ERSS User Manual - Transcend Engineering
Contents
1. S m S cm mS cm uS cm ohm m Note 7 0 07 70 70000 0 14 5 0 05 50 50000 0 2 4 8 0 048 48 48000 0 2083 Seawater 2 0 02 20 20000 0 5 1 0 01 10 10000 1 0 5 0 005 5000 2 0 2 0 002 2000 5 0 1 0 001 1 1000 10 0 05 0 0005 0 5 500 20 0 02 0 0002 0 2 200 50 0 01 0 0001 0 1 100 100 0 005 0 00005 0 05 50 200 0 002 0 00002 0 02 20 500 0 001 0 00001 0 01 10 1000 Drinking Water 0 0005 0 000005 0 005 5 2000 Range 0 0003 0 000003 0 003 3 3333 0 0002 0 000002 0 002 2 5000 0 0001 0 000001 0 001 1 10000 0 00005 0 0000005 0 0005 0 5 20000 De Ionized H2O ERSS Users Manual version 1 0 Page 16 of 162. 10 bit ADC coefficients set using C command output string lt CR gt id M 3 lt CR gt Stream calibrated FF rrr rrrrrr r resistance in ohms resistance and tt ttt lt CR gt t temperature in deg C temperature repeatedly idJM 4 lt CR gt Stream calibrated FF rrr rrrrrr r resistivity in ohm m resistivity and tt ttt lt CR gt t temperature in deg C temperature repeatedly id M 0 lt CR gt Stop streaming data FF 0K lt CR gt r resistivity in ohm m t temperature in deg C Calibration Commands id C 0 x lt CR gt Set value of R8 FF OK lt CR gt x resistance of R8 in ohms id C 1 x lt CR gt Set Calibration FF OK lt CR gt x Calibration Factor Factor id C 2 x lt CR gt Set Temperature FF OK lt CR gt x Temperature Gain deg Gain C mA id C 3 x lt CR gt Set Temperature FF OK lt CR gt x Temperature Offset deg C Offset id C 4 x lt CR gt Set Field Geometry FF 0K lt CR gt x Field Geometry Factor Factor id C 5 x lt CR gt Set value of R11 FF OK lt CR gt x R11 resistance ohms id C 6 x lt CR gt Set Resistance FF OK lt CR gt x Resistance Offset ohms Offset id B lt CR gt Display all FF formatted x Resistance Offset ohms The hardware protocol is two wire RS 485 Communications parameters are 9600 baud no parity 8 bits per word 1 stop bit The command terminator is an ASCII carriage return lt CR gt Hex 0D Decimal 13 Each sensor has
3. applied to a pair of outer electrodes and voltage is measured across a pair of inner electrodes using a very high impedance voltmeter as shown in the left hand figure below This results in a distribution of current and potential in the measured medium similar to that idealized in the right hand figure below The resistivity measurement using the Wenner style arrangement at the boundary of a theoretical infinite half space represents the average resistivity of a hemisphere of medium with radius equal to the electrode separation The geometric scaling yields the following expression for electrical resistivity p ERSS Users Manual version 1 0 Page 3 of 16 b S a bR 1 M In the above equation b is the outer electrode separation a is the inner electrode separation and R is the measured resistance For equally spaced electrodes e g b 3a this expression reduces to p 2zaR ERSS is designed for use with a user provided Wenner style four electrode array in contact with earth media The user is free to decide the size and spacing of the electrodes The ERSS supports a broad range of electrode spacings by allowing the user to adjust user configurable parameters both in software and in the values chosen for two physical resistors on the printed circuit board One of the software parameters is a Field Geometry Factor that in theory should equate to the value of 27a in the above equation but can be set to any value that calibr
4. 50 50 2 50 20 00000083 0 0000028 68 252 98 246 20 am aga 161 249 20 300 11 3 222 2 51 24 0 0000015 0 0000006 3 IIT INETEN 0 0000150 72 NECEM ERE Nes L2 Mr E Le ass s nc an e o jene e rm METTE a ET NN Bus x Lene ced Lasso go go 250 so Ee i ee ale m a e m 22 300 11 3 222 2 51 0 0000034 16 Pee AE ed POOR E aso so so 250 4o 25 126 o 0000075 36 i a RNC EN RES ERE E ERSS Users Manual version 1 0 Page 11 of 16 ao ug 222 25 4o 25 28 0000007 8 ss o8 246 zo 25 32 oooooorn o Lam 154 xe 249 20 25 19 o000002 6 os asa So so om oooog9 269 50 50 2 50 0 0000300 144 25 2 0 0000153 7 15 4 161 0 0000093 4 11 3 222 bg 0 0000068 3 135 0 0000150 7 161 8 0 0000046 2 202 2g 4o so 57 o 0000034 16 27 235 0 0000140 67 0 0000075 126 36 64 0 0000038 18 39 11 28 ERSS Users Manual version 1 0 Page 12 of 16 Appendix B Serial Communications Command Reference Command Function Action Response Value id V lt CR gt Report software FF x lt CR gt x software version number version number Setup Commands id l aa lt CR gt Set sensor ID FF OK lt CR gt aa 1 byte address id N hh lt CR gt Set Averaging FF OK lt CR gt hh hexadecimal number of Sample Siz
