
7.8.1.5.1 Zero or Tare (Option 0)	154
7.8.1.5.2 Offset (Option 1)	155
7.8.1.5.3 Zero Basis (Option 4)	157
7.8.1.5.4 Two-Point Slope and Offset (Option 2)	159
7.8.1.5.5 Two-Point Slope Only (Option 3)	161
7.8.1.6 FieldCalStrain() Demonstration Program	162
7.8.1.6.1 Quarter-Bridge Shunt (Option 13)	165
7.8.1.6.2 Quarter-Bridge Zero (Option 10)	165
7.8.2 Information Services	166
7.8.2.1 PakBus Over TCP/IP and Callback	167
7.8.2.2 Default HTTP Web Server	167
7.8.2.3 Custom HTTP Web Server	168
7.8.2.4 FTP Server	171
7.8.2.5 FTP Client	171
7.8.2.6 Telnet	171
7.8.2.7 SNMP	171
7.8.2.8 Ping	171
7.8.2.9 Micro-Serial Server	172
7.8.2.10 Modbus TCP/IP	172
7.8.2.11 DHCP	172
7.8.2.12 DNS	172
7.8.2.13 SMTP	172
7.8.3 SDI-12 Sensor Support	172
7.8.3.1 SDI-12 Transparent Mode	173
7.8.3.1.1 SDI-12 Transparent Mode Commands	174
7.8.3.2 SDI-12 Programmed Modes	177
7.8.3.2.1 SDI-12 Recorder Mode	177
7.8.3.2.2 SDI-12 Sensor Mode	184
7.8.3.3 SDI-12 Power Considerations	185
7.8.4 Subroutines	187
7.8.5 Wind Vector	188
7.8.5.1 OutputOpt Parameters	188
7.8.5.2 Wind Vector Processing	189
7.8.5.2.1 Measured Raw Data	190
7.8.5.2.2 Calculations	190
7.8.6 Custom Menus	193
7.8.7 Conditional Compilation	198
7.8.8 Serial I/O	200
7.8.8.1 Introduction	201
7.8.8.2 I/O Ports	202
7.8.8.3 Protocols	202
7.8.8.4 Glossary of Terms	203
7.8.8.5 CRBasic Programming	204
7.8.8.5.1 Input Instruction Set Basics	205
7.8.8.5.2 Input Programming Basics	206
7.8.8.5.3 Output Programming Basics	208
7.8.8.5.4 Translating Bytes	208
7.8.8.5.5 Memory Considerations	209
7.8.8.5.6 Demonstration Program	210
7.8.8.6 Testing Applications	211
7.8.8.6.1 Configure HyperTerminal	211
7.8.8.6.2 Create Send Text File	214
7.8.8.6.3 Create Text-Capture File	214
7.8.8.6.4 Serial Input Test Program	214
7.8.8.7 Q & A	220
7.8.9 TrigVar and DisableVar — Controlling Data Output and Processing	222
7.8.10 NSEC Data Type	223
7.8.10.1 NSEC Options	224
7.8.11 Bool8 Data Type	227
7.8.12 Faster Measurement Rates	231
7.8.12.1 Measurements from 1 Hz to 100 Hz	232
7.8.12.2 Measurement Rate: 101 to 600 Hz	233
7.8.12.2.1 SubScan() / NextSubScan Details	234
7.8.12.3 Measurement Rate: 601 to 2000 Hz	235
7.8.13 String Operations	236
7.8.13.1 String Operators	237
7.8.13.2 String Concatenation	237
7.8.13.3 String NULL Character	238
7.8.13.4 Inserting String Characters	239
7.8.13.5 Extracting String Characters	239
7.8.13.6 String Use of ASCII / ANSII Codes	239
7.8.13.7 Formatting Strings	240
7.8.13.8 Formatting String Hexadecimal Variables	240
7.8.14 Data Tables	240
7.8.15 PulseCountReset Instruction	241
7.8.16 Program Signatures	242
7.8.16.1 Text Signature	242
7.8.16.2 Binary Runtime Signature	242
7.8.16.3 Executable Code Signatures	242
7.8.17 Advanced Programming Examples	243
7.8.17.1 Miscellaneous Features	243
7.8.17.2 Running Average and Total of Rain	246
7.8.17.3 Use of Multiple Scans	246
7.8.17.4 Groundwater Pump Test	247
7.8.17.5 Scaling Array	250
7.8.17.6 Conditional Output	251
7.8.17.7 Capturing Events	252
7.8.18 PRT Measurement	253
7.8.18.1 PRT Calculation Standards	253
7.8.18.2 Measuring PT100s (100-Ohm PRTs)	257
7.8.18.2.1 Self-Heating and Resolution	257
7.8.18.2.2 PT100 in Four-Wire Half-Bridge	257
7.8.18.2.3 PT100 in Three-Wire Half-Bridge	259
7.8.18.2.4 PT100 in Four-Wire Full-Bridge	261
7.8.19 Running Average	263
7.8.20 Writing High-Frequency Data to CF	266
7.8.20.1 TableFile() with Option 64	266
