μniversal fire

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.

Advantages:

Disadvantages:

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.1 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

Materials

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

Results

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. The following table is the result:

Electrode Distance Electrode Length Expected K Measured K
2.54mm 7.4mm 1.9 2.2
5.08mm 7.8mm 0.98 1.7

Note that calculating K also has the effect of determining the probe offset as well. Probes will never measure accurately without being calibrated and there are a few options for doing it; one way is a single-point calibration that essentially determines the % offset. So looking at the above table, and assuming that fringing is having an impact on the value as well as the offset, they appear to be close to what you would expect. 1

I calculated the K value for both probes several times. Each time, it returned the same result to 4 decimal places.

I also tried using liquid tape to seal the probes. It didn’t work nearly as well; it was much more difficult to apply and the readings varied wildly.

The next thing I did was to keep both probes in the solution for several days while taking a measurement every 10 seconds. At the end, I noticed slight discoloration on one probe and the other probe appeared largely unchanged.

Conclusion

The most inexpensive EC probe is about $34.00 from AliExpress. I am fairly certain the probe is the same as nearly every other branded 10 K probe, there can’t be that many EC probe manufacturers out there. In comparison, I bought several hundred header pins for a few dollars and a box of breadboard wires cost a couple dollars as well. The nail polish can be had for around a dollar and it took only a few minutes to make the entire thing. I would estimate that one probe costs under $1.00 and will last, at a bare minimum, several weeks of heavy use before it needs to be replaced. So given those numbers, it would take years to cost more than the least expensive commercial probe and appears to be just as accurate.

This technique for making an EC probe seems to be easy and cost-effective for a K = 1 or less range. It may prove difficult to get higher K values with this particular method.

Footnotes:
  1. A third factor, known as fringing is relevant to calculating K, but measuring it is difficult and ignored.  2