Just Shake and Use!


What do the polders of Bangladesh, the shrimp farms of Vietnam, the rice paddies of the Philippines and the village wells of Indonesia all have in common with Australian irrigators?

The answer is that all of these folk are critically interested in salinity that, at certain levels, is toxic to stock, crops, soils and humans.

The trouble with salinity measurements is that they are an impenetrable maze of weird indigestible units: μS/cm, mg/l, ppt, ppm, mS/cm, dS/m and EC units – take your pick! What do these numbers mean?

Down in the MEA basement, fresh from tackling the design of inexpensive soil moisture measure tools for Africa (see the 'Chameleon' story on this blog), the idea of colourful displays had taken hold.

Why not a ‘Salt Stick’ for measuring salinity?

Salt Sticks
Salt Sticks


This gadget needed to transcend languages, educational levels, battery issues, solar power problems, sample collection issues, instruction manuals and the Internet.

Two more young interns – this time from Vietnam and China - were inveigled into the complex world of electronic design and prototyping. Electrodes needed to be made from titanium to avoid corrosion, salinity ranges needed to be changed simply for different applications (shrimps like salt, pigs don’t!), the device needed to be auto-ranging to cover the territory between 0.1 and 60 EC units and the Salt Stick needed to be built into a syringe so that collecting the sample and measuring it all happens in-situ.

All good, but the toughest task was the design of the power source. Imagine this device left on a shelf or in a dark boat locker for years on end. No solar charging, batteries gone flat and new ones unobtainable in emergency situations, all mains power systems remote or down. That only leaves human muscle power. So we designed the Salt Stick to be shaken (not stirred). A magnet flies back and forth inside a coil in the sensor’s body, charging a super-capacitor that powers the measurement and display for up to a minute.

When a true measurement has been made, one of six colours appears on the indicator built into the handle. The salinity level – a number between one and six - is read off by rotating a coloured ring to match the bright LED indicator. Even the colour blind – who see colours differently to the majority of us – can match the sensor output to a numerical salinity level by this simple method.

With special thanks to interns Bonnie Wong (Vietnam) and Ning Liu (China) who suffered much to debug the Salt Stick hardware and software.


Ning Liu

Ning Liu



09 December 2014
Legend! Not your average canoe builder...
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