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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

The goal of this post is to compare EC readings when the Atlas-Scientific EC probe (K=1) is submerged in:

• 2000µS
• 1413µS calibration solutions.

# 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. ## Comparison

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 2000 2059 2.91 1875 8.42 1413 1388 1.79 1254 15.68

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.