A commercially available device for underwater cave surveying is available to purchase called the Mnemo, in keeping with traditional cave survey methods it logs distance, depth and bearing of the line used in caves to guide cave divers.
Inspired by this concept I set about designing and making my own version, it is a work in progress and in its current form can log depth (via a pressure sensor), bearing, temperature, pitch and roll of the device (useful for assessing how still the device was during logging, inclination (pitch) combined with depth change can also be used to estimate distance between belays using basic trigonometry).
The line measurement aspect of the Mnemo might be more difficult to implement in British caves as the line diameter varies greatly from cave to cave and sometimes even within the same sump so I have ignored that bit for now until the rest of the measurements are proven to be of reasonable accuracy.
Housed in a waterpoof box I have:
Adafruit Feather M0 SD (control and data logging)
Adafruit DS32231 RTC (timestamping)
Adafruit BN0055 (9 DOF IMU)
Blueorobotics Bar30 (pressure sensor)
Small screen (data display)
IP68 Momentary Piezo switch
18650 Battery
Assorted resistors, capacitors and a power switch
The components are mounted on a custom made isolation routed single sided PCB and hand soldered onto header pins.
The BN0055 IMU was chosen as it does the complicated sensor fusion on the board and outputs a heading, pitch and roll solution (it can also output raw data if required but the maths and programming is beyond me). This is much easier and hopefully more accurate than having to read and compute data from the separate IMU components.
The device is powered on by activating the latching on/off switch accessed by removing a 3/8″ UNF regulator blanking plug from the side (must be done out of water). When the program starts the battery voltage is displayed before showing the calibration status of the three sensors which make up the IMU which are ; a gyro, an accelerometer and a magnetometer. It is important each sensor is calibrated before use but this doesn’t take very long and once calibrated this status is held until the device is powered off. In between survey shots the status of each sensor and the overall system status is displayed on the screen
Once ready the device can be aligned with the dive line next to a belay, the button can be pressed then after a short delay the device writes 10 values at 10Hz to the SD card, it then waits for the next button push. The screen does display the shot data momentarily but as the screen is small this is more for reassurance.
http://https://youtu.be/mTJUWdjrYCA
In this manner it could be used to replaced the compass and depth gauge readings taken by a diver, line distance still needs to be measured traditionally and noted.
By automating the bearing and depth measurement and recording aspect of underwater cave surveying I hope to speed up the process and increase the accuracy of the data collected, this should prove useful in resurvey projects of caves which are thought to be close by to other caves.
The device could also be reprogrammed and repurposed as a DPV navigation console, or mounted to a camera and used to provide accurate camera orientation and depth data to improve under water photogrammetry image alignment (inspiration for this idea was taken from https://youtu.be/YKw3lBXX6vM ).
Building this device was the first goal, testing and appraising its accuracy is the second goal (currently ongoing) then if suitable putting it to use in some projects is the third and main goal.
Links to the various parts used are shown below:
https://learn.adafruit.com/adafruit-feather-m0-adalogger