Bar vs PSI

I recently went on a dive in Thailand and all the DM's used SPG's in BAR. My gauges are in PSI so I had to convert when asked by the DM "air remaining". At first it did cause a bit of confusion but after a couple of dives I got used to converting in my head. The question I have is the DM's gauge had the red line starting at 50 bar or by my calculations about 725 psi. My gauge has the red line starting at 500psi or about 35 bar. Why the differance? What do most DM's use for a minimun air point?

Helipilot:

I recently went on a dive in Thailand and all the DM's used SPG's in BAR. My gauges are in PSI so I had to convert when asked by the DM "air remaining". At first it did cause a bit of confusion but after a couple of dives I got used to converting in my head. The question I have is the DM's gauge had the red line starting at 50 bar or by my calculations about 725 psi. My gauge has the red line starting at 500psi or about 35 bar. Why the differance? What do most DM's use for a minimun air point?

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In my opinion, we should use both 500PSI and 50Bar. This is done so that no matter what guage you are using, you will not have to do a mental conversion before knowing how much air is left in your tank. Calculation is harder to do when "under pressure" and a miscal can cause a shortened dive.

As a rule of thumb. follow the redline.

As for minimum air point. it will depends on the individual DM. For me, the minimum air point will be based on the depth, current and experience level of the divers. For example,for inexperienced diver, I will be more conservative and give them a higher minimum airpoint like 700PSI if we are going like 80ft depth and a current is going on. For experienced diver, i will follow the redline, either 50Bar or 500 PSI.

crabball:

As a rule of thumb. follow the redline.

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Holy crap, I can't believe I'm reading this. Have you ever heard of gas planning? Even for recreational! Follow the red line where? When you're at 30m or at 3m? Diving an LP72 or a HP120? Solo or with a buddy?

The 50bar/500psi are arbitrary figures designed to get new divers out of the water with some air in reserve. They're not hard and fast figures you MUST follow. As figured out by the first poster, they're not even the same amount of air even if used on the same sized tank. On some dives they're excessive, on others woefully inadequate. Do a search on the term "rock bottom" calculations and learn about planning you air usage PROPERLY.

Helipilot:

I recently went on a dive in Thailand and all the DM's used SPG's in BAR. My gauges are in PSI so I had to convert when asked by the DM "air remaining". At first it did cause a bit of confusion but after a couple of dives I got used to converting in my head. The question I have is the DM's gauge had the red line starting at 50 bar or by my calculations about 725 psi. My gauge has the red line starting at 500psi or about 35 bar. Why the differance? What do most DM's use for a minimun air point?

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I think the 500 psi minimum back on board is set so that you have some reserve left in case of an emergency on your way up. What you have to consider is, if it is an adequate amount of air to assist you and your buddy if a problem arises. This would depend on the circumstances of a dive, if it is a relatively shallow no decompression dive, then 500 psi should be adequate, if it is a deco or deep dive use the rule of thirds. If your finishing your 15' safety stop after a deep dive and you still have 600 psi of air remaining, heck, hang around for for another 300 psi of breaths to be that much more consevative.
Remember, it is always better to have too much than not enough!!

Tobagoman:

I think the 500 psi minimum back on board is set so that you have some reserve left in case of an emergency on your way up. What you have to consider is, if it is an adequate amount of air to assist you and your buddy if a problem arises. This would depend on the circumstances of a dive, if it is a relatively shallow no decompression dive, then 500 psi should be adequate, if it is a deco or deep dive use the rule of thirds. If your finishing your 15' safety stop after a deep dive and you still have 600 psi of air remaining, heck, hang around for for another 300 psi of breaths to be that much more consevative.
Remember, it is always better to have too much than not enough!!

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Like Roo said, it depends on may things, is 500 what you really want, what if you're in double 120's?

Thanks everyone for the great info.
I guess I should have been a bit clearer with my question. I was wondering why the gauges are marked differently. Is it a function of Bar vs PSI? Or is it just the choice of the gauge manufacturer? I agree that during a dive is not the place to be doing conversions in your head. I have since used a label maker and added some key BAR amounts on the glass.

Helipilot:

Thanks everyone for the great info.
I guess I should have been a bit clearer with my question. I was wondering why the gauges are marked differently. Is it a function of Bar vs PSI? Or is it just the choice of the gauge manufacturer? I agree that during a dive is not the place to be doing conversions in your head. I have since used a label maker and added some key BAR amounts on the glass.