5. Transcendev Electrical Resistivity Smart Sensor ERSS User Manual version 1 0 Table of Contents Theory of Operation Electrode Arrangement Polarity Temperature Effects Calibration ERSS Electrical Functionality Power Requirements Electrodes User Configurable Parameters ID I command Bipolar Excitation Rate P command Input Gain Range G command Averaging Sample Size N command R8 Value C command option 0 Resistance Offset C command option 6 Calibration Factor C command option 1 Field Geometry Factor C command option 4 Temperature Calibration Offset C command option 3 Temperature Calibration Gain C command option 2 Self Calibration Enable Disable U command factory use only FAST Step Mode Enable Disable F command factory use only Measurement Range Serial Communications Appendix A Measurement Range and Resolution as a Function of User Configuration Appendix B Serial Communications Command Reference Appendix C Definition of Field Geometry Factor Appendix D Equivalence of Electrical Resistivity to Conductivity ERSS Users Manual version 1 0 O 000900000000 4 4 444 A A ANA 00 ey e A mR 13 15 16 Page 2 of 16 Theory of Operation In theory the electrical resistivity of a material is defined as the resistance per length for a unit cross sectional area This yields units of ohms multiplied by area and divided by length or ohm distance Typical resistivity units are o
6. a 2 byte ID All commands begin with a sensor ID 00 is reserved ID All sensors respond to ID 00 regardless of their individually assigned ID To avoid data collisions this ID should only be used when only one sensor is attached to the RS 485 bus ERSS Users Manual version 1 0 Page 14 of 16 Appendix C Definition of Field Geometry Factor ASTM standard G57 06 gives the following equation for computing the resistivity of a soil medium from the measured resistance and electrode spacing using an equally spaced Wenner style four electrode array in a homogenous half space current is assumed to flow evenly through a hemispherical volume of soil with radius equal to the electrode spacing p 7 2zaR where Ris the resistance ohms a is the electrode spacing m pis the resistivity ohm m The ASTM standard indicates that the resistance R to be used is the voltage drop V across the inner electrodes divided by the current 7 applied to the outer electrodes resulting in a reformulation of the above equation as V 2na p r This is notably consistent with a solution derived by Avants et al 1999 1 Avants B Soodak D Ruppeiner G Measuring the Electrical Conductivity of the Earth American Journal of Physics 67 7 July 1999 See equation 8 in the cited reference ERSS Users Manual version 1 0 Page 15 of 16 Appendix D Equivalence of Electrical Resistivity to Conductivity
7. ation data indicate is appropriate for the specific use Polarity In a medium bearing mobile chargeable molecules specific attention must be given to the potential to induce electrophoresis and or polarize the medium through application of DC current For this reason measurements of electrical conductivity of earth materials are generally conducted using low frequency alternating sinusoidal current or rapidly alternating DC current step function usually around 100 Hz although frequencies anywhere between 20 and 1000 Hz have been reported in the literature The ERSS applies a bi polar excitation current to the medium reversing the polarity of the current at a user configurable rate of either 50 100 or 200 Hz The factory default frequency is 100 Hz Temperature Effects The electrical conductivity inverse of resistivity of seawater and other materials including marine sediments varies with temperature The chart below shows the magnitude of the temperature effect for seawater ERSS Users Manual version 1 0 Page 4 of 16 Effect of Temperature on Conductivity of Seawater 0 07 0 06 0 05 0 04 0 03 Conductivity S cm 0 02 0 01 0 5 10 15 20 25 30 Temperature C The ERSS measures and reports resistivity at the prevailing ambient temperature which should be independently ascertained if the user desired to compute a temperature compensated resistivity value The ERSS does not provide on board temperature compensa