7.8.20.2 TableFile() with Option 64 Replaces CardOut()	267
7.8.20.3 TableFile() with Option 64 Programming	267
7.8.20.4 Converting TOB3 Files with CardConvert	268
7.8.20.5 TableFile() with Option 64 Q & A	268
Section 8. Operation	273
8.1 Measurements	273
8.1.1 Time	273
8.1.1.1 Time Stamps	273
8.1.2 Voltage	274
8.1.2.1 Input Limits	275
8.1.2.2 Reducing Error	276
8.1.2.3 Measurement Sequence	277
8.1.2.4 Measurement Accuracy	278
8.1.2.5 Voltage Range	280
8.1.2.5.1 AutoRange	280
8.1.2.5.2 Fixed Voltage Ranges	281
8.1.2.5.3 Common Mode Null / Open Input Detect	281
8.1.2.6 Offset Voltage Compensation	282
8.1.2.6.1 Input and Excitation Reversal	282
8.1.2.6.2 Ground Reference Offset Voltage	283
8.1.2.6.3 Background Calibration	283
8.1.2.7 Integration	283
8.1.2.7.1 ac Power Line Noise Rejection	284
8.1.2.8 Signal Settling Time	286
8.1.2.8.1 Minimizing Settling Errors	287
8.1.2.8.2 Measuring the Necessary Settling Time	287
8.1.2.9 Self-Calibration	289
8.1.2.10 Time Skew Between Measurements	294
8.1.3 Resistance Measurements	295
8.1.3.1 ac Excitation	297
8.1.3.2 Accuracy of Ratiometric-Resistance Measurements	298
8.1.3.3 Strain Calculations	300
8.1.4 Thermocouple	301
8.1.4.1 Error Analysis	302
8.1.4.1.1 Panel-Temperature Error	302
8.1.4.1.2 Thermocouple Limits of Error	304
8.1.4.1.3 Thermocouple Voltage Measurement Error	305
8.1.4.1.4 Ground Looping Error	309
8.1.4.1.5 Noise Error	309
8.1.4.1.6 Thermocouple Polynomial Error	309
8.1.4.1.7 Reference-Junction Error	310
8.1.4.1.8 Thermocouple Error Summary	310
8.1.4.2 Use of External Reference Junction	311
8.1.5 Pulse	312
8.1.5.1 Pulse-Input Channels (P1 - P2)	314
8.1.5.1.1 High-frequency Pulse (P1 - P2)	315
8.1.5.1.2 Low-Level ac (P1 - P2)	315
8.1.5.1.3 Switch Closure (P1 - P2)	315
8.1.5.2 Pulse Input on Digital I/O Channels C1 - C8	315
8.1.5.2.1 High Frequency Mode	316
8.1.5.2.2 Low-Frequency Mode	316
8.1.5.3 Pulse Measurement Tips	317
8.1.5.3.1 Frequency Resolution	318
8.1.5.4 Pulse Measurement Problems	320
8.1.5.4.1 Pay Attention to Specifications	320
8.1.5.4.2 Input Filters and Signal Attenuation	320
8.1.5.4.3 Switch Bounce and NAN	322
8.1.6 Period Averaging	322
8.1.7 SDI-12 Recording	323
8.1.8 RS-232 and TTL	323
8.1.9 Field Calibration	324
8.1.10 Cabling Effects	324
8.1.10.1 Analog Sensor Cables	324
8.1.10.2 Pulse Sensors	324
8.1.10.3 RS-232 Sensors	325
8.1.10.4 SDI-12 Sensors	325
8.1.11 Synchronizing Measurements	325
8.2 Measurement and Control Peripherals	326
8.2.1 Analog-Input Expansion Modules	327
8.2.2 Pulse-Input Expansion Modules	327
8.2.3 Serial-Input Expansion Modules	327
8.2.4 Control Outputs	327
8.2.4.1 Digital I/O Ports	327
8.2.4.2 Relays and Relay Drivers	328
8.2.4.3 Component-Built Relays	328
8.2.5 Analog Control / Output Devices	329
8.2.6 TIMs	329
8.2.7 Vibrating Wire	330
8.2.8 Low-level ac	330
8.3 Memory and Final Data Storage	330
8.3.1 Storage Media	330
8.3.1.1 Data Storage	332
8.3.1.1.1 Data Table SRAM	333
8.3.1.1.2 CPU: Drive	333
8.3.1.1.3 USR: Drive	333
8.3.1.1.4 USB: Drive	334
8.3.1.1.5 CRD: Drive	334
8.3.1.1.6 Data File Formats	335
8.3.2 Memory Conservation	339
8.3.3 Memory Reset	339
8.3.3.1 Full Memory Reset	339
8.3.3.2 Program Send Reset	340
8.3.3.3 Manual Data-Table Reset	340
8.3.3.4 Formatting Drives	340
8.3.4 File Management	340
8.3.4.1 File Attributes	342
8.3.4.2 Data Preservation	343
8.3.4.3 External Memory Power-up	343
8.3.4.3.1 Creating and Editing Powerup.ini	344
