Mini pressure guages ?

[Wibble]

I don't consider myself a mathematical genius, but I am fairly certain your calculations are incorrect.

Oops; swap them aound typo. Editing now...

 

[stuartv]

Need to convert to bar: 3000psi = 206bar, 2700psi = 186bar.

Obviously it depends upon circumstances, but in essence yes. It's probably more relevant for a cave penetration rather than a wreck penetration (wrecks aren't as long as caves!) where you've ended up using more than your third of gas and are now eating into your reserves.

For a wreck penetration, you'd have worked out your max TTS (time to surface) and your minimum gas. If your "disaster" (post leak) happens when you're in deep and you realise that you're now getting close to your TTS / min gas, getting out is definitely time critical. That 20bar / 300psi could make all the difference. If it really is a long or complex penetration, you can simply leave the line there and collect it another day; same with a cave for that matter -- much better to have a story on the boat / in the pub than to scare yourself witless with worry because you're now short of gas.

(The example numbers you used would have been better if they were lower, e.g. at the turn point, so call it 1700psi / 117bar and 2000psi / 137bar - here the loss of 300psi / 20 bar is more significant)


Edit: typo 2700 & 3000 swapped

I specifically said deco gas. Nobody is talking about using only a mini SPG on your back gas.

So, 2700 in your deco gas, but mini SPG says 3000. What would you do differently if your gauge actually said 2700?

 

[johndiver999]

This is soo crazy. People take positions, or more correctly form opinions, based on questionable logic and then they have to go out of their way to invent situations or circumstances, regardless of how improbable, to defend their opinion.

The old button gauges were tiny and very difficult to read. I think a lot of people formed their opinion on "mini-gages" from them. However, the newer ones (which I assume were invented for the paintball industry) are cheap, legible and rugged.

 

[Wibble]

I specifically said deco gas. Nobody is talking about using only a mini SPG on your back gas.

So, 2700 in your deco gas, but mini SPG says 3000. What would you do differently if your gauge actually said 2700?

OK. Deco gas: you've lost some of it and your plan calls for a specific amount. Obviously up to you, but loosing some of that gas may mean you need to take some form of mitigation action which might well be get out now without stopping to collect your line. As was said earlier, 300psi/20bar is not a lot and unlikely to be critical. As you're doing your deco ascent, it's a lot easier to read a full size gauge; if for whatever reason you're getting short on that gas, you can take mitigating action such as "trimming" your deep stops to get up to the gas switch. Thus accurately knowing your gas pressure could well be important; and it's far easier to read a full size gauge.

Yes, it's a bit manufactured as you'd be helped by your team (unless diving alone) and you'd have factored in plenty of additional gas reserves for deco. However, if diving deep on a rebreather, you may well be pushing your bailout reserves and be closer to limits, so needing to know how much longer you've got to empty - especially if you've lost the shot line and everyone else and you're on your own.

 

[johndiver999]

When you are on deco and it gets hard to breathe, it is about time to come up, you don't need a fancy gauge to tell you that. LOL

 

[Wibble]

When you are on deco and it gets hard to breathe, it is about time to come up, you don't need a fancy gauge to tell you that. LOL

Skip a stop completely? Or 'trim' the previous stops because you knew the gas remaining?

 

[stuartv]

Skip a stop completely? Or 'trim' the previous stops because you knew the gas remaining?

I think you're kind of missing the point.

We're talking about using a mini SPG on a deco or BO cylinder. They MAY not be as accurate or precise as a bigger SPG. So stipulated. But, the ones I use are big enough and accurate enough (based on comparing with a real cylinder pressure checker) that I feel good about believing I will be able to read it and tell within 300 psi what my correct pressure is.

With that as my background, the point is this: I plan my dives conservatively. 300 psi should NEVER be the difference between completing my required deco or not. If it does, then, one, things REALLY went pear-shaped. More than one bad thing happened, at least. And, two, if I'm 300 psi short, then worst case is I cut my deco short by 300 psi worth of. And (referring back to point 1), that would mean I'm also out of ALL my other gases (except possibly any hypoxic mix I was carrying) AND have lost all of my team members (so I can't get gas from them). If I'm not out of all other breathable gases or I have a teammate with me, then I can get 300 psi of gas (or more, if it's a leaner mix) to finish my final few minutes of deco.

So, the reduced accuracy and precision of a mini SPG on a deco or BO cylinder doesn't seem like a significant real-world problem.

 

[agilis]

I've been diving for a long, long time, so perhaps I've lost touch with what people are doing these days, but the idea of using a small safety cylinder, what we called a bail-out bottle, for decompression fills me with horror. The gas needed for decompression should, I strongly believe, be factored into the dive plan from the primary cylinder(s). These little 15 or 20cf bottles are for emergencies, redundant sources of air. I've always thought that the rule of thirds was more than just a good idea: it was obligatory. Squeezing every minute possible from a dive by nearly depleting the main air source and relying on a small secondary source to complete any required decomp while doing ordinary sport diving is the hallmark of a fool.

 

[stuartv]

I've been diving for a long, long time, so perhaps I've lost touch with what people are doing these days, but the idea of using a small safety cylinder, what we called a bail-out bottle, for decompression fills me with horror. The gas needed for decompression should, I strongly believe, be factored into the dive plan from the primary cylinder(s). These little 15 or 20cf bottles are for emergencies, redundant sources of air. I've always thought that the rule of thirds was more than just a good idea: it was obligatory. Squeezing every minute possible from a dive by nearly depleting the main air source and relying on a small secondary source to complete any required decomp while doing ordinary sport diving is the hallmark of a fool.

Fortunately, I don’t think anybody in this thread has said anything like that...

 

[Mike1967]

Fortunately, I don’t think anybody in this thread has said anything like that...

Yeah I was wondering what he was on about too.