Measuring EC for hydroponics

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I'm trying to get an idea of the OHMS of resistance to millisiemens. 

 

From googeling 

.001 millisiemens per centimeter = 1 EC

 

200 millisiemens = 5 OHMS

 

so 1 ohm would be 40 millisiemens 

 

40 millisiemens / .001 would be 40,000 OHMS

I don't think that's right or a standard ohm meter could measure EC of a fluid.

 

So need some help understanding this.

 

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metron9 wrote:

so 1 ohm would be 40 millisiemens 

 

40 millisiemens / .001 would be 40,000 OHMS

Huh?  perhaps you meant 40,000 centimeters (shouldn't that be cubic centimeters?)

 

Jim

 

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I was going the wrong way, if 200 ms.cm of conductance is 5 ohms of resistance then 100 ms/cm has to be a larger resistance of 10 ohms

Dividing by 2 the ms/cm and multiplying by 2 the resistance I get this table.

 

Im still missing something though as I come up with  only about a mega ohm for 1 EC =.001 ms/cm

 

Millisiemens Resistance
200.0000 5.0000
100.0000 10.0000
50.0000 20.0000
25.0000 40.0000
12.5000 80.0000
6.2500 160.0000
3.1250 320.0000
1.5625 640.0000
0.7813 1,280.0000
0.3906 2,560.0000
0.1953 5,120.0000
0.0977 10,240.0000
0.0488 20,480.0000
0.0244 40,960.0000
0.0122 81,920.0000
0.0061 163,840.0000
0.0031 327,680.0000
0.0015 655,360.0000
0.0008 1,310,720.0000

 

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I'm trying to understand if I were to build an EC meter I would need to read a voltage through a WheatStone  bridge circuit I would think but I'm trying to understand what resistance values to use.

 

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Siemens are just the reciprocal/inverse of ohms.

 

    1 ohm  = 1 siemens

  10 ohms = 0.1 siemens

100 ohms = 0.01 siemens

 

etc.

 

--Mike

 

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First big challenge - measuring fluid conductivity generally  requires AC. That is because of corrosion that occurs on electrodes if you use DC. 

 

I built a number of conductivity sensors that were part of oceanographic research. Saltwater is, of course, a particularly extreme case, but is really just "more of the same" compared to hydroponics water with various fertilizers.

 

What I did was to build a 4-arm glass tube. There was a main tube with an electrode at each end plus two side arms, each with electrodes. Now, this could probably be made from plastic. All 4 electrodes were capacitor-isolated to prevent DC currents that might arise from small circuit voltage offsets or asymmetry in the drive waveform,  and such. A small AC current was passed end-end through the main tube. This current drive was specially designed to have a controlled amplitude. An instrumentation amplifier with capacitor-coupled inputs read the voltage between the electrodes in the two side arms. This output was used to servo the current drive to produce a constant amplitude difference between the two arms. The feedback voltage was a measure of the conductivity. The side arm electrodes are, essentially, "Kelvin contacts". The result was a voltage that was proportional to conductance.

 

In this application, salt water was also a help, because the conductivity varied only over a fairly limited range in the open ocean. There is a possibility for a much greater possible range in hydroponics, I would suspect. Also, conductivity does not directly translate into nutrient content as different ions (from nitrate vs from phosphate, for example) do not contribute equally to conductivity. 

 

There are also some very sneaky methods using inductive coupling of toroid inductors (water forms a single-turn secondary for what is effectively a transformer). This is really nice because there is no direct contact.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:
There are also some very sneaky methods using inductive coupling of toroid inductors (water forms a single-turn secondary for what is effectively a transformer). This is really nice because there is no direct contact.
Might be difficult though may be worth the effort of an experiment.

Concept :

Inductive sensing: How to use a tiny 2mm PCB inductor as a sensor - Analog - Technical articles - TI E2E Community

by 

Jun 10, 2014

...

 

https://e2e.ti.com/cfs-file/__key/communityserver-blogs-components-weblogfiles/00-00-00-03-25/2480.Image-1.PNG

Figure 1: The LC tank senses proximity of conductive targets

 

...

Possibility :

LDC1000 to measure liquid electrical conductivity - Sensors forum - Sensors - TI E2E Community

 

edit :

An alternative is to measure the pickup coil's impedance.

Is the coil's response dependent on the fluid's electrical conductivity?

Are the frequencies of the poles and zeroes dependent on fluid's EC?

Are the magnitudes of the poles and zeroes dependent on fluid's EC?

1 MSPS, 12-Bit Impedance Converter, Network Analyzer | Analog Devices (AD5933)

...

