Testing and calibrating water quality array – Part 2

After the data quality issue with the first array design, I contacted Atlas Scientific(AS) about our problems. Jordan at AS suggested that we make two changes: 1) connect the probe grounds on both the dissolved oxygen and pH probes, and 2) isolate the electrical conductivity probe from the rest of the probe using their power isolator. Since neither of these changes are documented on their website or support materials, I was frankly growing a bit concerned about the claims made on AS’s website — was this really our problem?

Potential issues/interactions:

1. One of our probe arrays was a year old; the DO probe was showing some non-linearity in its readings
2. The probes electronics were interactions with one another on the microcontroller board
3. The probes were interacting with one another in the water (very likely with the EC probe)
4. The power draw from the Bluetooth device was interacting with one or more of the probe’s electronics

We addressed issue one by using a third set of probes that were still in the box (purchased within a month of array #2) and retired array #1 for now. For two and three, we followed the suggestions of AS above. In addition, we switched from the 3.3v Teensy 3.1 microcontroller to the 5v Arduino Nano. Jordan at AS has expressed some concern that the probes kits may be underpowered at 3.3v and the power isolator would only work at 5v. Effectively, we rebuilt the array from scratch:

Rebuilt water quality array – round 2

As for issue #4, preliminary testing is showing that whether the Bluetooth is in “discovery” mode (light flashing, higher power draw) or connected impacts the EC value but not the other measurements. In “discovery” mode, the EC value is approximately 0.5 µS lower than when the device is either connected or unplugged (why? common ground? power drop?). This seems to indicated that the connected mode is similar to not having a Bluetooth device is likely to have minimal impact on the EC values. It does indicate, though, that calibration should only be conducted with the Bluetooth unplugged or connected to the phone app.