8. brated resistance values in ohms the Resistance Offset is subtracted from the total resistance so the result reflects only the resistance across the electrodes Calibration Factor C command option 1 The Calibration Factor accounts for the deviation from nominal values of R8 and other physical components in the ERSS measurement circuitry The Calibration Factor is a linear factor multiplied by the theoretical series equivalent resistance computed from the ratiometrically measured voltage drop across the inner electrodes to yield the actual series equivalent resistance from which resistivity of the medium is then calculated by application of the Field Field Geometry Factor Field Geometry Factor C command option 4 The Field Geometry Factor accounts for the geometry of the DC electrical field the sensor generates in the medium and gets multiplied by the measured series equivalent resistance to yield the resistivity value reported by the ERSS In theory as discussed above the Field Geometry Factor for equally spaced electrodes against an infinite half space should be 27ra in which a is the electrode spacing In practice the Field Geometry Factor often deviates slightly from theory due to the violation of theoretical assumptions For example the electrodes have a finite area in contact with the medium and the medium s 3D geometry does not match the idealization of an infinite half space The user must experimentally determine the Field Geometr
9. ctory default setting disables FAST Step mode and the user should not enable FAST Step Mode A calibration performed with FAST Step Mode disabled will not be valid with FAST Step Mode enabled and vice versa Measurement Range The measurement range of the ERSS is highly configurable through selection of the values of discrete resistors labeled R7 and R8 on the printed circuit board R7 controls the amount of current applied to the medium through the user supplied Wenner 4 electrode array factory default R8 controls the reference voltage applied to the on board analog to digital converter ADC as a result of the applied current The reference ADC voltage must be in the range of 1 9 to 2 6 volts In the ERSS factory default configuration the value of R7 is 681 ohms and that of R8 is 25 2 kilo ohms This combination results in a nominal excitation current of 98 micro amps and an ADC input reference voltage of 2 46 volts The excitation current in micro amps is determined by the formula 1 266511 R The reference voltage in volts is Vig IMRs In these two equations R and Rs refer to the values of resistors R7 and R8 respectively Metal film or thick film resistors with a temperature coefficient of less than 20ppm C ERSS Users Manual version 1 0 Page 9 of 16 are recommended to ensure temperature stability of the electronics The table in Appendix A shows the measurement range and resolution that will result from several combinatio
10. e samples to average for each reported result 02 to FE 254 decimal id P h lt CR gt Set Bipolar FF OK lt CR gt Value of h Rate Excitation Rate 800 25 Hz 400 50 Hz 200 100 Hz default 100 200 Hz id G h lt CR gt Set Input Gain FF OK lt CR gt Value of h Range Range 0 10 mV 1 20 mV 2 40 mV 3 80 mV default id U v lt CR gt Enable Disable FF OK lt CR gt v 0 disable default Internal Self v 1 enabled Calibration factory use only id F v lt CR gt Enable Disable FF OK lt CR gt v 0 disable default FAST Step Mode v 1 enabled factory use only id D lt CR gt Display settings FF formatted output string lt CR gt Measurement Commands id S 1 lt CR gt Report single integer FF vvvvvv iiii v 3 byte voltage 24 bit ADC ADC values for tttt lt CR gt i 2 byte current 10 bit ADC voltage current and t 2 byte temperature 10 bit temperature ADC id S 3 lt CR gt Report single FF rrr rrrrrr tt ttt r resistance in ohms calibrated resistance t temperature in deg C and temperature id S 4 lt CR gt Report single FF rrr rrrrrr tt ttt r resistivity in ohm m calibrated resistivity t temperature in deg C ERSS Users Manual version 1 0 Page 13 of 16 and temperature id M 1 lt CR gt Stream measurements of integer ADC values for voltage current and temperature FF vvvvvv iii tttt lt CR gt repeatedly v 3 byte voltage 24 bit ADC i 2 byte current 10 bit ADC t 2 byte temperature
11. h end of the measurement range Users are advised not to set this parameter higher than 254 as that is the largest number of samples for which a correct response can be guaranteed when the resistivity of the medium is at the upper limit of the measurement range R8 Value C command option o R8 Value is the value in ohms of the resistor labeled R8 on the ERSS printed circuit board that is used to sense the current that gets applied to the medium during measurement Since users are free to change the physical component R8 to customize the measurement range and resolution of the ERSS the user must specify the nominal value of R8 tot he embedded software A nominal value is all that is needed as the Calibration Factor accounts for any deviation from nominal in R8 as well as deviations in other electrical components on the board The function of R8 and guidance for selecting ERSS Users Manual version 1 0 Page 7 of 16 custom alternative values are discussed in the Measurement Range section of this manual Resistance Offset C command option 6 The Resistance Offset parameter accounts for the amount of electrical resistance sensed by the ERSS that is dues to the internal electrical losses and the wiring to the Wenner electrode array regardless of the electrical resistivity of the medium This resistance is in series with the medium and is thus additive with the series equivalent resistance due to the resistivity of the medium In reporting cali
