And now for the exciting part….comparing EC readings between the Ladybug Shield and Atlas-Scientific’s EZO EC Stamp. Yes, I designed the Ladybug Shield for a different use case than the EZO EC Stamp. But I thinking of the EZO EC Stamp as a “gold standard” of Arduino EC sensors. Also, I use an Atlas-Scientific probe so both measurements will be using the same probe.
The goal of this post is to compare EC readings when the Atlas-Scientific EC probe (K=1) is submerged in:
Thanks to Those That Went Before
A huge thank you to Chris Gammell for mentoring and teaching his excellent Contextual Electronics courses. A year ago I would not have dreamed I would be able to build the Ladybug Shield. I give Chris the credit for getting the EC circuit to work in key areas – like advising to measure Vin as well as Vout.
Another giant thank you to Ryan @SparkysWidgets. Ryan open sourced the design of the minipH and miniEC. I absorbed and evolved these designs into the Ladybug Shield. Ryan has been extremely helpful in my efforts.
Use a K=1 EC Probe
The protagonist of any EC measurements is the probe.
Conductivity probes that I am aware of have these dimensions:
Probes may vary in the distance between the electrodes. EC = (the distance between electrodes/area of an electrode plate) * conductance. The distance between electrodes/area of the electrode plate is known more commonly as the K constant, or just K. A probe with K=1 has 1cm distance between electrodes with 1 cm squared area of electrode plate. I started with a probe with a K=.1. I figured a shorter distance between the plates would amplify the incoming signal and therefore get better readings. I found however the design of the Ladybug Shield’s EC circuit was better suited for a K=1 probe.
Here is an image of the EC Vout when K=.1:
The gain loop is amplified to the point where the peaks are chopped.
Here is an image when K=1:
My probe has a K of .1/cm -> .1cm distance between electrodes/1 cm squared area = .1/cm. Since the electrodes are closer together, the probe can be used in lower conductivity solutions than a probe where the K=1. Until I know better, a probe with either K=1 or K = .1 should work.
Tables for EC values of vegetables have a nasty habit of leaving off the distance, assuming the probe has a K of 1. For example, a page that lists EC values for vegetables notes: Electro-Conductivity (EC) or Conductivity Factor (cF) can be expressed as… milliSiemens (mS). This is true if K = 1. When K = .1, EC = K *conductance. The table lists the EC value for lettuce to range from .8 to 1.2mS. When K = .1, EC = .1*.8 to .1*1.2 = .08 to .12mS
The Ladybug Shield and the A-S EZO EC Stamp used the same K=1 EC probe.
The table below shows the µS value for the EZO and Ladybug Shield when the EC probe is in a 2000µS and then 1413µS calibrated solution:
|Calibrated Solution||Ladybug||% difference||EZO||% difference|
The results seem too good to be true. The Ladybug results were very close to the calibrated solution value. Talk about a YIPPEE! moment….
The biggest change in the Ladybug shield to get better results was the reading of Vin (the shrunken signal generated through the Wien Bridge Oscillator). As I noted earlier, Since both the Vin and Vout are key variables in calculating the Gain – it makes sense to measure both.
In the case of the 2000 µS calibrated solution, Vin = 239mV Vout = 731mV. Gain = 731/239 = 3.058577406. R = 1000/(3.058577406-1) = 485.77235769 Ω EC = 1/485.77235769 = .002058577 S = 2059µS.
In the 1413µS solution, Vin = 237mV Vout = 731mV Gain = 566/237 = 2.388185654 = R = 1000/(1.388185654-1) = 720.364741646Ω EC = 1/720.364741646 = .001388186 S = 1388µS.
That’s It For Now
The EC circuit appears to be working. A Definite YIPPEE! moment.
Thanks for reading this far. Please find many things to smile about.