In trying to get our two units to agree, we noticed that the DS18S20 temperature probes seem to be off 0.5 to 0.75 C from one another. Given that all probes use temperature compensation (and the dissolved oxygen uses it for calibration), this slight difference could be exacerbating any differences we were noticing.
So, we placed both temperature sensors in a cooler of ice water to generate a manual, one-point temperature compensation for each of our units. For each unit, we let the probe sit in ice water for at least 5 minutes and then took five consecutive readings one minute apart from one another.
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?
- One of our probe arrays was a year old; the DO probe was showing some non-linearity in its readings
- The probes electronics were interactions with one another on the microcontroller board
- The probes were interacting with one another in the water (very likely with the EC probe)
- 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:
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.
Well, we’ve encountered our first big hurdle with our water quality mapping. We’ve been so focused on getting the technology connections to work (Arduino -> Phone app -> Server) and getting our kayak mount system to work, that we’ve neglected data quality a bit. We’ve been operating on the assumption that we don’t have any electronic inference issues in our device and that the accuracy specs from Atlas Scientific are true.
We found out that we’ve got a problem somewhere.
Before heading out to the Milwaukee River, our first “big” sampling destination, we headed up to the Burnt Village put-in on Highway N on the Bark River with both boats and a borrowed YSI probe to compare our numbers to. With everything mounted on the kayaks, we all got in the water and started comparing numbers. Our first disappointment was that the Atlas Scientific arrays didn’t match the YSI values at all — not even close. And, even more disappointing, the Atlas Scientific arrays didn’t match each other — even after having been calibrated using the same techniques and same solutions earlier that day.
Time to hit the drawing board…