Two VR3 or not Two VR3 that is the question

[Pierre Farrugia]

Originally Posted by AnneMarie

Hello Pierre

On the Inspiration, people dive on .70 i.e. low setpoint but have the VR3 set at 1.3. The VR3 therefore thinks the diver is breathing a higher PO2 than he in fact is. The diver changes up to high setpoint on the yellow peril on the bottom, then it matches the VR3, however the diver has taken on more nitrogen than the VR3 is aware of.

Clear as mud eh?

One day when I get more time I'll elaborate Pierre. Bye for now

Thanks Annmarie, everything is clear but first are there divers on here that do this? And if yes why just to avoid a switch on Vr3?

Regards and thanks

Pierre

[PrettyFishies]

Loads of people have emailed me about this so I'll post this instead of replying to all the mails.

I've had my VR3 since I first got my unit and have always dived with the fourth cell plumbed in - this is mainly due to the spurious electronics in the inspiration - the fourth cell is a useful redundancy to loss of electronics.

With regards to deco, the derivative of the algorithm is such that it will allow large ascents through relatively big pressure gradients - the so called "bend and mend" method, it makes up for this by throwing in a lot of shallow deco.

The deep stops are pretty critical - ignore these and the thing will go into a sulk - HOWEVER although it says use tables if you miss a deep stop, it will actually attempt best guess deco. For the deeper stuff I pad out the stops, this is generally done for the shallower deco after the deep 2 minutes stops - these deep stops generally get you offgassing from fast tissues whilst ongassing to slow tissues so I don't hang around down there at that point as this can result in joint bends on really deep stuff. In the shallow range, the VR3 will actually "reward" you for example, at 15 metres, if you stay there long enough it will clear the shallow deco completely and allow you to go to the surface. Clearly no one in their right mind would do this after long deep stuff and I throw in ad hoc stops in this range. I do not recommend the VR3 for sub 100m diving unless there is a high level of conservatism built in (you can do this in the settings) also for extra conservatism you can dive with it set to a lower PO2 than you are breathing on the RB - this only works when the fourth cell is not in use as you configure a constant set point then (unless you're jacking it up manually on the RB).

Everyone's physiology is different however I am generally very anal about deco due to my medical history - this was the driver for buying a second VR3 as I cannot afford to make even the smallest fuck up on my decompressions. Carrying bail out tables is a pain in the ass for all the different scenarios you could come up with and I have multiple trimixes with me now, everything plugged inboard so I can change gas input sporadically and have them preset in the VR3. Another note here - there are some proponents of using VR3 to gas switch to air on deco, this is fucking ludicrous given that extra nitrogen is then taken on and has resulted in people getting bent. If the VR3 is to be used for this, it's much better to switch to a weaker trimix for deco and keep some helium in the deco mix...you'll know the score here re counter diffusion etc.

Whilst the fourth cell is in use, the VR3 will calc the deco from this. Beware of descents on a low PO2 with the VR3 set to a higher setpoint, this has been a common error. The technical architecture of the VR3 looks complex in the manual however the unit is intuitive and easy to configure with multiple configurations which can be saved therefore it is easy to program it for the most common diving scenarios that you use.

I would be very very wary about using the VR3 as the primary source for deco calcs for shallow deco gas switching, there are more sophisticated ways of doing this - also for deep stuff on VR3 it's definitely wise to take a conservative standpoint.

CAVEAT - unless you're absolutely convinced that your own personal physiology is top notch (and no can really be) then diving with zero conservatism on the VR3 for mid range to deep stuff could be risky.

Apologies if I'm reiterating the same points someone else has made, haven't read the whole thread, in fact, one more point, the VR3 fourth cell has a tendency to drift down during dives on the Inspiration specifically - this is due in part to the positioning of the fourth cell and the subsequent moisture on the cell face and temperature differentials. One to be aware of.

Cheers

AnneMarie