Building a turbidity sensor – prototype I

Asked for what kind of sensor they would like to see added to the kit, participants indicated the interest to know more about the clarity of the stream water.

The SHMAK kit (see for example NIWA, 2008) also features the measurement of water clarity either with the Clarity Tube the Black Dish method. The training guide states that

“What you are looking for in your clarity results is any change over time. If there is a change to more turbid, then you then need to look for reasons.” (NIWA, 2008)

Hence a probe constantly measuring the clarity of the water appears to be a useful addition to the sensor family.

Research on other DIY turbidity sensors:

donblair (2015) provides a good  overview of turbidity, “how it’s usually assessed, and various approaches one might take to measuring it.” The Open Water Project Github repository features comprehensive documentation of their turbidity sensor design.

The most simple DIY design involves only two components, a light source aimed at a light-sensitive photocell (see for example Marchetto’s design as described in donblair (2015). With all components (an LED, an LDR and various resistors) at hand in the lab, the next step involves considering different enclosures and ways of waterproofing the components. An option suitable for Papawai Stream needs to work in relatively shallow waters and should obstruct the natural course of the stream as little as possible. Hence, a tube with a diameter of around 1-3cm, akin to the usually shallow depth of Papawai Stream would be a good first prototype. The tube should be dark to minimise ambient light impacting the sensor readings.

Prototype 1: Garden Hose enclosure

A relatively cheap and easy to recycle material that is suitable to be used in water is a garden hose. For the Prototype 1, I use a piece of garden hose of about 10cm.

Image showing a garden hose being cut with a craft knife
Preparing the hose

First I connected the LDR sensor and the LED on a breadboard to test the sensor readings via Serial. The Arduino code used for this version of the prototype can be found here.

Image showing the Wemos D1 mini microcontroller on a breadboard connected to an LDR (light dependent resistor) and a white LED.
Wemos D1 breadboard setup

It turned out that a 2K resistor for the LDR and a relatively low brightness value for the white LED shows a consistent change in the tube.

analogWrite(ledPin, 64);

These values are good enough for general testing of the design and will likely need to be adjusted to the conditions in the field.

Image showing the inside of the garden hose containing the white LED pointing at the LDR
Image of the inside of the garden house with the LDR and LED

I cut the hose in half to position the components inside and used transparent sellotape to attach the two halves back together. This design generally worked but required some work making sure that the electonics don’t short circuit.

Image showing a top down view of the garden hose and the breadboard
Breadboard Setup including garden hose

The next challenge is to waterproof the design. For this first iteration, I chose hot glue to seal the exposed wires of the components, similar to Marchetto’s design (as cited in donblair, 2015, see image).

The four wires sticking out of the submerged part of the sensor need careful waterproofing. While hot glue generally works, it runs the risk to break once set. A flexible waterproof sealant would be safer.

Image showing the garden hose and components in a vice covered in hotglue
Attempt of using hot glue to attach the sensor and LED to the use

I will redo the design with proper cables connected to the sensors for more safety and the ability to test the sensor submerged in water.

Component List:

Hardware:

  • Computer with USB interface
  • Wemos D1 mini (know known as LOLIN
  • 1 LDR
  • 1 2k Resistor
  • 1 3mm white LED

Software

Arduino… Tools… Board showing list of ESP8266 boards

 

 

donblair. (2015, August 25). Turbidity 001. Retrieved January 18, 2019, from publiclab.org/n/12168
Kelley, C., Krolick, A., Brunner, L., Burklund, A., Kahn, D., Ball, W., & Weber-Shirk, M. (2014). An Affordable Open-Source Turbidimeter. Sensors, 14(4), 7142–7155. https://doi.org/10.3390/s140407142
NIWA. (2008, December 17). Training notes. Retrieved January 28, 2019, from https://www.niwa.co.nz/our-science/freshwater/tools/shmak/manual/15trainingnotes
Open Water Project. (n.d.). Open Water Project. Retrieved January 28, 2019, from https://github.com/OpenWaterProject