Conductivity Sensor

Hi,

I'm constructing a Hydroponics system, and need to monitor the conductivity of the nutrient solution.

Any suggestions on what to use for the sensors for stable long-term readings?

Thanks,

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Luhan Monat: luhanis(at)yahoo(dot)com
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Luhan Monat
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Luhan Monat: luhanis(at)yahoo(dot)com
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Luhan Monat

The picture is a week out of date. I've gone to nylon ties with a piece of mousepad between the jars and the PVC. We are running over 100 here every day lately - the rubber bands were almost totally decomposed in about 2 weeks.

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Luhan Monat: luhanis(at)yahoo(dot)com
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Luhan Monat

Thanks, thats very informative.

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Luhan Monat: luhanis(at)yahoo(dot)com
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Reply to
Luhan Monat

Platinum is traditional, and graphite has its devotees.

One of platinum's advantages is that it has much the same coefficient of thermal expansion as soft soda glass, which can be handy.

------- Bill Sloman, Nijmegen

Reply to
bill.sloman

I may go for the 'poor mans' version of platinum/titanium - stainless steel is one thing I have in stock here.

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Luhan Monat: luhanis(at)yahoo(dot)com
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Luhan Monat

Hello Luhan,

I believe corrosion resistance is really important. Titanium is one of the materials used. An example:

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Regarding your hydro tree in the picture, what happens when one of the rubber bands snaps?

Regards, Joerg

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Joerg

Hello Martin,

And I thought the pros use duct tape.

Regards, Joerg

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Joerg

My favorite kind of sensor is a toroidal transformer type. It is completely enclosed in insulating material and is very resistant to fouling. Unfortunately, the electronics are fairly expensive. I think that Cole Parmer has some units for $360. But it might be fun to try to make one of these sensors from simple parts.

The concept is that you excite a toroidal core (high permeability) with AC through a winding. Then you place a second toroid beside that one with another winding. The two cores are enclosed in insulation, with a hole passing through both cores. When submersed, the first core induces 1 turn's worth of voltage around the liquid loop that passes through the hole. The current that voltage moves through the liquid is sensed by the second core, acting as a current transformer. You amplify the AC current from the second core, rectify it, and the result represents the conductivity of the solution.

Reply to
John Popelish

Hey, that sounds interesting. I see why it would work.

Alternativly, maybe using the fluid as part of an 'air core' inductor. Conductive water may change the inductance. Easy to test by using the inductor in an oscillator configuration and using the Micro as a crude frequency counter.

Also, making an 'air core' capacitor using two closely spaced insulated plates. Capacitors are known to be sensitive to the dielectric used between the plates.

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Luhan Monat: luhanis(at)yahoo(dot)com
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Luhan Monat

The pro version will use cable ties or gaffer tape........

martin

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martin griffith

Hi Joerg

Started life in the film industry, at Ealing Studios, So I call it gaffer tape

martin

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martin griffith

This all sounds a bit hairy, because the capacitance between two electrodes in a liquid depends on the nature of the ions in the solution. At audio frequencies the impedance between the electrodes can be dominated by the Warburg impedance, which has equal resistive and reactive components, both of which decreases as the square root of the frequency.

At higher frequencies, the double layer capacitance tends to dominate, but that varies with the specific ions involved, not necessarily in the same was as the conductivity.

------------ Bill Sloman, Nijmegen

Reply to
bill.sloman

Circuit losses were probably due to the lowered resistive component that is part of the resonance formula. Hence, it probably has much the same flaw as conductance systems as the electrodes become contaminated.

By the way, I'm defining 'success' of a hydroponic system as one that needs attention once per week or less often. My previous system used a horizontal section of 2 inch PVC pipe with holes every few inches on top for plant placement. This actually works well, but leaves a big mess of roots inside the pipe. The current jar system is designed to allow easy replacement of plants over time.

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Luhan Monat: luhanis(at)yahoo(dot)com
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Luhan Monat

A Co I once worked for used coaxial tubes as a capacitor for a liquid level sensor, with conductive liquids.

The designer didn't use the change in capacitance though, he used the change in the tuned circuit losses. In fact he kept the amplitude constant with an agc loop, and measured the change in agc voltage.

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Tony Williams.
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Tony Williams

If the nylon dies, I'll definitely go to the steel hose clamps.

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Luhan Monat: luhanis(at)yahoo(dot)com
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Reply to
Luhan Monat

Use stainless steel hose clamps - cheap and readily available from automotive suppliers. Nylon ties die fairly quickly if exposed to UV. So do most tapes.

Ted

Reply to
Ted Edwards

Very interesting! What frequency do you use/recommend and why?

Ted

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Ted Edwards

I haven't actually built one of these, yet, so I haven't thought too much about that. This is the way commercial toroidal conductivity probes work. I think I once measured a Yokagowa unit and it operated around 5 kHz. I would choose a frequency that allowed the two high permeability cores to operate with low losses. If the excitation core has high losses, they subtract a little of the voltage per turn the liquid receives. If the current transformer core has losses, it subtracts from the conductivity signal. I think any frequency in the mid to upper audio range will work pretty well with a pair of 10,000 relative permeability ferrite cores. If you have a couple tape would permoly or hypermoly cores, the frequency could be lower.

Reply to
John Popelish

Why so tall? And where do the plants go?

And where do you buy a jar of "Medium"? ;-)

Thanks, Rich

Reply to
Rich Grise

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