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
- 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
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.
||Range in µS
||0.5 to 400
||10 to 2000
||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:
||1.5 mS (1,500 µS)
||53 mS (53,000 µS)
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
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....
Read more »