mark f l davison
08-05-2004, 11:47
VR3, CNS and PPO2
Although I have set this a separate post it follows on from my post ?VR3 Manual Errors - High PPO2 Alarm and CNS Calculations?
The VR3 will not show a high PP02 alarm until you reach 1.8 PPO2, this has been acknowledged by Delta P and it appears they have no plans to change this. Contrary to what you might expect setting the MOD for any gas has no effect on when you get the high PPO2 alarm, the manual gives the impression that it does on page 32 where it states ?Exceeding the MOD will trigger a PO2 alarm? . Page 43 states under PO2 Alarm. ?This alarm occurs when an open circuit gas or Diluent has exceeded 1.6 PO2?.
I guess quite a few of you are asking so what. Most Nitrox trained divers are taught at some length about CNS and OTUs/UPTDs but did a few calculations for the exam and promptly forgot them. This is common because most diving agencies stick with low maximum PPO2, typically 1.4. At 1.4 the CNS clock runs at 0.65 per minute, which has the effect that it is almost impossible to get a CNS loading of 80% or more. You would need to be at this limit for 123 minutes to get to 80%. So it is hardly surprising that most divers don?t consider CNS and OTUs when diving, but do consider MOD, which of course is PPO2 dependant. However what you may not have picked up on your Nitrox course is that the CNS clock is based on an expediential graph and this results in the following CNS loadings:
PPO2 1.4 = 0.65% CNS per minute
PPO2 1.5 = 0.83% CNS per minute
PPO2 1.6 = 2.22% CNS per minute
PPO2 1.7 = 10.0% CNS per minute
PPO2 1.8 = 50.0% CNS per minute
PPO2 1.82 = 100.0% CNS per minute
So when the VR3 alarm kicks in you are getting 50% CNS per minute, so in 2 minutes you have collected 100% CNS. The CNS limited is adjustable on the VR3 from 80% to 200% (BSAC / IANTD is 80%) should you exceed your CNS limit the VR3 advises you to take an air break, even though you have no air switched on. So in practise the first alarm you might see is air break alarm.
Whilst I cannot do any thing but agree with Delta P?s statement ?Our view is that it is the diver's responsibility to analyse the gas in his cylinders and keep to his desired MOD?. I would add that it is a diving computer manufacturer?s responsibility to print an accurate manual.
My view is that a good nitrox computer will give warnings about high levels of PPO2 and when a diver is about to exceed his/her given MOD for any given gas. In most circumstances we know the bottom depth and select the gas accordingly, but there are situations where the bottom depth is not finite drift dives and wall diving. Additionally there are circumstances when you become preoccupied, photography or in an emergency situation are examples when a timely computer warning of excessive PPO2 can be highly beneficial.
I would suggest that Delta P looks at adopting a the TOD (Target Operating Depth) and MOD concept. This would allow the user to select a TOD, the point at which the user is advised to make a switch and the MOD is when the user receives a high PPO2 alarm. The MOD and TOD could be relational and expressed as PPO2, so if you wanted to set a TOD of 1.30 and MOD of 1.4, then the user could select a MOD ? TOD value of 0.1. This could be set globally by default and exceptionally individually. The advantage of a global option is that every time you change a gas the MOD and TOD would be set automatically to the users set PPO2.
Finally I would like to offer some advice to VR3 users. If you are diving where it might be possible to exceed your MOD, wall diving, shark watching etc take a watch with a depth alarm and set it to your MOD. If you don?t have a watch with this function then you can of course activate AIR on you VR3 so if you exceed the MOD for your Nitrox mix you?ll get an alarm to advise you to switch to air. You then take this as your cue that you have exceeded your MOD. Remember switch alarm will activate when you reach the MOD you set in the gas set up, not 1.8 PPO2.
BSAC and most diver training agencies advise a maximum PPO2 of 1.4, although I would not be surprised if this post does not initiate some comments on what are safe PPO2s.
For all my comments on the VR3, I use it all the time I think it is a highly innovative product that technical divers appreciate, I know I do.