12. hm cm or ohm m While geophysicists and engineers are accustomed to thinking in terms of resistivity physical oceanographers generally prefer the reciprocal property of electrical conductivity due to its directly proportional relationship with salinity Electrical conductivity EC is expressed in Siemens 1 ohms per unit length A typical unit of EC is mS cm At 20 C the conductivity of seawater is 48 mS cm 0 048 S cm This corresponds to a resistivity of 20 Q cm 0 2 Q m The ERSS reports resistivity in units of ohm m A table in Appendix D of this manual presents the equivalence between resistivity in ohm m and conductivity expressed in several different units for values over a broad range Electrode Arrangement Attempts to measure electrical conductivity of porous media or solutions using a two electrode arrangement are problematic because the voltage drop due to the medium must be measured on the same two electrodes that are supplying the current This results in measuring the sum of the resistivity of the medium and that of the contact between electrodes and soil This is particularly troublesome in highly electrolytic solutions in which the induced current can stimulate surface chemistry reactions at the face of the electrodes to create scaling and other skin effects In 1915 Wenner introduced the use of a linear four electrode array that has become the standard approach in the earth sciences ever since In this arrangement current is
13. ior This functionality is implemented for factory calibration and quality assurance purposes only The user is cautioned against enabling internal self calibration as enabling this functionality results in application of a unipolar excitation current to the medium for several milliseconds which in a most media of geophysical interest introduces a polarity bias to subsequent measurements that requires a long period of continuous measurement to neutralize In practice the user should never enable internal self calibration In addition the user has the option change discrete resistors on the ERSS printed circuit board to customize the measurement range and resolution according to the instructions in the Measurement Range section of this manual below FAST Step Mode Enable Disable F command factory use only The AD7730 embedded in the ERSS actually performs hundreds to thousands of samples of input voltage for each cycle in which the applied current polarity is reversed Each sample thus utilized in the multi sample average reported to the user see Averaging Sample Size above is actually the result of digital filtering performed on this high sampling rate data stream FAST Step Mode Enable Disable controls how the on chip digital filtering algorithm compensates for rapid step changes in the input voltage This functionality is implemented for factory use only to configure the ERSS for applications in certain highly capacitive media The fa
14. ircuitry and connection to the electrodes and then to use liquid solutions of known resistivity to experimentally determine the Field Geometry Factor for the device in which the ERSS is installed ERSS Users Manual version 1 0 Page 5 of 16 ERSS Electrical Functionality The electrical resistivity smart sensor ERSS consists of an analog front end and a digital back end both located on a single PCB The analog front end produces and senses alternating polarity signals using H bridges composed of MOSFET dual N and dual P transistors A LM234Z IC produces a controlled current that is proportional to the resistance of a current setting resistor R7 This current is alternated via an H bridge and passed through the outer electrodes of the Wenner array and a current sense resistor R8 which produces a nominal differential voltage of 2 5v used as the reference voltage for measuring the voltage drop across the inner electrodes of the Wenner array An Analog Devices AD7730 IC digitizes the differential voltage produced across the inner Wenner electrodes provides filtering of the digitized signal and provides a pair of complementary control signals to the MOSFET H bridges The AD7730 s measurement of voltage drop across the inner electrodes is ratiometric to the reference voltage which is derived directly from the applied current via current sense resistor R8 Therefore thermal drift and other sources of instability potentially affecting the applied curre
