In my previous test, I used a fixed resistor value – 200Ω – to simulate an E.C. Probe measurement. In this post I will use an E.C. probe placed in a nutrient bath. Using the E.C. probe will be a great start in realistic testing.
The Conductivity Probe
The first thing is to determine which probe to use. A conductivity probe measures the conductivity between NaCl and water between two electrodes:
The water+salt is in contact with the electrodes. The (shrunken) Wien Bridge AC Circuit is fed into the probe. A current between the electrodes is generated whose amount is dependent on the resistance of the water+salt (Ohm’s law).
Notice the K constant = [distance between the electrodes]/[area of electrodes] is important in determining the conductivity. Conductivity probes are sold based on the probe’s value for K.
I will be using E.C. measurements to determine the amount of nutrients when hydroponically growing vegetable and herbs. According to the tables found here, the conductivity range to measure for vegetables and herbs is 0.8 to 5.0 mS. According to the image above, these values align in the middle of the range for a K=0.01 and at the lower end range of K=0.1 conductivity probe.
I have on hand a K = 0.1 conductivity probe that I used earlier when playing with Atlas Scientific’s EZO EC stamp.
The Nutrient Bath
In order for the E.C. circuit to determine the E.C. value of a nutrient bath, it determines the Vout of the op amp in which Vin+ is an AC Signal with a ~.2Vpp and a Vin- voltage divider in which the bottom resistor is what is needed to be figured out. The inverse of the resistance is the E.C. value. The range I am interested in is: 1/0.0008S to 1/0.005S = ReC = 12.5K Ω to 200Ω.
Vpp of the ECv AC
Vout of the gain loop op amp is ECv – a sinusoidal wave. Vout cannot be bigger than the rails. In order to avoid two power supplies (an additional power supply for negative voltage values), I raised ground from 0 to 2.5V and refer to this as VGND. All op amp readings in the E.C. circuit are relative to 2.5V of a 0 – 5V power source. The implication is the Vpp of the ECv must be less than 2.5V.
The largest Vpp will happen when ReC is smallest – for measuring vegetables and herbs this is 200Ω.
In the schematic, the Known Resistance is currently set through a 5K POT – represented by POT1 and POT2:
The Vpp of the Vin+ will be amplified by the gain. The gain = 1 + [Known Resistance (5K POT)]/ReC If the Known Resistance = 1KΩ and ReC = 200Ω, the Gain = 1 +1000/200 = 6. If instead the 5K POT is set to a resistance of 4K, the gain = 1 + 4000/200 or 21.
Looking back at the image from the scope of the Vpp of the AC signal after it was shrunk by the voltage divider:
Vpp = .288V.
If a gain loop of 21 is used, the Vpp = 21*.288V = 6.048V. If a gain loop of 6 is used, the Vpp = 6*.288V = 1.72V. Given the calculations, setting the 5K POT to 1K should work well for ECv readings.
To test, I need a solution that has a conductance of 5mS. In order to measure the E.C., I will calibrate with an inexpensive TDS meter. The TDS meter (unfortunately) gives readings in PPM. As noted in this table, all meters must convert from E.C. to TDS. The challenge however is how much is added to the mS in order to get PPM. I am assuming 500PPM per 1 mS. So if I want to use with an E.C. of ~ 5mS, the TDS meter will read ~ 5*500 = 2,500PPM. I will use the lowest level of calibration solution that I have 10,500µS = 10.5mS. I will water this down with filtered water. Ideally I should be using a pure water since filtered water carries additional salts. In addition, results will not be scientifically accurate because I will be using an inexpensive TDS meter for comparison.
The bath I will be using measure 3,070PPM. Converting the PPM value to E.C. = 3070/500 = 6.14mS. This should be close enough to the 5mS upper conductivity range for pH values used in hydroponically growing vegetable and herbs.
With the probe in the bath
- Step 1: calculate the known gain. I am using the same value for R as I did in this post: Known Gain = 1 + 989/200 = 5.945
- Step 2: find the measured gain. Vrms = .87V. Vin = Vout/[Known Gain] = .87/5.945 = Vin = .15V. Measured Gain = Vout/Vin = .87V/.15V = 5.8
- Step 3: find E.C.: Rec = 989/4.8 = 206Ω. E.C. = 1/206 = .0049S = 4.9mS PPM = 4.9*500 = 2,450PPM
The result 4.9mS or 2,450PPM is lower than the reading on the TDS – which is 3,070. I am encouraged the measured value is so close to what is read on the TDS. There are many factors that cause the difference. Including:
- The solution does not have salt evenly distributed. Thus the electrodes of the probe are getting a different salt concentration between its plates than the meter.
- The TDS meter is inaccurate. There are many reasons this could be true: 1) it is the most inexpensive I could find. Thus the parts and material that make up the circuit most likely introduce inaccuracies. 2) the meter converts from E.C. to PPM. Any conversion can introduce error. 3) the temperature is not considered. The temperature has a significant effect on the E.C. reading. I do not address temperature within this test. 4) the probes/electrodes in the probes are very different. I am not sure what the electrodes are in the TDS meter (and what the K constant is).
- I am not using a calibrated solution to determine the Known Gain. Any potential deviation due to inaccuracies caused by the probe are not accounted for.
- The E.C. circuit is not adjusted within any accuracy limit. I have not focused on any % accuracy. Currently the circuit is strung together with a bunch of wires and adjusted through POTs.