Monitoring Oxygen and Carbon Monoxide in Dive Tanks
I'm convinced of the benefit of analyzing tanks for O2 percentage and the presence of CO (carbon monoxide).
On my last dive trip I found that my Cootwo was giving really wonky readings (O2 randomly off by 5~10%, despite having a good sensor, recent calibration, etc) and the CO sensor was due for replacement/calibration. With Divenav out of business (or at least, out of the business of hardware products -- maybe they still have a presence with their apps), I began looking for alternatives.
I decided to use off-the-shelf O2 and CO devices, packaging them to use the same gas supply in order to simultaneously analyze for both gases.
I picked up an O2 analyzer that was designed to pass an active flow of gas across a sensor, instead of being made to measure ambient O2 in still air and a CO sensor, and tried connecting them in-series to the low-pressure BCD hose, using the pressure-reduction fitting from my old Cootwo. I immediately found two problems.
When connected to the BCD fitting, the pressure was enough to pop any of the hoses.
Even with the tank open just a tiny crack to reduce the pressure, the gas flow rate was so high that the meters were inaccurate, for example, showing that a tank of air had 30%+ oxygen.
My solution was to add a way to reduce the gas pressure and flow.
I'm calling it the Frankenstein Ugly Carbon-monoxide Kit & Oxygen Reader, but I may need to change the acronym for marketing purposes.
The gas flow starts on the left, with a low-pressure Schrader valve fitting to connect to a BCD, stolen from my Cootwo. That goes to a flow reducer, then into the O2 analyzer, and ends in the CO analyzer.
And now for the parts breakdown and details.
Cootwo fitting, short length of plumbing pipe with glued caps at each end, spare rubber hose. The pipe has several 1/8' holes drilled to release air (more details below).
I tapped the end cap at 3/8' NFT (fine) thread to accept the BCD fitting.
The other end of the flow limiter (expansion) tube was also tapped at the same pitch, to accept a 3/8-threaded to barbed hose fitting. The torn O-ring was an unnecessary attempt to prevent either set of plastic threads from stripping when tightening. What's not shown here is a small tapered plug inserted inside the barbed fitting, with a hole (~1/16') to further limit air flow out to the analyzers.
The oxygen detector, from Forensics Detectors -- this product is made for divers, yet it's not waterproof, though they also sell waterproof O2 and CO detectors. I contacted them to suggest a combined O2 and CO unit (like the Cootwo), but haven't gotten a response yet. Also shown are another 3/8'-to-barbed hose (1/4' ID) connector and a 3/8-to-1/4 barbed hose adapter.
I tapped threads in each end of the O2 detector. Easy work, but the plastic feels quite brittle, so it was a slow process.
Assembled, the O2 analyzer now has a hose fitting on each end:
The 3/8-to-1/4 hose adapter threads into the tapped O2 analyzer -- despite screwing a barbed fitting into the threads the plastic deforms enough that it is tight enough fit for this application. The analyzer ships with a soft rubber 'boot' with a small hole. It's designed to fit over the outside of either end of the rigid blue plastic pipe and allows the analyzer to be held tightly to a tank without a 1st stage regulator. Since I tapped the inside of the plastic, the rubber piece can still be used.
I picked up a low-pressure gas flow regulator/analog meter. The outlet valve can be adjusted to set the flow rate from 0 - 1.5L/m, and the bubble in the tube provides a nice analog meter of the gas flow.
Based on Sensorcon and Forensics literature, it looks like the gas sensor on each meter will be fine getting a supply at about 0.5L/min.
I had drilled 4x 1/8' holes in the expansion pipe, thinking that would be a good starting point to bleed off gas before supplying it to the O2 and CO detectors. To check the flow rate, I put the regulator downstream of the expansion tube, with the flow rate regulator valve fully open, using it as a meter to see the gas flow.
With 4 holes in the expansion tube, the meter on the flow regulator showed a flow rate of ~0.1L/m. Closing 3 of the 4 holes gave a steady flow rate of almost exactly 0.5L/m. After testing, I removed the flow meter/regulator from the contraption