Wow, thanks a lot for the literature, and also for the new resources. Public Lab seems to have some really good stuff and I will for sure be lurking around there.
I see that DC voltage is not recommended for use in conductivity measurements for electrochemical reasons, as well as 5v is high and is not required for successful conductivity measurements.
With this, I see that using AC voltage, sine waves, is the practice. I have found a few things online about duping sine waves with microcontrollers, but nothing that seems to be solid. Have you had any practice with this? Can the Mayfly be used to produce low-voltage alternating current?
If so, what other mechanisms are needed? Look-up tables?
I am quickly reaching the ceiling on my know-how. Can this be done?
Initially my aim was to produce an inexpensive, yet robust, logger that will spend significant amounts of time deployed and could detect and record the presence of water. But it would seem that I am working towards measuring TDS, which I am very much okay with. If at the end of this project I can’t seem to make a reliable, and deployable TDS sensor, I would have at least made a device that can determine the presence of water.
The majority of the time, these sensors will not be exposed to water and will spend most of their time loitering in dry stream beds, at least that’s what we think.
I was pleased to see that temperature is a critical metric when determining TDS. I initially planned to use temperature sensors to determine the presence of water, so Ive got a few on hand. Lastly, my current probe consist of one DS18B20 with the electrodes straddling it. The temp sensors are cheap Asian market style, 5 for $10 sorta thing. They seem to work well and follow the oneWire library set.
I am assuming the metal casing around the senor is stainless, is it possible that the temp housing could be providing ions the electrodes.
For the probe housing, I am currently using 1.5″ PVC parts. That decision was solely driven by the size of the cable and the pass-thru gland needed to achieve a water-tight seal from the enclosure. I was worried about voltage drop, so I sized way up to 18awg, from the 22(?) of the off-the-shelf jumpers/sensors parts.