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Scuba Forum / General / January 2007The 50 bar fallacy
El Stroko Guapo wote: > For recreational diving at the OW level, > "start your ascent at 50 bar" is all the gas planning that is *essential*, > along with a knowledge that deeper = proportionately more air consumption Respectfully, I disagree. A 50 bar reserve in an AL80 or a 10 liter cylinder is adequite for sport diving, as it relates to about 500 liters of gas. However, if you teach someone "start your ascent at 50 bar" and they are diving with a 6 liter or a 7 liter cylinder they are going to come horribly short. If you are diving with a 6 liter cylinder and you are at 50 bar you only have 300 liters of gas left! Conversely, if you are diving with a 15 liter cylinder, a 500 liter reserve equates to 33.3 bar, and there's no need to panic when you hit 50 bar, and even at 40 bar you can start to think about your ascent. I think people are cleverer than what we (as instructors) give them credit for - giving students a simple rule to remember which doesn't hold true in all instances is also performing a disservice! Having said all of this, on training dives I prefer that my students surface with 50 bar in a 10 liter or 12 liter cylinder, and generally we start our ascent around 70 or 80 bar. Our cylinders are mostly 232 bar, so we still get a good dive and we run out of bottom time or get too cold to do longer dives anyway ... rgds monty > El Stroko Guapo wote: > > For recreational diving at the OW level, [quoted text clipped - 24 lines] > bar, so we still get a good dive and we run out of bottom time or get > too cold to do longer dives anyway ... "Start your ascent at 50 bar" is one of those easy-to-remember general rules that provides a good margin of safety for OW diving, with very few exceptions, I think. You're right that it will provide different amounts of gas depending on the cylinder size, but it's so generous that it will almost always provide a good safety margin in OW diving. Doesn't your point about small tank size pertain more to the issue of avoiding a serious mismatch of tank size to the individual's air consumption rate? Also, it's easy enough to acknowledge the students' intellect and ability just by explaining the exceptions while stressing the value of simple, general rules until experience is gained. I think most students would accept that approach pretty readily, having been given credit for being able to think for themselves, unless, of course, the instructor is particularly condescending, bullying or dogmatic. Just my humble opinion, of course. Kindest regards. Dave C >>El Stroko Guapo wote: >> [quoted text clipped - 50 lines] > > Dave C The attributions got a little messed up there, I don't speak bars and litres. But on this side of the pond we tell students blahblah at 500 psi, more than enough gas unless yer on the Titanic with a SpareAir. 50 bar is half again as much gas! esg > The attributions got a little messed up there, I don't speak bars and > litres. 50 bar = 725.1887 psi 1 Liter = 0.03531467 cu ft However, I don't understand how you can get a 500 liter reserve from a 15 liter cylinder... > But on this side of the pond we tell students blahblah at 500 psi, more > than enough gas unless yer on the Titanic with a SpareAir. 50 bar is > half again as much gas! With dubs it's the rule of thirds, with single tanks, no stop recreational diving, you are supposed to surface with a 500 psi reserve, but depends upon whom you ask. One of my buds just got back from a mediocre day of diving off Monterey where the captain checked everyone's SPG when they got out, and admonished them with the threat of not letting them do a second dive if they didn't hit his steps with at least 500 psi showing on the SPG. I would have had to get out, and remove my reg to swap tanks without turning the gas off first, just so he could hear it go "feh". Needless to say, he wont be going back out on that boat (name purposely, instantly forgotten). > However, I don't understand how you can get a 500 liter reserve from a 15 > liter cylinder... 