Just bought a pony system, need help with answers...

[cornfed]

Karl_in_Calif once bubbled...

It would be a bad idea ... if that was your sole bail out gas

I thought that's what we were talking about... my mistake...

 

[MikeS]

DA Aquamaster once bubbled...
Mike,
I agree with every thing you except maybe this part....
In my opinon if you still have to consider if a saftey stop is warranted, you need to make the stop and stay unitl the stop is done even if the SPG is bouncing on the peg.

The ascent rate from 15 feet to the surface is just as important in not more so than the safety stop itself. I believe that chances of DCS are less with a continuous slow ascent rate to the surface than with an aborted safety stop and an OOA controlled swimming ascent. Not to mention that a bolt for the surface is the more likely outcome if you actually run out of air.

It’s not a black and white question; three minute safety stop or not. My point is that by monitoring your air pressure you can make an informed decision as to the length of the safety stop while insuring that you start up from the safety stop with sufficient air to ascend at a 30 ft/min rate.

Most SPG's are not horribly accurate at the low end of the scale and tend to be a little conservative.

If your SPG is not accurate to 25 psi you should get a better one! Additionally, the smaller the tank the more accurate the SPG becomes in terms of volume.
For example, to get from 15 feet to the surface at 30 ft/min with a SAC of 1.0 you need 0.5 cu. ft. That’s approximately 20 psi in a AL80 but 50 psi in an AL30.

So with a 30 cu/ft pony I plan on starting for the surface with no less than 100 psi (50 psi for ascent, 25 psi for possible gauge error, and a 25 psi margin.

In any case it is a really good idea to get away from the math and best guestimate exercise and actually try some normal ascents with your pony to see what it can do really do. It builds confidence and also builds the skills you need to actually do it when things go south on you.
While practice is import the math is just as important. It’s impractical if not impossible to try every scenario. By using math to build a model you can examine other scenarios based on you actual experience. For example, the numbers from an ascent from 80 feet should allow a good estimate from 100 feet.

Karl,

I am unable to follow your logic. On a discussion based on using a bail out bottle you’ve covered going to 185 feet on air, 100 ft/min ascent rates, and are now talking about using a pony for staged deco. IMO you should use a bit more caution keeping in mind that new divers may be reading this. Someone might get hurt by following your advice.

Mike

 

[Doc Intrepid]

1. if you're still following this thread, don't take any nitrox mix past its Maximum Operating Depth (MOD). If you should need to use the pony as intended: e.g. an emergency bail-out bottle, it won't do you any good if you are deeper than the MOD of the EAN in the pony. Therefore, the gas you put in your pony should be at <1.6 (I'd make that 1.4) at the deepest depth you plan on attaining during the dive. Or just use air.

2. if we're discussing anything else besides your setting up a pony bottle for NDL recreational diving following an OOA experience you had during a wall dive to 112' fsw, please let me know.

3. it would be most unwise to take a single tank (with or without a pony) below 130' fsw. Remember, the pony does NOT figure into your gas planning.

4. the entire point of carrying a redundant air source such as a pony is to avoid having to make a rapid, controlled swimming ascent directly to the surface in the remote event that equipment failure (not pilot error) results in an OOA. Therefore, plan for a safe 30 fpm ascent rate making all safety stops. Thats why you carry the pony to begin with. Do not ascend at rates of 60 fpm or 100 fpm directly to the surface.

5. DA Aquamaster and Mike are both right. The math is important to establish what your needs (theoretically) should be. The practice is important to establish what your performance capability (practically) is at this time. (It can change with practice.)

6. Ideally you should be able to get at and manipulate the pony. Some divers attach it to their single tank. I suggest a more useful alternative is to sling the pony, so you can get at it or - in an extreme situation - hand it off to another diver.

Hope this thread has addressed most of your initial questions.

Doc

 

[rakjar]

I am reading each post and I believe I am getting some good information. However, there are some questions I am now having which did not concern me as much before.

1. Again, how do I caculate my SAC rate. Do I need to get a cylinder and breath off it at the surface to see how long it takes to lose a certain psi? I am not sure, could use some help.

2. The abbreviations being used. I know some of them, not all. Can someone come up with a list so I know what you are taking about.

3. When using rental cylinders, do I try to rent the same size and type each time? This goes into knowing what the cf of that particular cylinder is. If I change the place I rent cylinders, how do I find what the cf of that cylinder is from that shop. The reason I ask is becauce I have rented from the shop I was certified through and they rent me the steel 95 (fat bastard) I have heard them called. I have also rented from another shop, steel 95, which was thinner but a little longer.

By the way, after the fill of the, fat bastard, I had a starting psi of around 2500 to 2600 and with the thinner 95's I had a starting psi of almost 3100. I believe for the money, starting with the 3100 is a better deal but can someone explain why that is?

Is it just the shop filling the cylinder or can the thinner/longer cylinder hold more then the fatter/shorter 95's?

 

[Uncle Pug]

we are having a discussion about this very thing here.

Click on the link and join us.

 

[cornfed]

Thanks "Ron"