2015 July

Testing and calibrating water quality array – Part 3

With the rebuilt arrays, time for more in-depth testing… Again, I’m growing more dubious of the claims of Atlas Scientific and how easy it is to obtain high-quality metrics from their probes.

For this test, I was interested in seeing the influence of each probe on the other in the same body of water. In other words, does one probe change the value of another when they’re near each other in the water? With our kayak mount, we can place probes on either side of the boat and wanted to know the optimal configuration.

Setup:

• Arduino Nano connected to I2C board with pH and DO, EC through PWR-ISO, and Bluetooth (HC-06). Ground wire bridge between PGND on pH and DO (as per Atlas-Scientific instructions)
• All probes EZO version in I2C mode
• Bluetooth in connected mode with phone app
• All probes calibrated via Altas-Scientific methods. 3 point for pH (4,7,10), 1 point for DO (100%), 2 point for EC (dry and 84µS)
• Three buckets of equal amounts of tap water (~3.5 gallons) drawn from tap at same time and let setting for two hours

Procedure:

1. Calibrate probes immediately before test
2. Place each probe in separate buckets to start.
3. Record values for 5 minute with each of the following combinations (isolated, in pairs, all three):

Table of testing probe combinations (number of sample bucket probe placed in)

Readings	pH	DO	EC	Temp
0-41	1	2	3	2
42-85	2	2	3	2
86-129	1	2	3	2
130-172	3	2	3	2
173-215	1	2	3	2
216-258	1	3	3	2
259-300	1	2	3	2
301-343	2	2	2	2
344-388	1	2	3	2

 

It’s clear that we are getting interference from some of the probes. In particular, the pH and DO probes influence one another. When they’re both first added to the same vessel, their values are thrown off by a considerable amount for several readings and then both stabilize. The DO readings are close to initial readings (off by ~0.5) after 2-3 minutes; however, the pH values stay considerably lower (by ~1.25).

And, somewhat as expected, the DO probe also appears to be influence by moving it from one vessel to the next (so, it does matter which probe is moved to another vessel). As it’s exposed to the air, it rises approximate half a point and then gradually declines to its previous value. Note that this effect alters values for almost the whole 5 minute sample period.

I did some additional ad-hoc testing of the each probe moving through the water. Only the DO probe shows any noticeable changes in values, and it’s showing about a 1 mg/l increase if any air bubbles are present when moving.

Lessons:

• pH and DO can’t operate near each other — they’ll go on opposite sides of the boat. The connected grounds suggested by AS didn’t eliminate the interaction.
• The isolated EC didn’t appear to influence any other probe. I assume this is due to the power isolator, but this hasn’t been explicitly tested.
• All three probes can’t work near each other — need to separate
• DO needs to stay underwater — any surfacing can influence quite a few readings
• temperature doesn’t appear to influence any of the other probe’s values

We’ll go with DO and temperature on one side of the boat and pH and EC on the other.

Water quality array – temperature compensation

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.

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:

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.