An internet search will bring up a number of devices people have created to help when calibrating a DistoX2, mostly these seem to require the use of a 3d printer or a trip to the local plumbing store. I have made a rotary cradle out of some scrap plastic material that allows the the heading of the Distox2 to be maintained whilst rolling the unit to collect the required shots for calibration to be performed.
DistoX2 mounted in plastic jig
It is two discs (cut on a lathe whilst sandwiched and bolted together so they are exactly the same size) which clamp around the outside of the Disto body. The front has a large hole for the laser and the rear has a small hole in which the rear reference point is to be aligned with.
Front end with clearance for laser beam, rear hole to align with reference point
Before use the front end (whilst maintaining the position of the reference point to the rear) must be moved so that the laser exit is inline with the centre of the circle of plastic. This was done via trial and error, the jig and Disto was pointed at a wall around 4 m away and rotated until the laser described as small a circle as possible (nearly a dot) on the wall.
Once satisfied I then collected the 56 shots required for calibration. I prefer doing this in my garden, i’m yet to get better results using targets on a cave wall.
Using a plastic mitre to keep heading constant for flat shotsNon metallic objects used for angled shots wood held together by dowels not metal !
Once the shots have been collected they are grouped then analysed in Topodroid, i’m very happy with the results and would definitely use this jig the next time I calibrate the disto
There are a number of different commercially made rebreathers available on the market which vary greatly in design, most manufacturers publish their test data so I thought it would be interesting to tabulate it to allow easy comparison (though not all tests are directly comparable). I have made the simple division of absorbent weight Vs. time/ litres of carbon dioxide to give an indication of efficiency and how this changes with depth, something I haven’t seen discussed anywhere else.
Rebreather scrubber duration review table
It is interesting to see the effect of increased depth, the scrubbing capability really deteriorates when the 100 m tests and the 40 m tests are compared. Adding extra insulation or testing in warmer waters also has a huge effect on duration as well as the obvious one: breathing less.
One of the previous posts on this website details the device I have assembled in the hope to speed up underwater cave surveying and at the same time make it more accurate than using the traditional divers compass, depth gauge and slate.
Using the Adafruit BN0055 ‘9 DoF IMU’ inside a waterproof housing as the tilt compensated compass should give a reasonable degree of accuracy but just how accurate is it going to be ?
To find out I ran some tests using a DistoX2 for comparison.
A small wooden jig was constructed that allowed easy foresight and backsight alignment of the home built device and the DistoX2 so that comparable shots could be easily collected.
Disto X2 in wooden Jig, Plastic pegs used for alignmentSurvey box in wooden jig, axis of BN0055 co-incident with alignment of plastic pegs
The sizing of the recess in the wood is such that when the box is rotated for the foresight/ backsights and pushed up against the right and left hand edges the sensor of the BN0055 is in approximate alignment with the plastic pegs used to align the DistoX2, the BN0055 is mounted around 90° out from the long axis of the box so its raw reported bearings is around 90° different.
Forty comparable foresight and backsight shots were taken with both devices and the data entered into a spreadsheet. The first task was determining the average difference between the DistoX2 data and the box data (I should come up with a decent name for this device…) The average difference between the two was 89.56°.
The Raw data from the box was then corrected by 89.56° and re-compared to the DistoX2 data. Average difference to Distox2 and Standard deviation values were calculated.
The foresight and backsight differences were also calculated to give a quality check for the shots as the jig wasn’t moved until foresight/ backsights were taken with both devices. The DistoX2 foresight/ backsight differences were far smaller than those calculated for my home made device.
These tests were conducted on a flat surface so further tilted tests will be done to assess this aspect, overall I am happy with the results so far, I have been able to buy an off the shelf sensor and without any complicated calibrations or maths have a sensor that is able to report magnetic bearing to within a few degrees of a DistoX2.
Assuming the tilted performance isn’t much worse then any large errors underwater will come from the ferrous metal equipment carried by the diver (or in the sump) and the ability of the diver to align the device with the dive line which is another challenge itself which needs thinking about.
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.
Device assembled and ready for testing
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.
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.
Test data
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.
