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)

Testing probe interaction/interference with three vessels

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.