8.3.4.4 File Management Q & A	347
8.3.5 File Names	347
8.3.6 File System Errors	347
8.3.7 Memory Q & A	348
8.4 Telecommunications and Data Retrieval	348
8.4.1 Hardware and Carrier Signal	349
8.4.2 Protocols	350
8.4.3 Initiating Telecommunications (Callback)	350
8.5 PakBus Overview	351
8.5.1 PakBus Addresses	351
8.5.2 Nodes: Leaf Nodes and Routers	351
8.5.2.1 Router and Leaf-Node Configuration	352
8.5.3 Linking PakBus Nodes: Neighbor Discovery	353
8.5.3.1 Hello-message (two-way exchange)	354
8.5.3.2 Beacon (one-way broadcast)	354
8.5.3.3 Hello-request (one-way broadcast)	354
8.5.3.4 Neighbor Lists	354
8.5.3.5 Adjusting Links	354
8.5.3.6 Maintaining Links	354
8.5.4 PakBus Troubleshooting	355
8.5.4.1 Link Integrity	355
8.5.4.1.1 Automatic Packet-Size Adjustment	355
8.5.4.2 Ping	356
8.5.4.3 Traffic Flow	356
8.5.5 LoggerNet Network-Map Configuration	356
8.5.6 PakBus LAN Example	357
8.5.6.1 LAN Wiring	357
8.5.6.2 LAN Setup	358
8.5.6.3 LoggerNet Setup	361
8.5.7 PakBus Encryption	363
8.6 Alternate Telecommunications	364
8.6.1 DNP3	364
8.6.1.1 Overview	364
8.6.1.2 Programming for DNP3	364
8.6.1.2.1 Declarations	364
8.6.1.2.2 CRBasic Instructions	365
8.6.1.2.3 Programming for Data-Acquisition	366
8.6.2 Modbus	367
8.6.2.1 Overview	367
8.6.2.2 Terminology	368
8.6.2.2.1 Glossary of Terms	368
8.6.2.3 Programming for Modbus	369
8.6.2.3.1 Declarations	369
8.6.2.3.2 CRBasic Instructions - Modbus	369
8.6.2.3.3 Addressing (ModbusAddr)	370
8.6.2.3.4 Supported Function Codes (Function)	370
8.6.2.3.5 Reading Inverse-Format Registers	370
8.6.2.4 Troubleshooting	370
8.6.2.5 Modbus over IP	371
8.6.2.6 Modbus tidBytes	371
8.6.2.7 Converting 16-bit to 32-bit Longs	371
8.6.3 Web Service API	372
8.6.3.1 Authentication	372
8.6.3.2 Command Syntax	373
8.6.3.3 Time Syntax	375
8.6.3.4 Data Management	375
8.6.3.4.1 BrowseSymbols Command	375
8.6.3.4.2 DataQuery Command	379
8.6.3.5 Control	385
8.6.3.5.1 SetValueEx Command	385
8.6.3.6 Clock Functions	387
8.6.3.6.1 ClockSet Command	387
8.6.3.6.2 ClockCheck Command	389
8.6.3.7 Files Management	391
8.6.3.7.1 Sending a File to a Datalogger	391
8.6.3.7.2 FileControl Command	392
8.6.3.7.3 ListFiles Command	394
8.6.3.7.4 NewestFile Command	398
8.7 Support Software	399
8.8 Using the Keyboard Display	399
8.8.1 Data Display	402
8.8.1.1 Real-Time Tables and Graphs	403
8.8.1.2 Real-Time Custom	403
8.8.1.3 Final-Storage Tables	405
8.8.2 Run/Stop Program	406
8.8.3 File Display	407
8.8.3.1 File: Edit	407
8.8.4 PCCard (CF Card) Display	409
8.8.5 Ports and Status	409
8.8.6 Settings	410
8.8.6.1 Set Time / Date	411
8.8.6.2 PakBus Settings	411
8.8.7 Configure Display	411
8.9 Program and OS File Compression	411
8.10 CF Cards & Records Number	414
Section 9. Maintenance	417
9.1 Moisture Protection	417
9.2 Replacing the Internal Battery	417
9.3 Repair	420
Section 10. Troubleshooting	423
10.1 Status Table	423
10.2 Operating Systems	423
10.3 Programming	423
10.3.1 Status Table as Debug Resource	423
10.3.1.1 CompileResults	424
10.3.1.2 SkippedScan	425
10.3.1.3 SkippedSlowScan	425
10.3.1.4 SkippedRecord	425
10.3.1.5 ProgErrors	426
10.3.1.6 MemoryFree	426
10.3.1.7 VarOutOfBounds	426
10.3.1.8 WatchdogErrors	426
10.3.1.8.1 Status Table WatchdogErrors	426
10.3.1.8.2 Watchdoginfo.txt File	427
10.3.2 Program Does Not Compile	427
10.3.3 Program Compiles / Does Not Run Correctly	427
10.3.4 NAN and ±INF	428
10.3.4.1 Measurements and NAN	428

