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rmssetc

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I'm convinced of the benefit of analyzing tanks for O2 percentage and presence of CO (carbon monoxide).

I was a kickstarter supporter of Cootwo, and still think it was a great concept and form factor. I was unhappy with the lack (and tone) of support from Divenav in the past. 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. There are a couple of great threads on true DIY builds for an analyzer ( Nitrox/Trimix & CO analyzer and DIY Touch Screen Trimix Analyser), and either of those would probably be a better solution than my hack, but that's never stopped me before from building my own Frankenstein.

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. Here's the finished product, which I'm calling Frankenstein Ugly Carbon-monoxide Kit & Oxygen Reader, but I may need to change the acronym for marketing purposes:

20211009_162558.jpg


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 Sensorcon CO analyzer.

And now for the parts breakdown and details.


20211009_162057.jpg

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).

20211009_162116.jpg

I tapped the end cap at 3/8" NFT (fine) thread to accept the BCD fitting.

20211009_162143.jpg

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.

20211009_162303.jpg

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:
20211009_162333.jpg


I tapped threads in each end of the O2 detector. Easy work, but the plastic feels quite brittle, so it was a slow process:
20211009_162314.jpg


Assembled, the O2 analyzer now has a hose fitting on each end:
20211009_162420.jpg

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.
20211009_162633.jpg

Based on Sensorcon and Digital 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.
20211009_162753.jpg

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.

Bill Of Materials (all prices approximate)

 

rmssetc

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Update: I have been using the contraption for the last 14 days of diving. Works fine. Most of the tanks have been within 0.4% of the pre-existing label, with a couple of outliers that suggest that the analyzer is accurate. Two tanks out of ~30 showed minimal CO (1 or 2ppm). I'd call that within the error of the equipment. The CO meter works as expected when it was not on a tank and a boat fired up next to us in the harbor--40ppm briefly, then 20s to teens.

After 2 weeks of my using it, several other people on the boat borrowed it to check their tanks...the boat & DMs did not offer an analyzer until then.

I'll post more of a follow up in the thread about whether people in the "real world" check tanks, or only in training.
 

Agro

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What is the CO limit for breathing air? The www gives 2ppm - 15ppm, which is a difference....
 

Scuba-Lad

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Update: I have been using the contraption for the last 14 days of diving. Works fine. Most of the tanks have been within 0.4% of the pre-existing label, with a couple of outliers that suggest that the analyzer is accurate. Two tanks out of ~30 showed minimal CO (1 or 2ppm). I'd call that within the error of the equipment. The CO meter works as expected when it was not on a tank and a boat fired up next to us in the harbor--40ppm briefly, then 20s to teens.

After 2 weeks of my using it, several other people on the boat borrowed it to check their tanks...the boat & DMs did not offer an analyzer until then.

I'll post more of a follow up in the thread about whether people in the "real world" check tanks, or only in training.
Wow, so if I'm understanding this correctly, not only did they not offer to check for CO (which most places seem not to do anyway) , but they also didn't offer a Nitrox analyser either? Or do you mean an O2 analyser to check standard tanks of air?
 

rhwestfall

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Nice little project. My solution (on my compressor) is a tee in the line feeding the meter with a check valve on the other leg to vent excess pressure/flow. I have a flow limiter fed off a second leg of my compressor fill line (which is a male DIN so it could be put on a tank valve) that feeds a flow meter, then the tee and then the meter.

Note: for anyone using the flowmeter, you need to keep it upright or your flowrate value is incorrect.
 

rmssetc

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Wow, so if I'm understanding this correctly, not only did they not offer to check for CO (which most places seem not to do anyway) , but they also didn't offer a Nitrox analyser either? Or do you mean an O2 analyser to check standard tanks of air?

Yes. I was on two boats, with other people diving Nitrox.

In both cases, the rental tanks had been labeled by the fill station (not run by either dive op) before being put on the boat, well before the divers arrived.

Neither boat offered an O2 analyzer to check the O2 content for Nitrox divers.

On one boat, when people saw me checking my tanks, the DM & captain each pulled out a Nitrox analyzer, so they had them handy, but unless a customer asked for one, there was no offer and tanks weren't being checked in front of divers. I don't find that at all unusual -- it seems to be the norm and is accepted by other divers.
 

rmssetc

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What is the CO limit for breathing air? The www gives 2ppm - 15ppm, which is a difference....

I don't think there's any published limit for CO in SCUBA air. There are safety limits for industrial workers, but they are much higher than I'd want for breathing gas under pressure.

For my casual (depth above 30meter, warm water, low effort, no deco) dives, my personal limit is if the analyzer is showing 4PPM -- I figure the analyzer has a margin of error of about 1PPM, so that 4PPM reading might really be 5PPM -- out of my comfort zone.

See:
AnotherThreadAboutCOanalyzers
 

rhwestfall

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For my personal requirements, it better be 0.

YMMV
 

zebu14

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Thanks for sharing this nice project.
Very interesting for me as I'm actually setting up a DIY nitrox analyzer.
 
https://www.shearwater.com/products/swift/

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