So I dug my teeth into the problem. This was around the beginning of February 2011, and my first task was to find out just how many different ways there actually were to detect water movement. I had some sense of this from the units we had already used in the past:
- Mechanical methods, such as rotating impellers, veins etc. (like the RCM7’s Trish had used in her PHD work)
- Electromagnetic flow sensors (like the InterOcean S4 or the Ott Nautilus) that detected the movement of ions in the water, or flux line deflection.
- Ultrasonic systems (like the Falmouth Scientific ACM & the later model RCMs)
And of course there were many, many more listed on wikipedia including:
- Optical flow meters, using reflection of light from particles.
- Thermal mass flow sensors, using heat conduction.
- Vortex flow meters, using turbulence and vibration.
- Pressure based systems, that rely on the Bernoulli principle
I even thought of a method based on inductive coupling (that still might work) based on building a transformer with a hollow core through which water flowed. I hoped the inductance of the core would change (and therefore the voltage on the secondary winding) with the water velocity. But it seemed that I would also need a conductivity reading because the inductance would also change with whatever was dissolved with the water (probably much more so than flow) so I would have to separate the changes caused by salinity from those caused by water flow. This might even be possible, but it just seemed way too complicated.
And I was realizing that many of these approaches were simply out of my league, as my electronics background was fairly modest. So then I concentrated on the apparently simpler physical and mechanical methods, using a process of elimination based on major criterion Trish had outlined to find the the most promising one.
A propeller based system seemed pretty simple to build, and modifying one of the many hall effect magnetic flow sensors already on the market seemed like the easiest approach. But I had enough experience diving those caves to remember all the organic “ick” that could accumulate near the Cenote openings, so I was not really keen to use anything with a little propeller on it.
The thermal loss systems involved heating an element to a constant temperature and then measuring the electrical power that was required to maintain the heated element at temperature. So they seemed pretty easy to make, as they were basically just a resistor in line with something measuring the temperature. In fact, there were plenty of cheap anemometers already out there based on mass flux that I could modify, but they were physically pretty small so I worried about fouling, and I had a sense that just throwing electrical power away as heat would draw too much juice for long term deployments, especially in an underwater environment.
Then I came across the Salamander Sensor Project. They were working on flow sensors too! And they had already open sourced a velocity sensor design based on a really simple flexible resistor strip. I was pretty excited, and thought that I had reached the end of my quest right there. But as I read into it, I discovered that they had been concentrating on streams, and other surface accessible water flows, with wireless data transmission. And I had a feeling that those flex sensor strips were just not going to like being put under significant pressure all the time. So none of it was really suitable for complete submersion in flooded caves, possibly thirty meters down, and hundreds of meters away from any surface access.
However, the fact that some other group had already come up with a water velocity measuring device was really encouraging, and the simplicity of the flexi sensors appealed to me, as they had so few moving parts. So I dug a little deeper into the concept, and found the same physical principal being applied all over the place like, for example, in the old Johnson Airspeed indicators, used by barnstormers back in the 30’s. All I had to do was figure out how to use the same “flow-pressure causing displacement” principal in a design that was small, robust, and most importantly, completely submersible for long periods of time….hmmmmm.