15. ns of values for R7 R8 Input Gain Range and electrode separation distance Serial Communications The ERSS communicates with a host data logger or computer via RS 485 serial communications using a 2 wire interface The baud rate is 9600 with 8 bits per byte one stop bit and no parity 9600 N 8 1 No other baud rates are supported at this time All serial commands their meanings and syntax are listed in Appendix B Serial Communications Command Reference ERSS Users Manual version 1 0 Page 10 of 16 Appendix A Measurement Range and Resolution as a Function of User Configuration User configurable parameters appear under italicized column headings Factory default value appears against a gray background The column representing the resolution of one bit in ohm meters is the minimum possible electrical resistivity the sensor electronics can register The last column represents the relative resolution in parts per million at the resistivity of standard seawater Electrode Max Resolution ppm Spacing Input of 1 bit resolution Resistivity ohm m at 0 2083 ohm m ohm m 240 os a7 254 so 19 358 oo000m3 102 sao so so 250 So w m ooo ss Pes asa os oss so os foooooss as Lam 154 161 249 So w so o 0000035 17 300 11 3 222 2 51 19 43 0 0000026 12 2490 as prp o 179 1330 2 50 0 0000057 681 NES 0 0000029 249 0 000018 Pa el eee el ned 0 0000013 2490 95 27 20 1330
16. nt source do not affect the measurement All that must be known to infer the series equivalent resistance of the medium from the ADC result is the value of the current sense resistor R8 The 5 volt AD7730 is supervised by a Texas Instruments MSP430 microcontroller residing on the 3 3 v digital end of the PCB A Texas Instruments MSP430 microcontroller controls the AD7730 via a 3 wire SPI interface The MSP430 includes a serial UART which through a transceiver provides the RS 485 communications between the smart sensor and an external controller such as data logger or personal computer running a terminal emulation program Each time a measurement of electrical resistivity is reported by the ERSS it consists of an arithmetic average simple mean of a user specified number of samples 2 to 254 taken by the AD7730 at a user specified rate of 50 100 or 200 Hz The user need only specify these parameters once prior to acquiring measurements as they are stored in internal non volatile memory on the ERSS In addition to averaging number and sample rate the serial communications command set allows the user to set the ERSS address and to specify a number of other calibration and data acquisition parameters also stored in non volatile memory Power Requirements The basic ERSS board requires regulated 5 volt DC power applied to pads 1 V and 4 GND of the position labeled JIN on the printed circuit board However some models are shipped with a Mic
17. rochip MCP1702 LDO regulator wired to the power inputs at the pads labeled JIN on the board This regulator will accept from 3 to 13 volts across the pre wired red V and black GND leads The ERSS will draw a peak current of up to 25 mA while acquiring measurements Electrodes The ERSS is designed to interface to the sensed medium via a user supplied array of four equally spaced Wenner style electrodes Users are free to determine their own electrode size and spacing as the ERSS can and must be programmed to account for the effect of electrode geometry ERSS Users Manual version 1 0 Page 6 of 16 User Configurable Parameters Through the serial communications interface the user has control of the parameters listed below Refer to the section above on Theory of Operation for an understanding of these parameters ID d command This parameter sets the ID of communications to which the ERSS will respond The ID should be set only when a single ERSS is being communicated with and no other ERSS are on the RS 485 bus The ID is a two digit hexadecimal value Both digits must be specified ID oo is reserved and all units will respond to commands issued to this ID so it should likewise be used only with one ERSS on the RS 485 bus Once a new ERSS ID is assigned the unit will respond only to communications tagged with that ID and to 00 the global address FF is also reserved for communication directed at the controller AII units address
