Conductivity via Analog Electrical Resistance topic

Introduction

This is for a very basic conductivity circuit built with a single resistor and an old power cord. DC power is briefly supplied across the power cord causing the water to act as one of the resistors on a voltage divider. Knowing the voltage of the other resistor in the divider, we can calculate to resistance from the water (and then its electrical conductivity) based on the drop in volage across the divider.

For this to work, the power across the circuit MUST be turned off between readings. If the power to the circuit is left on the water will become polarized and the values will not be valid. The water temperature (if used) must be supplied separately for a calculation.

The Circuit

One pole of the power cord wire is connected to the ground of the main logger board. The other pole is connected to the sensor power supply via a resistor of known resistance (R1) and then to an analog pin to measure the voltage.

So the circuit is:

The above diagram and the calculations assume the resistance of the analog pins themselves on the Arduino is negligible.

Calculating the Conductivity

First, we need to convert the bit reading of the ADC into volts based on the range of the ADC (1 bit more than the resolution):

\[meas\_voltage = \frac{analog\_ref\_voltage * raw\_adc\_bits}{ANALOG\_EC\_ADC\_RANGE}\]

Assuming the voltage of the ADC reference is the same as that used to power the EC resistor circuit we can replace the reference voltage with the sensor power voltage:

\[meas\_voltage = \frac{sensor\_power\_voltage * raw\_adc\_bits}{ANALOG\_EC\_ADC\_RANGE}\]

Now we can calculate the resistance of the water, knowing the resistance of the resistor we put in the circuit and the voltage drop:

\[R_{water\_ohms} = \frac{meas\_voltage * R_{series\_ohms}}{sensor\_power\_voltage - meas\_voltage}\]

Combining the above equations and doing some rearranging, we get:

\[R_{water\_ohms} = \frac{R_{series\_ohms}}{\frac{ANALOG\_EC\_ADC\_RANGE}{raw\_adc\_bits} - 1}\]

The conductivity is then the inverse of the resistance - multiplied by a measured cell constant and a 10^6 conversion to µS/cm.

\[water\_conductivity = \frac{1000000}{R_{water\_ohms} * sensor_{EC\_Konst}}\]

The real cell constant will vary based on the size of the "cell" - that is, the size of the plug on the power cord. You can calculate the cell constant for each power cord sensor you use following the calibration program.

For one AC Power Cord 12t with male IEC 320-C8 connector the cell constant was 2.88.

References

• For the sensor setup and calculations: https://hackaday.io/project/7008-fly-wars-a-hackers-solution-to-world-hunger/log/24646-three-dollar-ec-ppm-meter-arduino
• For temperature compensation: https://link.springer.com/article/10.1023/B:EMAS.0000031719.83065.68

Build flags

• -D MS_PROCESSOR_ADC_RESOLUTION=##
  • used to set the resolution of the processor ADC
• -D MS_PROCESSOR_ADC_REFERENCE_MODE=xxx
  • used to set the processor ADC value reference mode

Sensor Constructor

Example Code

The analog electrical conductivity sensor is used in the menu a la carte example.

Classes

class AnalogElecConductivity

Class for the analog [Electrical Conductivity monitor](Conductivity via Analog Electrical Resistance)

class AnalogElecConductivity_EC

The Variable sub-class used for electrical conductivity measured using an analog pin connected to electrodes submerged in the medium.

Sensor Variable Counts

The number of variables that can be returned by the analog conductivity sensor

#define ANALOGELECCONDUCTIVITY_NUM_VARIABLES = 1

Sensor::_numReturnedValues; we only get one value from the analog conductivity sensor.

#define ANALOGELECCONDUCTIVITY_INC_CALC_VARIABLES = 0

Sensor::_incCalcValues; we don't calculate any additional values though we recommend users include a temperature sensor and calculate specific conductance in their own program.

Configuration Defines

Defines to help configure the range and resolution of the home-made conductivity sensor depending on the processor and ADC in use.

#define RSERIES_OHMS_DEF = 499

The default resistance (in ohms) of the measuring resistor. This should not be less than 300 ohms when measuring EC in water.

#define SENSOREC_KONST_DEF = 1.0

Cell Constant For EC Measurements.

Sensor Timing

The timing for analog conductivity via resistance.

#define ANALOGELECCONDUCTIVITY_WARM_UP_TIME_MS = 2

Sensor::_warmUpTime_ms; giving 2ms for warm-up.

#define ANALOGELECCONDUCTIVITY_STABILIZATION_TIME_MS = 1

Sensor::_stabilizationTime_ms; we give just a tiny delay for stabilization.

#define ANALOGELECCONDUCTIVITY_MEASUREMENT_TIME_MS = 0

Sensor::_measurementTime_ms; we assume the analog voltage is measured instantly.

Electrical Conductance

The electrical conductance variable from a home-made analog sensor.

#define ANALOGELECCONDUCTIVITY_EC_RESOLUTION = 1

Decimals places in string representation; EC should have 1.

#define ANALOGELECCONDUCTIVITY_EC_VAR_NUM = 0

Sensor variable number; EC is stored in sensorValues[0].

#define ANALOGELECCONDUCTIVITY_EC_VAR_NAME = "electricalConductivity"

Variable name in ODM2 controlled vocabulary; "electricalConductivity".

#define ANALOGELECCONDUCTIVITY_EC_UNIT_NAME = "microsiemenPerCentimeter"

Variable unit name in ODM2 controlled vocabulary; "microsiemenPerCentimeter" (µS/cm)

#define ANALOGELECCONDUCTIVITY_EC_DEFAULT_CODE = "anlgEc"

Default variable short code; "anlgEc".

Define documentation