Question Shearwater Teric CNS% calculation and Air Integration

[theminidiver]

Hello,

Does Teric AI take account of breathing rate ou tank's air consumption to calculate the oxygen toxicity (CNS%)?
Or does it show the same CNS% value with or without air integration?

I'm a average recreational diver with AWO+Nitrox certification who wants take the Tec 40 certification in order to have a proper training to plan and dive deco dives.

Today I use the simple, but yet useful, for the majority of rec dives, Cressi Leonardo dive computer. Which it is not suitable for deco dives, multiples gases, etc.
So I'm looking for the Shearwater Teric or Peregrine.
And as far as my research, the Peregrine is enough for simpler deco dives. I don't intent to take deeper Tec certificates, with Trimix or rebreather in the next couple of years. So the Peregrine seems ok for my needs.
But one of the downsides of the Peregrine is that is does not have Air Integration. And since my main concern is safety, I want the most rich and precise informations to do safe deco dives.
I can live without air integration, the SPGs seems enough.
But my question is, does Teric (or any other computer with AI) make better calculations regarding oxygen toxicity while monitoring my air consumption through the AI transmitter? Or the Teric's CNS% calculation doesn't take account info about my breathing rate?

Best regards,

 

[rongoodman]

CNS exposure limits are calculated by the length of exposure and the percentage of O2 in the breathing gas, not the amount of gas used, so AI isn't going to affect the calculation.

 

[Wibble]

... And since my main concern is safety, I want the most rich and precise informations to do safe deco dives...

The decompression algorithms (e.g. Bhulmann + Gradient Factors) are conservative and can be adjusted if you have some predilection to being more conservative -- change the GF's to do longer stops.

Unless you're diving beyond the recommended partial pressures of oxygen (1.4 for bottom, 1.6 for decompression gas) the CNS or "oxygen clock" will not be an issue for short/shallow decompression dives.

 

[mderrick]

Does not compute.

%CNS calculations are based on exposure to a specific pressure of oxygen and time. Your question seems to imply there is a relationship between rate of respiration and a PO2. Just to be clear, there is no relationship between computation of %CNS and rate of respiration.

Although widely taught by IANTD, the science behind %CNS is considered by many experts to be questionable at best and completely invalid by some -- particularly the concept of a surface interval 'credit' used to reduce the calculated exposure.

 

[Wibble]

Does not compute.

%CNS calculations are based on exposure to a specific pressure of oxygen and time. Your question seems to imply there is a relationship between rate of respiration and a PO2. Just to be clear, there is no relationship between computation of %CNS and rate of respiration.

Although widely taught by IANTD, the science behind %CNS is considered by many experts to be questionable at best and completely invalid by some -- particularly the concept of a surface interval 'credit' used to reduce the calculated exposure.

CNS (central nervous system oxygen toxicity) is about partial pressure of Oxygen and time. Same with pulmonary oxygen toxicity (aka the Oxygen Clock).

It's not pressure per-se, it's the partial pressure of oxygen at a particular depth which we all see as exceeding the MOD (Maximum Operating Depth) for a specific oxygen percentage. This is a simple formula:

MOD (as atmospheres) = target PPO2 / percentage of oxygen in the mix​

Example: air is 21%. Bottom gas max partial pressure is 1.4
Therefore MOD = 1.4 / 0.21 = 6.66 atmospheres
Convert to metres (subtract 1 from ata) 6.66ata = 56.6 metres (which is about 186.78 feet)

Same for 32% gives a MOD of 33.7 metres.


Oxygen Toxicity - DIVER magazine



If doing shallow decompression dives or short ones (e.g. under an hour of deco) doesn't really register against the CNS and Pulmonary toxicity unless you're using pure oxygen for your deco mix and hanging around at 6m/20ft so running it at a PPO2 of 1.6. One of the reasons most people start with accelerated decompression using well under 100% -- such as 50% or 80%. 80% is just as quick to get out and you don't run up the oxygen clock.

OT: When running much longer decompression times rebreather divers tend to use lower max PPO2 settings (known as a setpoint). For very long deco times the CNS is often pushed beyond the limits, but never deep. Decompressing is generally a very relaxed time busy doing nothing which probably mitigates a high CNS.

 

[mderrick]

CNS (central nervous system oxygen toxicity) is about partial pressure of Oxygen and time. Same with pulmonary oxygen toxicity (aka the Oxygen Clock).

It's not pressure per-se, it's the partial pressure of oxygen at a particular depth which we all see as exceeding the MOD (Maximum Operating Depth) for a specific oxygen percentage. This is a simple formula:

MOD (as atmospheres) = target PPO2 / percentage of oxygen in the mix​

Example: air is 21%. Bottom gas max partial pressure is 1.4
Therefore MOD = 1.4 / 0.21 = 6.66 atmospheres
Convert to metres (subtract 1 from ata) 6.66ata = 56.6 metres (which is about 186.78 feet)

Same for 32% gives a MOD of 33.7 metres.


Oxygen Toxicity - DIVER magazine



If doing shallow decompression dives or short ones (e.g. under an hour of deco) doesn't really register against the CNS and Pulmonary toxicity unless you're using pure oxygen for your deco mix and hanging around at 6m/20ft so running it at a PPO2 of 1.6. One of the reasons most people start with accelerated decompression using well under 100% -- such as 50% or 80%. 80% is just as quick to get out and you don't run up the oxygen clock.

OT: When running much longer decompression times rebreather divers tend to use lower max PPO2 settings (known as a setpoint). For very long deco times the CNS is often pushed beyond the limits, but never deep. Decompressing is generally a very relaxed time busy doing nothing which probably mitigates a high CNS.

Not sure if you were generally expanding on my post or not. Partial Pressure of Oxygen (aka PPO2) is a more limited phrase that assumes diving a mixture of gases. A more encompassing term is "pressure of oxygen" which I chose specifically because it works even when the gas being breathed is pure oxygen, and is independent of depth (or altitude) or gas mixture. While formulas can be used to derive the PO2 from other knowns, for the purposes of computing %CNS all that is needed is a PO2 and the length of time, together with the NOAA exposure tables. I can't see that MOD comes in to play at all for calculation of %CNS.

 

[Wibble]

I can't see that MOD comes in to play at all for calculation of %CNS.

Just the PPO2 does. If you're deeper than the MOD for a gas the CNS goes up very quickly.

(The first time I had a CNS warning on a Shearwater was when the dive was moved to a deeper site and despite dumping a load of gas and air-topping on the boat, the mix was still too hot for that wreck. Hung around on the deck talking to the blennies and other critters -- off my face with narcosis too! Of course my deco gas that day was 100% oxygen rather than my preferred 80%, so even that pushed up the CNS. I lived, although my chest felt as if I'd been smoking a pack of ciggies.)

 

[theminidiver]

Thank you all for the information
I was sure if my breathing rate goes up, my %CNS also goes up because "more O2 getting in"

 

[inquis]

On average, your lungs are about half full. Fast or slow matters not.

 

[theminidiver]

On average, your lungs are about half full. Fast or slow matters not.

Don't we "absorb" more O2 when breathing faster and deeper (deeper like deep breath, not deeper in water) like when we are running?