18. their responses to ID FF Bipolar Excitation Rate P command Bipolar Excitation rate is the frequency at which the ERSS reverses the polarity of the current it applies to the medium It is the same frequency at which measurements of the medium s response to the applied current are also made Each time the polarity is reversed the sample of resistance obtained is actually the result of numerous Input Gain Range G command The Input Gain Range parameter adjusts the full scale input range of the internal 24 bit analog to digital converter ADC in the ERSS Higher resolution of measurements can be obtained at the expense of decreasing the maximum resistivity that can be measured by selecting a lower Input Gain Range Values of 80 40 and 20 millivolts are available The factory default is 80 millivolts Averaging Sample Size N command This is the integer number of samples of resistivity in the medium that are taken and averaged each time a measurement is reported To cancel of any bias in the polarization state of the medium an even number of samples is always processed thereby ensuring that both excitation polarities are equally represented Valid values include all even integers from 2 to 254 If the user specifies an odd integer for Averaging Sample Size the embedded software rounds up to the next even number Larger values than 254 can be specified but run the risk of accumulator overflow if the resistivity of the medium is at the hig
19. tion because the relationship of resistivity to temperature varies as a function of the medium and for many media is not well known If ERSS attempted temperature compensation it would be incorrect most of the time since it can neither sense what medium it is placed in nor know the relationship of resistivity to temperature in that medium Calibration The ERSS reports measurements in user selected units of either resistivity ohm m resistance ohm or the hexadecimal integer value from the integral 24 bit analog to digital converter ADC The reported value of resistance or resistivity is determined by application of a linear calibration equation that relates the integer output of the ADC to known values of resistors placed in contact with the electrodes and or to known values the resistivity of aqueous KCl calibration solutions of known ionic strength The ERSS stores its calibration equation and coefficients internally and applies them each time data are reported Calibration parameters for determining series equivalent resistance of the medium from the ADC output of current and voltage measurements include Resistance Offset and Resistance Gain The series equivalent resistance is multiplied by a linear Field Geometry Factor to yield the resistivity of the medium Recommended practice for calibration is to use high precision resistors of known value to determine the Resistance Offset and Calibration Factor parameters for the internal ERSS c
20. y Factor for their own electrode array and application environment by first calibrating for resistance and then measuring resistance in a medium of known resistivity and finally dividing the known resistivity of the medium by the ERSS measured series equivalent resistance to yield the Field Geometry Factor Temperature Calibration Offset C command option 3 The ERSS includes a low resolution temperature sensor on the printed circuit board available for monitoring the temperature of the electronics It uses a 10 bit ADC and can resolve temperature differences of only about 0 39C The offset is the temperature at which the voltage across the zener diode drops to zero This theoretically corresponds to zero Kelvin or 273 159C Users are advised to ignore this setting Temperature Calibration Gain C command option 2 This is the temperature response slope of the zener diode in degrees C per change in integer ADC value Accounting for the on board reference voltage and amplification of the temperature signal this value comes out to about 0 3 It is precisely determined and entered into memory during factory calibration of the ERSS Users are advised to ignore this setting ERSS Users Manual version 1 0 Page 8 of 16 Self Calibration Enable Disable U command factory use only The AD7730 embedded in the ERSS has the ability to execute an internal self calibration routine that compensates for minute changes in on chip analog circuit behav
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