Telemetry overlay on video & Bubble physics?


(Alex Satrapa) #1

I’m building a bubbler-style pneumatic water level sensor[1]. The simple functionality of this sensor is to release a stream of bubbles at the bottom of the vessel, measure the air pressure in the tube, convert air pressure to height of water column.

While I have the basic electronics sorted out (along with some code[2] that reveals just how clueless I am), I still have to wrap my head around the physics, specifically the differences in pressure that will be seen during the formation and separation of bubbles at the end of the bubbler tube.

Does anyone know of convenient software that will allow me to take “high speed” video (240fps from an iPhone for example), along with rows of data from the sensor, and produce an animated line graph superimposed on the video playing back at “normal” frame rates of, say 30fps?

I plan to synchronise the video with the telemetry using LEDs visible in the video stream.

Also, does anyone understand the physics involved? When attempting to solve the Young-Laplace equation[3] to derive maximum pressure from radius and surface tension, I end up with a bubble on a 4mm tube producing a pressure equivalent to 0.5cm of water (about 0.007psi, with PSI being the units reported by my pressure sensor).

My plan with the high speed video is to record a few seconds of bubbles being formed and then escaping the tube, overlaid with a scrolling graph showing the measured surface tension, just to confirm where maximum and minimum pressures are observed.

[1] High level design of bubbler level sensor from Benoit Drooghaag, “Rain water tank level,” http://playground.arduino.cc/Main/Waterlevel
[2] Arduino code and Fritzing designs for my bubbler water level sensor: https://github.com/AlexSatrapa/Arduino/tree/master/Bubbler
[3] Young-Laplace applied to bubbles forming at the mouth of a capillary immersed in liquid from Krüss, “Bubble pressure tensiometer,” http://www.kruss.de/services/education-theory/glossary/bubble-pressure-tensiometer/


(Jack Pittar) #2

I have done this, but many years ago with different hardware that pre dates that available today.

I found the flow rate of the air had a big influence on the reading.
If you want accurate readings (needed if you want to measure flow in and out of the tank) you will have to compensate for the offset drift in the sensor, and in the case of an absolute sensor, for changes in the atmospheric pressure over time.
If you have no check valve, and the pump leaks a bit, the air in the tube will leak out and allow you to get a zero reference.
To take an accurate reading, you need to calibrate the zero reading, then run the pump until the reading levels off, then take the real depth reading after the pump has stopped.
Jack.