15 liters is the volume of the inside of the tank, not its capacity at its rated pressure (like we do it here). A 15 liter tank with 100 bar of gas in it has about 1500 standard liters of gas. > > However, I don't understand how you can get a 500 liter reserve from a 15 > > liter cylinder... > > 15 liters is the volume of the inside of the tank, not its capacity at > its rated pressure (like we do it here). A 15 liter tank with 100 bar > of gas in it has about 1500 standard liters of gas. I thought this whole metric thing was supposed to make more sense... an 80 cf tank holds 80 cf of gas, but a 15 liter tank can hold 1500 liters of gas? . > I thought this whole metric thing was supposed to make more sense... > > an 80 cf tank holds 80 cf of gas, but a 15 liter tank can hold 1500 liters of gas? More. If it's a 200 bar tank, it holds roughly 3000 liters at 200 bar. It makes gas matching with different size tanks for cave diving really easy compared to our system. Liters times bars approximately equals how much gas you have. If Dick has double 95's jacked to 3300 and Jane has double 80's jacked to 3600, who has more gas? "When you metric SOB's build a metric spaceship, fly it to the moon with 3 guys in it, land, get out and walk around, and then come back to earth in it, I'll think about changing." - Cobber, on the old Techdiver list > More. If it's a 200 bar tank, it holds roughly 3000 liters at 200 bar. Or 105 feet. > It makes gas matching with different size tanks for cave diving really > easy compared to our system. Liters times bars approximately equals how > much gas you have. If Dick has double 95's jacked to 3300 and Jane has > double 80's jacked to 3600, who has more gas? Assuming dick is diving LP 95's, he has more gas hands down. Dick has 261.2 feet of gas, Jane has 184.8 > "When you metric SOB's build a metric spaceship, fly it to the moon > with 3 guys in it, land, get out and walk around, and then come back to > earth in it, I'll think about changing." - Cobber, on the old Techdiver > list Classic. In the machine shop we call it "FM"; f.cking Metric. >>More. If it's a 200 bar tank, it holds roughly 3000 liters at 200 bar. > [quoted text clipped - 19 lines] > > f.cking Metric. Did you read my reply to Jim ( IIRC)? Matthias > Did you read my reply to Jim ( IIRC)? Probably, but I dont recall... Please re-reply. Has to be good. >>Did you read my reply to Jim ( IIRC)? > [quoted text clipped - 3 lines] > > Has to be good. I referred to the doomed Mars mission. (http://www.wired.com/news/technology/0,1282,31631,00.html) Though not intending to make a judgement about which system is better ;-) Matthias > > Please re-reply. > > [quoted text clipped - 5 lines] > Though not intending to make a judgement about which system > is better ;-) Oh, that is rich. I think I do remember that exchange. 60k vs. 150k. It's always the little sh.t that gets you. In my trade, we often get models that are in metric, which when imported into CAM software are 25.4 or 0.03937 out of scale. Accumulation of error, etc. As you know, by the drawings we have shared, I can set it up to dual dimension. Not a machinist I know here has metric mikes or scales. Buying metric cutting tools is usually an order as needed basis. We sit with metric drawings and convert all the dimensions as determined by tolerance. For instance, when we have a surface to cut, we will choose a readily available imperial ball mill, and constrain the toolpath to make multiple passes and create the metric radii. With modern machine tools, and the current travel speeds available, the difference in actual runtime is usually measured in seconds. The best part (play on word) is that we usually end up with a better finish and appearance than if we had driven the contour to the radii with the matching metric tool, due to chatter, etc. Some times you eat the bear, sometimes the bear eats you. > . > > I thought this whole metric thing was supposed to make more sense... [quoted text clipped - 6 lines] > much gas you have. If Dick has double 95's jacked to 3300 and Jane has > double 80's jacked to 3600, who has more gas? well, 10% of 95 is 9.5, so Dick would have 104.5 x2 = 209 cf 20% of 80 is 16, so Jane has 96 x 2 = 192 cf Asuming of course that PV=nrT holds true for the specified pressures and that V and T are close enough to constant that they can be ignored. Not really that hard. I do see that the metric version makes the calculations somewhat easier, but it still doesn't make sense to be able to fit 3000 litres into a 15 liter containter... I think someone may have made a mistake in calculation in there somewhere. :-) > I thought this whole metric thing was supposed to make more sense... > > an 80 cf tank holds 80 cf of gas, but a 15 liter tank can hold 1500 > liters of gas? It makes it easier as long as you stay in the same units of measurement... If someone says that they