Testing DIY EC sensors in Papawai Stream


moving The kit from lab to field


The crates make it easy to transport the probes and could also play in the role when installing the probes to keep the electronics in place and safely stored outside of the water during use. Additionally, I am carrying a backpack with my Thinkpad laptop, and a bag with additional equipment such as the TDS reader, batteries and USB cables and other necessities such as extra jars, paper towels, cable ties, pens, and a notebook. I also take a little camping stool and my smartphone with me. I transport the equipment by car to the Prince of Wales Park and further carry it from the carpark to the install site.



I follow the stream upstream and reach the site that I have scouted earlier as a possible install location. The stream flows on the right side of the walkway. I walk one minute to reach the install location further upstream where the path meets the water. I have placed the probes in reused jars, but I have not waterproofed them yet, so I temporarily propped the jars up with bubble wrap to protect the electronics from humidity and insects while making them easily accessible during the testing.

Papawai test Install site showing crates with probes next to stream
Setting up

The Raspberry Pi is still running, and the Moturoa_Transmissions network is up. I start unrolling the cables and connect the probes one by one to their batteries. I make sure the crate is in a stable position, so it does not create a hazard once I launch the probes into the stream water.


Looks weird. Two electrical plugs in the water.



2 EC probes and temperature probe submersed in stream water


accessing the sensor data


Plants are already losing some bits and pieces on my keyboard.




This is exciting.


I don’t see the temperature in the feed, so maybe the battery I used is not working. It does not have an indicator light so I do not know whether it is working or not. I will swap the power supply to the spare solar battery I brought along. This one is clearly discharging according to the LED, so I should see a temperature value soon.

Oh yeah, watertemp here we go. 14.2



concluding the experiment



Observations and recommendations for next install:

Design development and install


Equipment and electronics

  • Bring all spare batteries just in case.

Hygiene, health and safety:




Lab EC-sensor tests

Before using more EC probes in the field and gathering data from different parts of the stream I tested them in a controlled environment in the lab.

Experiment 1: 

The two versions tested use the same materials, same length of chord and same 560 Ohm resistor. The first test involved a jar filled with tap water, immersing both sensors in the jar and monitoring the data via mosquitto_sub.

Jar filled with tap water for testing two DIY EC probes

Note: The pink tape wrapped around one probe was an attempt to avoid the probe mistaken with a power plug on the lab table, which poses a severe hazard. The probes are stored away safely when unattended to ensure health and safety.  This version of the probe uses the Live (L) and neutral (N) prong of the plug. To improve the safety of the probe the live (L) prong should be removed and Neutral (N) and Ground (⏚) should be used for measuring the electric conductivity. 

The terminal output shows the EC probes publishing the measured values under the topics motorola/ec and moturoa/ecrua. While the test recording was done, the DHT11 sensor was also active in the lab, publishing air temperature (moturoa/atemp) and air humidity (moturoa/ahumid). Output showing values returned by two EC probes (moturoa/ec and moturoa/ecrua) and DHT11 sensor showing air temperature (moturoa/atemp) and air humidity (moturoa/ahumid)

This first test showed a deviation of around 10 between both probes, behaving relatively consistent. The next test would require measurements in the stream to see whether the probes return coherent readings from flowing  water.

Experiment 2:

Due to bad weather and high winds it was too dangerous to conduct testing in the field. However, to get a better idea of the consistency between the two probes I went to an easily accessible part of the stream outside of the forested area and collected two samples of stream water. 

Back in the lab I pour the first sample into a clean jar that is big enough to contain all three sensors. I prepared a paper sheet for keeping experiment notes, starting with date/time, location of sample taken, last weather and readings from the TDS meter at the beginning and end of the test.

  1. I boot the Raspberry Pi (the Pi acts as Wi-Fi Access POint hosting the Moturoa_Transmissions network and acts as the MQTT-broker).
  2. Connect laptop to Moturoa_Transmissions and start log with timestamp
    mosquitto_sub -v -h -p 1883 -t '#' | xargs -d$'\n' -L1 sh -c 'date "+%D %T $0"' > data.log
  3. Immerse probes into water sample and activate by connecting the Wemos D1 micro controllers to power supplies (USB batteries).

The raw data of both test results can be found on the development repository.