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I don't understand why an analog guage doesn't have both markings, such as mph/kph markings on speedometers.

For me this is an issue, although I live in the US , I do most of my diving in places using bar/meters, however I plan on diving in the US as well.

*sigh*

MoonWrasse:

I don't understand why an analog guage doesn't have both markings, such as mph/kph markings on speedometers.

For me this is an issue, although I live in the US , I do most of my diving in places using bar/meters, however I plan on diving in the US as well.

*sigh*

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Some SPGs (such as OMS G240) do have both BAR and psi markings. I’m sure that many divers (including myself) find that useful but an equal number of divers may feel that having both BAR and psi on the same gauge is confusing...

Helipilot:

Thanks everyone for the great info.
I guess I should have been a bit clearer with my question. I was wondering why the gauges are marked differently. Is it a function of Bar vs PSI? Or is it just the choice of the gauge manufacturer? I agree that during a dive is not the place to be doing conversions in your head. I have since used a label maker and added some key BAR amounts on the glass.

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Hi!

As an American diving at the Red Sea, I've had very similar experiences. See what I wrote here.

I think the fact that Imperial gauges are usually red-lined at 500 psi, while metric are at 50 BAR, is purely convenience... the former usually have their major markings every 500 psi, while metric gauges have them every 50 BAR.

There's nothing magic about either figure. Divemasters that have groups consisting of divers of various experience levels need to address the common denominator, and I guess just pick the red-line figure most divers have on their SPGs as the "amount of air I want to see in your tank when you're back on the boat."

You've already figured out that a rough conversion is psi divided by 15 = BAR... or easier, 2/3 of psi and drop a zero. But I agree with crabball; even those calculations are hard to do underwater, when under even a little bit of pressure (like the DM wanting to know how much air you've got left ).

I think it might be nice to have a gauge that's marked in both units; but I tend to agree with vie, that it might also be confusing.

The best goal, of course, is to be allowed to take total responsibility for your own air management, without having to report to the DM. That's often not possible, of course, especially when you're required to be with a DM or guide for whatever reason.

In that case, most DMs around the world should be familiar enough with Imperial gauges that they'd be flexible and accommodate you.

If they insist on being signalled pressures in BAR, I think your label-maker solution might be a good one. Interpolate as best you can in order to make the DM happy by showing him/her however many fingers when they ask; but still make your own decisions.

Another option, if you're close enough, might be to just show them your SPG so they can judge how far away from your redline you are. But again, be responsible for your own air.

Sooner or later, as you gain experience and/or the DM gains confidence in you, you'll find that they do start to trust your judgement.

--Marek

Scubaroo:

Holy crap, I can't believe I'm reading this. Have you ever heard of gas planning? Even for recreational! Follow the red line where? When you're at 30m or at 3m? Diving an LP72 or a HP120? Solo or with a buddy?

The 50bar/500psi are arbitrary figures designed to get new divers out of the water with some air in reserve. They're not hard and fast figures you MUST follow. As figured out by the first poster, they're not even the same amount of air even if used on the same sized tank. On some dives they're excessive, on others woefully inadequate. Do a search on the term "rock bottom" calculations and learn about planning you air usage PROPERLY.

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Hi Scubaroo,

I think you have either mistaken my reply or my language skill is not good enough to convey what i am trying to say. Anyway, the question is what does DM normally use as a min guide.

You are absolutely right about the dive/gas planning portion. The amount of air left in your tank before you decide whether to ascend or not should correspond with your depth and your SCR, and this is a variable component. I mean everyone has a different SCR and thus theoretically requires a different amount of air left in the tank to surface safely. Thus, there is no way to judge where is the minimum point. The DMs will normally estimate based on the divers consumption so far and the depth they are at at the moment to decide what is the min. point.

That said, I think all cert. agencies also says that the bottom line is to be on the boat with 500PSI or 50 Bar as a minimum in case of any unforeseen circumstances e.g boat no longer there when you surface or you have to do a long surface swim to the boat through a choppy sea.

Thus, the divers or the DMs will try to surface before the needle hits the redline.That's why i say follow the redline.

Maybe i should have phrase it another way. sorry for the miscomm.