Know your PCO2 (Why Rebreather have no CO2 sensor)

decoweenie

Gilles,

Are you French ?

This is my last post since you don't even seem to attempt to at least listen to almost everyone is saying. This reminds me of your discussion with DrMike about calibrating with wet sensors.

I am not going to go into the details of the incident, but the diver did pass out from hypercapnia and not hypoxia. Just for the record since I was there and did the check at the time of the incident (in the water) and on the boat (after the rescue).

If you keep ignoring all of the warnings, and continue to push your scrubber to learn the signs/symptoms of hypercapnia, I wish you all the best.

__________________
"...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..."

- Rebreather World PM

Gilles

No I am not French. I am obstinate only when confronted with unjustified antagonism, very common on this forum.

I had no discussion with DrMike about wet sensors. I recall only an error I made, aknowledged, and then corrected. You are maybe confused with a different argument about manual flight.

There is/was no insinuation about the above-mentioned fatality. Who knows what the poor bloke (God rest his soul) went through before he died. Did he have a BOV installed upon his system? Any other details you can share?

Most of us that dive CCR inadverdently push our scrubbers, whether we mean to or not. I have done so (inadverdently), and by repeating the potential mis-steps I have made, I established a clear correlation for myself, consistent with the descriptions provided by others. I risk antagonism to share with you and all.

No need for this to be your last post. Your statements/responses remain intelligent and induce thought/discussion. The very purpose of this forum. Let us all not be YES persons

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Gilles
http://www.dirrebreather.com

decoweenie

Apologize, my mistake then...

You must have meant to question (narked@90) John's post about his friend fatality incident. I don't know anything about it...

__________________
"...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..."

- Rebreather World PM

narked at 90

Hi all, i don't want to go into my friends death at this point so that discussion will have to wait for time to pass for his family
i posted to show that you don't always have the same symptoms to co2 like getting the feeling of impending doom before you pass out and game over.
i would like to see a usable co2 sensor in the loop how much use it would be...well i'll reserve judgement on that one, but our body's are a poor indicator of time left before pass out so why would you want to push it?
sure some time we go over the times stated in the manuals but i don't have any intention pushing my scrubber to brake though just to see what it's like and where that barrier is..i know what it's like and don't want to go there again, the risks are just to high and your ability to cope is just not there, so there becomes just one out come to that game.
kind regards john routley

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Ellie

Hi.

First post here, thank you for the interesting reading.

Since a diveable pCO2 monitor is not currently available, how about monitoring the diver instead? A pulse oximeter is a fairly simple (solid state!) device that could trigger a warning when a diver's heart rate exceeds X or blood oxygen saturation falls below Y.

... Or would it be one more thing to distract a diver, eat batteries, and break?

Ellie

iain-hsm

Ellie

C02 detection has been discussed here before, two years ago we had both a forum discussion and some limited disclosure at DEMA 05

Of the pulse oximeter data monitoring this is used in LEBA (long endurance) military rebreather designs but is better known used in hyperbaric chambers. PPC02 is also mil spec only at present as is units for monitoring bubble formation.

These are some of the monitor conditions proposed in the NGUBA next generation ........As at present the decompression program VVAL 18 and 21 limits the operational duration.

The basic idea is to monitor all mechanical and physiological condition and performace of the diver and cut the table in real time underwater (deco obligation) to the match the physiological diver conditions.

One other area to look for is :

Some divers are more at risk of decompression sickness than others. Recreational divers using tables designed for the military place themselves at higher risk because such tables were designed using highly trained and physically conditioned divers who have different physiological characteristics from the average UK diver, who tend to be heavier than the average population, and cannot be described as elite sports participants. This risk is well described in observational studies, and also in the original caisson studies of tunnel workers.

A study of 40 recreational divers examined the link between various physical characteristics and the number of venous bubbles produced after a 35 metre dive using the COMEX 1987 decompression table with a bottom time of 25 minutes and stops at 6 msw for 3 minutes, and at 3 msw for 15 minutes. As expected, half of the divers had detectable cardiac bubble signals but a very clear relation was found between the bubble count and the age of the diver, weight, percentage body fat, and an inverse relation was demonstrated with physical fitness measured by VO2 max.

Statistically significant differences were confirmed between the group with no bubbles, and those with the highest bubble grades for age, weight, and VO2 max (but not body fat). Previous research has been performed on pigs where a measure of bubbling tendency was performed before starting a training programme to increase their aerobic fitness. Similar work has been performed in rats, and this confirms that the bubbling tendency is reduced as aerobic fitness increases.
The conclusion from these studies must be that divers should be encouraged to maintain or improve their level of fitness. This is concerning because there are signs that the fitness of divers and the general population is declining with increases in body mass index and the number of smokers. In the future dive computers may be programmed to include risk assessment with divers entering their individual characteristics including weight, age, sex.

Until then divers need to take responsibility for their own risk reduction and should be educated accordingly.

Circulating venous bubbles in recreational diving: relationships with age, weight, maximal oxygen uptake and body fat percentage. Carturan D, Boussuges A, Burnet H, Fondarai J, Vanuxem P, Gardette B. Int J Sports Med 1999;20:410-414

narked at 90

hello Ellie, the system you propose has been talked about before and at face value seems to have merit but by the time your body accumulated co2 and triggered an alarm it is probably to late, as you all ready know you have a hit or have passed out..
what we need is live alarms of rising co2 below the fresh hold of danger rather than co2 levels we have in our body's already,
kindest regards john routley
Home
ps welcome to the mad house!!!!!

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iain-hsm

John.
Just a small correction to your comments. The pulse oxymeter Ellie described is originally a medical devise that measures only the oxygen 02 in the blood. Basically it is a clip your wear on your finger or ear lobe and it is only the oxygen level within the blood that is measured.

However for the military the diving carbon dioxide C02 detector measures the breathing gas at the inhale side of the breathing loop. This is extremely accurate devise, it has to be as the maximum percentage of carbon dioxide C02 allowed is only 0.5% for C02 in a rebreather.

Although the C02 sensor can measure up to 8.00Kpa it is the accuracy of the C02 gas measurement at the lower levels that is critical at (+/- 0.2Kpa or 0.2% at 1 bar) the other accurate measurement required is resolution accurate to 0.01 kPa or 0.01% at 1 bar.
The accuracy limit set for the controller within 0 to 3.00 kPa is within +/-0.2kPa
The reaction time for the controller is 5 seconds to 80% of final reading.
The power up from a cold start is 80 seconds to 100% final reading.

This is no were near the massive 8 to 10% of C02 needed in a breathing loop before you pass out. Quite the contrary it is exactly what you suggested in you post. "A live alarm of rising C02 below the threshold of danger". Iain

jdicediver

For information, what BOV have you settled for?

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Regards;


"Some people shouldn't dive...others shouldn't even get wet".

Gill Envy

ok, maybe this is a crazy thought but what about the value of monitoring co2 on the exhale side of the loop coupled with monitoring respiration cycles. I know it's counter intuitive but since the limiting factors are dwell time and volume of co2 absorbtion of sorb, maybe a pre alarm alerting a diver to an aproaching risk threshold where breakthrough is likely to occur due to highish levels of co2 production and rapid Respiration/decreased dwell time would be more valuable and succesful at preventing co2 hits than trying to detect more infinitesimal levels on the inhale side... that once detectible have already reached dangerious levels.

useful direction?
g

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Gill Envy

...Because I wasn't born with gills!
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