Although I have set this a separate post it follows on from my post ?VR3 Manual Errors - High PPO2 Alarm and CNS Calculations?
The VR3 will not show a high PP02 alarm until you reach 1.8 PPO2, this has been acknowledged by Delta P and it appears they have no plans to change this. Contrary to what you might expect setting the MOD for any gas has no effect on when you get the high PPO2 alarm, the manual gives the impression that it does on page 32 where it states ?Exceeding the MOD will trigger a PO2 alarm? . Page 43 states under PO2 Alarm. ?This alarm occurs when an open circuit gas or Diluent has exceeded 1.6 PO2?.
I guess quite a few of you are asking so what. Most Nitrox trained divers are taught at some length about CNS and OTUs/UPTDs but did a few calculations for the exam and promptly forgot them. This is common because most diving agencies stick with low maximum PPO2, typically 1.4. At 1.4 the CNS clock runs at 0.65 per minute, which has the effect that it is almost impossible to get a CNS loading of 80% or more. You would need to be at this limit for 123 minutes to get to 80%. So it is hardly surprising that most divers don?t consider CNS and OTUs when diving, but do consider MOD, which of course is PPO2 dependant. However what you may not have picked up on your Nitrox course is that the CNS clock is based on an expediential graph and this results in the following CNS loadings:
PPO2 1.4 = 0.65% CNS per minute
PPO2 1.5 = 0.83% CNS per minute
PPO2 1.6 = 2.22% CNS per minute
PPO2 1.7 = 10.0% CNS per minute
PPO2 1.8 = 50.0% CNS per minute
PPO2 1.82 = 100.0% CNS per minute
So when the VR3 alarm kicks in you are getting 50% CNS per minute, so in 2 minutes you have collected 100% CNS. The CNS limited is adjustable on the VR3 from 80% to 200% (BSAC / IANTD is 80%) should you exceed your CNS limit the VR3 advises you to take an air break, even though you have no air switched on. So in practise the first alarm you might see is air break alarm.
Whilst I cannot do any thing but agree with Delta P?s statement ?Our view is that it is the diver's responsibility to analyse the gas in his cylinders and keep to his desired MOD?. I would add that it is a diving computer manufacturer?s responsibility to print an accurate manual.
My view is that a good nitrox computer will give warnings about high levels of PPO2 and when a diver is about to exceed his/her given MOD for any given gas. In most circumstances we know the bottom depth and select the gas accordingly, but there are situations where the bottom depth is not finite drift dives and wall diving. Additionally there are circumstances when you become preoccupied, photography or in an emergency situation are examples when a timely computer warning of excessive PPO2 can be highly beneficial.
I would suggest that Delta P looks at adopting a the TOD (Target Operating Depth) and MOD concept. This would allow the user to select a TOD, the point at which the user is advised to make a switch and the MOD is when the user receives a high PPO2 alarm. The MOD and TOD could be relational and expressed as PPO2, so if you wanted to set a TOD of 1.30 and MOD of 1.4, then the user could select a MOD ? TOD value of 0.1. This could be set globally by default and exceptionally individually. The advantage of a global option is that every time you change a gas the MOD and TOD would be set automatically to the users set PPO2.
Finally I would like to offer some advice to VR3 users. If you are diving where it might be possible to exceed your MOD, wall diving, shark watching etc take a watch with a depth alarm and set it to your MOD. If you don?t have a watch with this function then you can of course activate AIR on you VR3 so if you exceed the MOD for your Nitrox mix you?ll get an alarm to advise you to switch to air. You then take this as your cue that you have exceeded your MOD. Remember switch alarm will activate when you reach the MOD you set in the gas set up, not 1.8 PPO2.
BSAC and most diver training agencies advise a maximum PPO2 of 1.4, although I would not be surprised if this post does not initiate some comments on what are safe PPO2s.
For all my comments on the VR3, I use it all the time I think it is a highly innovative product that technical divers appreciate, I know I do.