  • Frequency resolution of 27 bits (<0.1 Hz)

...

due to Impedance Spectroscopy (PJRC)

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Fri. Jul 12, 2019 - 08:02 PM
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I think measuring the resistivity of a fluid is challenging.

 

Typically one would use an AC current and measure the voltage across the electrodes.

The current is AC to minimize electrolysis of the electrodes.

That's the easy part.

 

For a relative measure of resistivity, that's all one needs.

If one is after accurate measurements, (i.e. exact numbers, not some random scale), then the problem becomes more challenging.

The issue is this, if you have a big long trough of water and nutrients in which you are growing plants, you can't just stick two electrodes in the water bath.

The measurement depends upon the shape, surface area, orientation, and separation of the electrodes, etc.

 

If you were just spot checking with measurements, taken manually, once in a while, the easy method is to build a "test chamber".

Think of a small glass cylinder with a 2 cm diameter, and a 5 cm length, (or whatever), and a metal plate at each end that makes up the "end plates".

Fill the cylinder with the solution, and pass your current through the tube.

As there is a uniform current density, and the cross sectional area and separation distance are known, one can calculate the actual resistance of the fluid.

 

Think of the photos of "magnetic lines" from sprinkling iron filings on a sheet of paper covering a magnet.

The magnetic strength field varies from location to location.

That is analogous to the current flow if you just stick to random shaped electrodes in the water trough.

You will get a measurement, but it won't be calibrated to any real-world units.

 

One could build a "test chamber" into the main water trough, with care and careful design, for continuously measuring the fluid's resistivity.

The goal is to make sure that the fluid is exchanging with the main fluid bath, and yet control the measurement current path, (and current density uniformity).

 

All doable, just not a 5 minute project.

 

JC  

 

 

Edit:  Man, I must type slower than everyone else...

 

 

Last Edited: Fri. Jul 12, 2019 - 04:55 PM
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ka7ehk wrote:
First big challenge - measuring fluid conductivity generally  requires AC. That is because of corrosion that occurs on electrodes if you use DC. 

Jim is correct, I too used AC to make a water purity sensor!  Nice description Jim!

 

Flyover Jim

 

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Thanks all for the replies.

I am going to do some experiments.

 

I did buy a BlueLab Monitor but as usual i wanted to figure out how it works.

https://www.google.com/search?ei=ouIoXaXcCpaztQb06IBA&q=bluelab+guardian+monitor&oq=bluelab+guar&gs_l=psy-ab.1.0.0l10.9852.18359..20552...1.0..0.94.921.11......0....1..gws-wiz.......0i71j0i67j0i10.ilMCpC-u6Ik

 

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The inductive conductivity sensors I have seen are about the size of a standard bagel. They are potted in some kind of waterproof coating. Typically single layer of wire wound on the core. 

 

Hmm, my memory isn't so good. Here is a link to an interesting page with a variety of conductivity sensors. Inductive is at the bottom. THAT uses TWO cores, in which the water forms a single turn link common to both cores. 

 

https://thecavepearlproject.org/...

 

It also describes the "contact" system I described, near the top of the page. Slightly different than I remembered, but I had it mostly (well, half) right.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Fri. Jul 12, 2019 - 08:54 PM
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"a standard bagel"... now I have heard everything! cheeky

Ross McKenzie ValuSoft Melbourne Australia

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Experiment: used some solder for a probe and a 1k resistor in series across a 12Volt DC supply.

Measuring across the 1k resistor I get 7.65 volts at 10 ma immersing in an EC (read by my EC meter) of 2.9

and about 1 volt in RO water with a zero EC reading. 

 

Diluting the EC 2.9 solution by 50% with RO water gives a 6.65V reading and an EC reading of 1.5

 

Using this simple measure one could measure their initial mix of fertilizer for a voltage standard of the correct mix

and adjust to that voltage over time as the system used nutrients and effects of evaporation.

 

I have a hydroponic grow tower (the same ones at Disney Epcot) and it got up to 9 EC after 3 months of running.

A simple device like this would have told me how far off the initial mix I was getting.

 

Plants look great though even at that high EC reading.

 

Going to test using a 12 volt AC wall Wart to see if it causes electrolysis as I can see the electrolysis eating the solder on the probe.

This would allow constant measuring but spot checking with DC voltage would be better than nothing.

 

 

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Sorry, realize that the terms "standard" and "bagel" do not often appear together. I was going use "doughnut" but they come in an even greater variety of sizes.

 

Jim

 

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Is that a metric bagel?

hj

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No, its a Polish-American bagel!

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Top with diced tomato and onion and you have a picobagel.

 

--Mike

 

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Hate to see what a nanobagel would be!

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Easily confused with a cheerio.

 

JC