Soil PH "homemade" sensor

Hi there,

I was hoping I could retrofit an existing soil ph/humidity sensor with an arduino board but I'm struggling figuring out how to do the ph part, hopefully folks on this forum can shed some light or suggest how to proceed...

I'm using the following sensor:
http://www.amazon.com/gp/product/B00XXU3CDG

I took it apart and connected the soil humidity part to my arduino w a R=10K, I get some data back on A0 (0 for dry and 250 I think when in water) - all good so far I think and I can map this between 0 and 100 to get soil humidity.

Now, the Ph part:

  • I got 3 calibration solutions (ph4/7 et 10) and I was thinking I would just get the analog reading and map it out like the humidity. Unfortunately it seems that's not how you get the Ph :frowning:
    I found some details on how to compute the ph from reading from an existing ph probe Sign in - Google Accounts but that uses the glass probe w the calibrated solution inside

Any suggestions/pointers on how I could get this to work?

Thanks!

Xavier

A data sheet of the sensor would be helpful.

Do you know what is inside the probe ? How is the PH measured ?

Wikipedia says that you need to use proper probes. This suggests it is not a home mad device.

Weedpharma

@DrDiettrich: Sorry no sheet - I just got the probe and took it apart.

@Peter_n: My understanding is the probe is composed of 2 electrodes - I took the probe apart and although I haven't destroyed it you can see the soil humidity having 2 electrical connections - the ph part has 2 cables coming back as well through a 3 pin jumper.

Do I really need a reference electrode to compute the Ph? The device I got from amazon is very cheap so I have a feeling they're just getting some electrical value that is being converted/computed to PH - thoughts?

thanks again for the help,

X

No thoughts, I would like to know how they do that. Perhaps two different metals, and the voltage generated is according the PH (like a battery). Or they reverse the voltage and check the difference in current.

Cheap ? This is cheap :o http://www.ebay.com/itm/121285596674
This has better photos : http://www.ebay.com/itm/361249697920
The leads are two different metals, and they both have an isolated tip.

As far as I know, even PH meters with such a glass sensor can be very cheap. I think this is the cheapest : http://www.ebay.com/itm/390647140904
The range is a lot better, 0-14 PH.

@ Peter_n: yes that was my thinking as well - I have a 10K resistor on the circuit and measuring in a ph=7 and ph=4 solution but that's not giving me much delta to work with. I'll try putting a bigger resistance and see if I can get better measurements. oh well, time to go through some pdfs to learn about the relationship between pH and conductivity...

pH electrodes are extremely high impedance voltage sources. You need a special amplifier to use them. There are a few home brew designs on the web, google "diy ph meter".

jremington, can you give a link ? I can only find diy amplifiers that use a glass probe. Those are indeed extremely high impedance. But this is about some silly an not accurate way to determine more or less the PH with two metal probes.

I think @ jremington may have point in all cases, from doing some more reading it seems in all cases the signal is very low and needs to be amplified. I took apart the PCB for the device I bought and although it's hard to follow the trace I was able to spot a LM324DG which I suppose is used for the ph...I'll post more updates if/when I get it to work....

Who knows how the original sensor is supposed to work? Did you try it out before you took it apart?

I have trouble believing that a sensor, advertised to require no batteries, can give reliable measurements of pH.

@jremington: yep I tried it out of course. Even tried it in a ph 4 calibrated solution and it reported 4.5 (close enough to ensure my blueberries will grow…)

fyi, the sensor I’m using does use a 9V battery (http://www.amazon.com/gp/product/B00XXU3CDG) just got some op amps to try out and see if I can amplify the signal coming out of the prob…

You took it apart???
An extremely delicate elecrode with a glass membrane - I'm assuming you only meant the cap portion.
As mentioned by others, the pH electrode is a very high impedance device and handling 60mV per pH unit output at picoamp levels needs some special techniques.
If this is an unamplified electrode i.e. just bare glass electrode, you need to use an electrometer type op amp as close up to the electrode as possible (I have successfully used Macxim MAX 407 in the past) - use it as a unity gain buffer amp then you you can run the output using any old two-core cable rather than tricky coax.
If it is amplified already and there is an op amp in the cap, then it should be as simple as firing up with a suitable voltage and measuring the output.
Remember that this is effectively a "dual rail" output with 0mV at pH 7 then +60mV per pH unit for pH's below (acidic) 7 and -60mV per unit above 7.
So for a single rail op amp, you need to privide an offset.
The amps I put together ran on a small lithium cell of 3.6V and drew 2.4 microamps - all ground loop problems eliminated, lawnmower cable up to 100-metres.
You will also need to do temperature compensation as the approximate output of 60mV per pH unit is temperature dependent and what might look like a pH change, might actually be temperature changing.
The standard lab technique for soil pH involves making up a suspension os 1 part soil in 2 parts deionised water.
There again, if it's not a standard glass electrode, that's another story

Peter_n:
No thoughts, I would like to know how they do that. Perhaps two different metals, and the voltage generated is according the PH (like a battery). Or they reverse the voltage and check the difference in current.

Cheap ? This is cheap :o http://www.ebay.com/itm/121285596674
This has better photos : http://www.ebay.com/itm/361249697920
The leads are two different metals, and they both have an isolated tip.

As far as I know, even PH meters with such a glass sensor can be very cheap. I think this is the cheapest : http://www.ebay.com/itm/390647140904
The range is a lot better, 0-14 PH.

I wonder how these probe works.
Reading here maybe they use the Antimony electrode. There are also ISFET solid state probes but it seems that they are expensive.