Preferred mix in a pony

[Gil57usa]

Plain old air. I only use nitrox in stage bottles and back gas.

 

[rongoodman]

It's handy having at least one tank with air in it to calibrate my O2 analyzer.

 

[Dan Watkins]

I have a somewhat related question, I have an Oceanic VT3 dive computer that can switch between 3 gasses. Though it does not allow me to designate a lower FO2 than what GAS 1 was. So if I had 32% O2 in my primary tank, it won't allow me to designate 21% 02 in my pony as GAS 2, it would have to be 32% or more.

Anyone know what the reasoning behind this would be?

 

[SteveAD]

I have a somewhat related question, I have an Oceanic VT3 dive computer that can switch between 3 gasses. Though it does not allow me to designate a lower FO2 than what GAS 1 was. So if I had 32% O2 in my primary tank, it won't allow me to designate 21% 02 in my pony as GAS 2, it would have to be 32% or more.

Anyone know what the reasoning behind this would be?

could just be that gas 1, gas 2, and gas 3 are supposed to be set from leanest to richest. check your manual can 2 be richer than 3?

 

[Dan Watkins]

GAS 1 can be set from 21% to 50% FO2
GAS 2 and 3 can be set from 21% to 100%, but the instructions state:

"The VT3 is programmed to prevent FO2 for GAS 2 and 3 from being set at values lower than the FO2 Set Point for GAS 1. GAS 2 and 3 can only be set to values equal to or higher than the FO2 Set Points for GAS 1 and 2, respectively."

So if I set my GAS 1 to 32%, GAS 2 and 3 cannot be set lower than 32%. If I set GAS 2 to say 36%, GAS 3 cannot be lower than 36%.

I just wondered what the reasoning behind this restriction is... Is there generally some concern to switching to a gas with a lower FO2 than the primary tank?

I imagine I could designate GAS 2 as my primary tank, but GAS 1 is set to be the primary default, and I cannot actually switch to GAS 2 until I am under water.

 

[ianr33]

I just love it when manufacturers try to tell me how to dive

Sure,it would be normal to switch to higher,rather than lower ppO2's during deco,but having a computer that wont let you do it the other way ? Thats NUTS.

 

[Dan Watkins]

I will ask this question in the Oceanic forum to see what they say...

 

[in_cavediver]

First, my opinion on the pony for Rec dives is max FO2 of 32%. Thats 1.6PO2 at 130. Anything lower is fine. I wouldn't sweat these details. The use of the pony is to get you ascending to a safety stop, not to extend the dive.

Now, use the pony like a stage - then it ought to be the same as backgas or a suitable bottom gas. Depends on how much work you want to do deco status wise. I prefer simpler in most cases which means same mix as backgas. (and my 'pony' when used like this is either a 63 or 80)

As for computers - VT3. The gas restrictions are the stupidist things I have heard. I own a VR3 and I like the ability to change my gases whenever to whatever I want, even under water or to horrific PO2's. I expect it to keep giving me its model information for whatever gas I select, even if I violate it's best profile. (Its a complicated procedure to update gases so its unlikely to be accidently done).

 

[Dirkadiver]

I always dive my backgas mix in my bailout. To me, it just makes sense. Granted you're only using it to get to the surface, but think about the what if's. I always consider very worst case scenario when planning dives. Having the same gas could eliminate confusion in an otherwise stressful situation.

 

[The Chairman]

Air and only air in a pony. The problem with MOST divers who use a pony is that they end up factoring this in their bottom time calculations. These are the people who seem to like having a NitrOx mix in their pony. If it is only for an emergency, then air is always your safest bet.