NASA's solid state CO2 detector: suitable for rebreathers?

[Tassie_Rohan]

Folks -

I came across this post from NASA marketing their solid state CO2 detector chip, and was wondering if any rebreather manufacturers are looking at incorporating them into their units - or if they are suitable at all?

• Solid state CO2 detector, using carbon nanotubes and a metal oxide
• Apparently accurate in humid environments.
• Appears to avoid the interference between CO2 and H2O found in infra-red CO2 detectors.
• Doesn't need as much calibration as IR sensors.

The boffins explain their sensor in more detail here.

To my untrained eye (and brain) this sensor looks like a long-awaited step forward in CO2 detection, but I haven't seen any discussions on its use in rebreathers. Your thoughts?

Cheers,
Rohan.

 

[Mod63]

From the paper: "Based on the results of the humidity study, we need to remove the moisture from CO2 in the air stream.". So IMHO no use in rebreathers (yet). Not to mention what Dr. Simon Mitchell has stated as prerequistes to have a proper and usefull reading in a rebreather.

 

[Capt Jim Wyatt]

I would rather have no CO2 sensor in my RB than one that *MAY* be inaccurate due to a humid environment.

 

[davehicks]

There are already commercially available CCR units for sale using solid state sensors. It looks like in the next year more units will become available at an even lower cost than the existing units.

 

[bubblemonkey2]

What is the issue with moisture exactly?

For a device measuring infrared transmittance, it seems like the absorbance wavenumber spectra are different enough between CO2 and H2O that it should be possible to tell them apart, run calibrations etc.

Good chance that something does work? But perhaps nobody in their right mind is going to stand behind a new gadget that could be blamed for new accidents.

Might give some people a risky temptation to dive their canisters significantly longer because they are "watching the CO2?"

But I guess it would be nice to have the data anyway. Maybe it could help in some cases--including post-accident analysis. Or maybe your aiCCR will auto-bailout

 

[tryerson64]

The unwanted response to O2 (see Fig. 10a in the cited article) may be problematic, especially when diving nitrox or trimix gases.

 

[Capt Jim Wyatt]

here are already commercially available CCR units for sale using solid state sensors.

O2 not CO2.

 

[mr_v]

I would rather have no CO2 sensor in my RB than one that *MAY* be inaccurate due to a humid environment.

That a million times over. If you maintain your unit and do not engage in questionable scrubber saving practices, you will be okay.

A solid state O2 sensor is another story though.

 

[Dr Simon Mitchell]

That a million times over. If you maintain your unit and do not engage in questionable scrubber saving practices, you will be okay.

Hello Mr V,

Unfortunately this is not true. You are assuming that the only cause of CO2 problems is scrubber failure or bypass leading to CO2 inhalation. That is one way in which hypercapnia can occur, but its certainly not the only or even the most important one. It is also possible to become hypercapnic simply because we don't breathe enough to eliminate the CO2 being produced. This occurs, in some people more than others, when you combine increased work of breathing with exercise; a combination that occurs in most dives. The mechanisms and consequences, and the monitoring required to detect it are all explained in this presentation. If you want to fully understand this issue I would recommend you watch it.

Simon M

 

[mr_v]

Hello Mr V,

Unfortunately this is not true. You are assuming that the only cause of CO2 problems is scrubber failure or bypass leading to CO2 inhalation. That is one way in which hypercapnia can occur, but its certainly not the only or even the most important one. It is also possible to become hypercapnic simply because we don't breathe enough to eliminate the CO2 being produced. This occurs, in some people more than others, when you combine increased work of breathing with exercise; a combination that occurs in most dives. The mechanisms and consequences, and the monitoring required to detect it are all explained in this presentation. If you want to fully understand this issue I would recommend you watch it.

Simon M

Dr Mitchell,

I always appreciate your input!

My comment was a bit tongue-and-cheek. Of course, it is highly possible to not breathe correctly and, given other variables, e.g., load and gas density, end up with a CO2 hit. I experienced that but not while diving. Here is the question: is the latest CO2 sensor technology reliable enough? The way I see it, unless we can measure the delta and figure out how much CO2 a body retains, we are still dealing with a very rough terrain; hence, my comment.

Not going cheap on scrubber, knowing appropriate gas densities, and breathing correctly still seems to be the best available approach to a typical CCR diver. I hope things change soon.

Cheers!