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This project seeks to create a simple EC measurement device based on the work of Michael Ratcliffe. Michael used the two prongs of a US style AC plug as the probe tips to measure the EC of a hydroponic nutrient solution.
The plugs have a geometry that allows for EC measurements in the 1us to 1mS range. The probes have a value called the cell constant which relates the geometry of the probe to the electrical conductivity. The probes need to be compensated for the difference in temperature from the reference temperature (25C).
See Michaels blog for more detail ... www.MichaelRatcliffe.com/projects
An Arduino is used to drive voltage onto the probe and determine the EC of the solution
US plugs are available in many different metals, but ones for export are typically solid metal, with a hard exterior coating. The probes are typically 1.7cm long, .6cm wide and are suitable for EC measurements in the 1uS to 1mS range.
In the original design -
One probe tip is connected to a digital output pin on the Arduino. The other probe tip is connected to ground. The first output is driven high (5V). The voltage at the junction of the probe tip and the sense resistor is measured. The voltage drop is in the ratio of (1/EC) of the solution and the resistance of the sense resistor.
The hardware used is similar, but an extra resistor and an extra analog input allows the use of a square wave drive to the solution. See Schematic below:
The goal of using the square wave is to reduce fouling on the probe. This is achieved by changing the polarity of the measurement each time a reading is accumulated by swapping which pin is positive and which pin is ground. The analog input is sensed at the top of the resistor in the ground leg and the voltage used to determine the sense current (Is) using Is=Vs/Rs. The resistance, Rec, of the solution can then be calculated using Rec=Vec/Is, where Vec is the voltage across the prongs. The uncompensated EC (ECu) is calculated using ECu=Kcell/Rec, where Kcell is the cell constant. The temperature sensed by the 18B20 is then used to compensate using EC = ECu/(1+(TemperatureCoef/100)*(T−25)).
Is = Vs/Rs
Rec = Vec/Is
ECu = Kcell/Rec
EC = ECu/(1+(TemperatureCoef/100)*(T−25))
Pure water 0.055 µS/cm
Deionised water 1 µS/cm
Rainwater 50 µS/cm
Drinking water 500 µS/cm
Industrial wastewater 5 mS/cm
Seawater 50 mS/cm
1 mol/l NaCl 85 mS/cm
1 mol/l HCl 332 mS/cm
Cell constant = Kcell = s/A
where
s = separation in cm between the electrodes (prongs)
A = area in cm2 of the electrodes (prongs)
A = 2 * ((l * w) + (l * t)) // Side faces (two)
+ w * t // + End
- 2 * (pi * r^2) // - Holes
+ (2 * pi * r) * t // + Internal annulus
where
l = length of prongs
w = width of prongs
t = thickness of prongs
r = radius of holes in prongs
Experimentation suggests that the areas on the edges and the reverse faces only contribute to fringe effects, that is, they are negligible. The formula is simplified to:
A = ((l * w) + (l * t)) // Side face
- (pi * r^2) // - Hole
In practice, the cell constant is determined by measuring a calibrated solution with a known ECcal and the value is derived using the measured Rec:
Kcell = ECcal/Rec
EC = (Kcell / R) / (1 + (TemperatureCoef/100) * (T−25))
By measuring the conductivity of a sample at temperature T1 close to Tref (25°C) and another temperature T2 coefficient by using the following equation:
θ = (κT2 - κT1) * 100 / (T2 - T1) * κT1
κT2 = Conductivity at T2
κT1 = Conductivity at T1
T2 should be approximately 10°C different from T1
The temperature coefficients of the following electrolytes generally fall into the ranges shown below:
Acids: 1.0 - 1.6%/°C
Bases: 1.8 - 2.2%/°C
Salts: 2.2 - 3.0%/°C
Drinking water: 2.0%/°C
Ultrapure water: 5.2%/°C
Hana [USA] PPMconversion: 0.5
Eutech [EU] PPMconversion: 0.64
Tranchen [Australia] PPMconversion: 0.7
The PPM value can be calculated using:
ppm = Ec * PPMconversion