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Section 7.

Installation

7.5.4 Ground Looping in Ionic Measurements

When measuring soil-moisture with a resistance block, or water conductivity with

a resistance cell, the potential exists for a ground loop error.

In the case of an

ionic soil matric potential (soil moisture) sensor, a ground loop arises because soil

and water provide an alternate path for the excitation to return to CR1000 ground.

This example is modeled in the diagram, figure

Model of a Ground Loop with a

Resistive Sensor

(p. 92).

With Rg in the resistor network, the signal measured from

the sensor will be:

where

is the excitation voltage

is a fixed resistor

is the sensor resistance

is the resistance between the excited electrode and CR1000 earth

ground.

is the source of error due to the ground loop. When R

is large, the error is

negligible.

Note that the geometry of the electrodes has a great effect on the

magnitude of this error. The Delmhorst gypsum block used in the Campbell

Scientific 227 probe has two concentric cylindrical electrodes. The center

electrode is used for excitation; because it is encircled by the ground electrode, the

path for a ground loop through the soil is greatly reduced. Moisture blocks which

consist of two parallel plate electrodes are particularly susceptible to ground loop

problems. Similar considerations apply to the geometry of the electrodes in water

conductivity sensors.

The ground electrode of the conductivity or soil moisture probe and the CR1000

earth ground form a galvanic cell, with the water/soil solution acting as the

electrolyte. If current is allowed to flow, the resulting oxidation or reduction will

soon damage the electrode, just as if dc excitation was used to make the

measurement. Campbell Scientific resistive soil probes and conductivity probes

are built with series capacitors to block this dc current. In addition to preventing

sensor deterioration, the capacitors block any dc component from affecting the

measurement.