EC_Salinity Probe Interface

Monitor hydroponic nutrient levels, salinity levels in aquariums or the ocean, saltwater pools, soil salinity, or water quality

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Monitor hydroponic nutrient levels, salinity levels in aquariums or the ocean, saltwater pools, soil salinity, or water quality

Electrical Conductivity in Siemens
Total Dissolved Solids in PPM
Salinity in PSU and PPT

Any probe can be used, including a handmade one.

This project was inspired by yet another attempt at growing some vegetables in my backyard. Living in the desert, my previous attempts met with failure so I looked at hydroponics. I soon found that hydroponics provide a rich environment for automation and monitoring. One of the things you can monitor is the nutrient content of your water; it is typically done with an Electrical Conductivity meter. A quick search found lots of cheap hand-held EC meters, but none that could be controlled by a computer/processor or otherwise automated.

Some quick searching on the internet found these two articles:

Three Dollar EC - PPM Meter [Arduino]

Salinity Measurement

Which provided enough information to get started on my own project, especially the first link.

Along the way, I decided I would see this project all the way through from start to finish, learn some new things like Doxygen, the ins and outs of git and GitHub, and whatever else seemed useful to end up with a finished-looking product.

The hardware is very simple and doesn't need much explanation; I used an ATTiny85 as an I2C slave, created an Arduino library, and published it for use in the library manager.

  • 1 × ATTiny85 Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers
  • 5 × Resistors/Capacitors

  • Open Source Hardware Association

    μFire07/16/2017 at 00:29 0 comments

    I might be a sucker for badges, but I also agree with the mission of the Open Source Hardware Associate. It's an easy process where you essentially self-certify that your design is open source and you get permission to use their OSHW logo with your unique ID number.

    Here's mine

  • Making Your Own EC Probe

    μFire07/13/2017 at 17:54 0 comments

    Making Your Own EC Probe

    For many applications, a hand-made EC probe will work just as well as a commercial one. With light use and cheap materials, it is possible you will never spend more on making (and re-making) one than you would have buying one. This article will focus on making a 2-electrode EC probe.


    • Significantly cheaper than buying one
    • With light use, it can last a long time
    • Can be made much smaller and less visually noticeable


    • Requires time to design and create
    • Less accurate and more prone to fouling
    • Higher K values are harder to obtain
    • Higher maintenance

    Design Considerations

    An EC probe is, at its simplest, just two electrodes; in a DC application, one sends out a pulse of electricity, the other receives it and measures it. The resulting number is the raw conductance of the sample being measured. To turn that conductance measurement into the SI standard Siemen unit, it needs to be multiplied by the cell constant of the probe, typically referred to as K.

    K is determined by the area of the probe divided by the distance between them. A probe with electrodes that are each 1 cm square, placed 1 cm apart will have a K = 1 ((1 * 1) / 1 = 1).

    Know What You Want to Measure

    While you could use any given K value to measure any given solution, the results may only be accurate for a very small range around the calibrated point. This might be acceptable if you only need an indication of the solution being out of range, but if more accurate results are needed, it is important to match the K value to the expected range you want to measure in.

    K Range in µS
    0.1 0.5 to 400
    1 10 to 2000
    10 1000 to 200,000

    Measuring below 0.5 µS is difficult due to the impact of system capacitance and beyond the scope of this document.

    Examples of commonly measured solutions:

    Solution Conductivity
    Pure Water 0.05 µS
    Tap Water 50 µS
    Hydroponics 1.5 mS (1,500 µS)
    Ocean Water 53 mS (53,000 µS)

    Probe Construction


    Commercial probes are commonly made from platinum, titanium, gold, and carbon or some other non-reactive material. They are also relatively expensive or difficult to work with, or both.

    Since the EC_Salinity Probe uses DC, a two-electrode probe will pass electricity from one probe to the other, the sending probe will eventually lose material, while the receiving probe will foul from chemical reactions taking place. This can be prevented somewhat by expensive chemical processes or materials. But as stated above, with light use, a very simple and inexpensive probe will likely last for awhile.

    A more commonly available material for making probe electrodes is male terminal headers. Some are gold plated, and while it may be the smallest amount of gold, it’s better than nothing. The headers come with spacers, usually 2.54mm apart. They are 0.64mm square and about 6mm in height. So theoretically, spaced 2.54mm apart, our breadboard wire EC probe should be K = 1.51 ((0.64mm * 6mm) / 2.54mm), and spaced two spaces apart, we end up with a K = 0.76. You can also adjust the length fairly easily, although since we are dealing with millimeters, it’s difficult to get exactly the length you want.

    Putting it Together

    To keep things simple, I will make two probes using two pieces of breadboard wire with two male headers. To ensure the solution comes into contact with only the parts of the electrode I choose, I will paint some nail polish on all of the outer-facing sides of the pins. To apply the nail polish, I indiscriminately painted it over all surfaces and then used a knife to remove it from the inner-facing electrodes. I made sure that any exposed metal was covered as well as any junctions or spots that water might collect in. The end product looking like this Breadboard EC Probe


    Then I used the shell example to test the two probes. I let them sit in a 2.77 mS calibration solution for about 10 minutes then ran "calk 2.77" in the shell....

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