have an 80 cu-ft tank, you really don't know what the physical size of the tank might be... It might be physically smaller and at a higher pressure or it might be quite a bit larger and at a lower pressure... If it is at a lower pressure, you can choose to increase the pressure and get more gas out of it... If at a higher pressure, you might not have a compressor that can boost it much more... With a tank measured by its liquid volume, it's a simple conversion to do to determine who much gas you put into it by just measuring the pressure in bar that you fill it to... A 15 liter tank at 100 bar will give you 1500 liters of gas... A bar is approximately 14.7 psi, so at 100 bar, the tank is at 1470 psi... Let's look at it from the point of determining how much gas volume is left in a tank after reading your SPG... For the US, let's say that we see on our SPG a measurement of 2300 psi and we know that our tank hold 80 cu-ft at 3000 psi, so the calculation is as follows: RemainingVolume = 80 cu-ft / 3000 psi * 2300 psi = 61.3 cu-ft For the metric system, let's say that we see 135 bar on our spg and we know that we have a 15 liter tank, so the calculation is as follows: RemainingVolume = 15 liters * 135 = 2025 liters One less mathematical operation to perform if you are using the metric system... Another interesting tidbit on the metric system is that you seldom see fuel economy figures for cars quoted as kilometers per liter -- you see them as liters per 100 kilometers... Now, Canada seems to be a bit confused with regards to the metric system... Although you see the metric system in many things, you still see land sold by the acre (instead of the hectare) and houses measured in sq-ft (vs sq-meters)... > It makes it easier as long as you stay in the same units of measurement... > [quoted text clipped - 4 lines] > increase the pressure and get more gas out of it... If at a higher > pressure, you might not have a compressor that can boost it much more... Most AL80's only hold 77 cu ft... > > It makes it easier as long as you stay in the same units of measurement... > > [quoted text clipped - 6 lines] > > Most AL80's only hold 77 cu ft... It's a rounding error. They're common in standard measurements. :-) > Most AL80's only hold 77 cu ft... Which further complicates things in that not all 80 cu-ft tanks are created equal... <grin> >>I thought this whole metric thing was supposed to make more sense... >> >>an 80 cf tank holds 80 cf of gas, but a 15 liter tank can hold 1500 >>liters of gas? > > It makes it easier as long as you stay in the same units of measurement... That's why I always use Big, Medium, and Small which corelates perfectly with Full, Half Full, and Empty. esg >>>However, I don't understand how you can get a 500 liter reserve from a 15 >>>liter cylinder... [quoted text clipped - 6 lines] > > an 80 cf tank holds 80 cf of gas, but a 15 liter tank can hold 1500 liters of gas? It can hold nearly any amount, depending on pressure. Our normal fill pressure is 190 bar at normal water temp, relating to a fill to 225 bar when the compressor stops. Because of that, our gauges are calibrated in bar. So you can calculate amount of gas easily by calculating pressure times wtaer capacity. Your's are in cubic feet? Matthias > > However, I don't understand how you can get a 500 liter reserve from a 15 > > liter cylinder... > > 15 liters is the volume of the inside of the tank, not its capacity at > its rated pressure (like we do it here). A 15 liter tank with 100 bar > of gas in it has about 1500 standard liters of gas. Come on man, I was jess goofin... 1500 liters is a small bottle; 52.972 cu ft... -- > Come on man, I was jess goofin... I realized that right as I hit the send button. > With dubs it's the rule of thirds, with single tanks, no stop > recreational diving, you are supposed to surface with a 500 psi reserve, > but depends upon whom you ask. Nawh, with doubles and not in open water, it's the rule of thirds... Doubles in open water are no different than single tanks... 500 psi if you're feeling generous, 250 psi if you're not... On the other hand, we went for many years with just the philosopy that our J-valve would tell us when it was time to head for the surface... > Nawh, with doubles and not in open water, it's the rule of thirds... > Doubles in open water are no different than single tanks... 500 psi if > you're feeling generous, 250 psi if you're not... Discipline. Dammit. >>The attributions got a little messed up there, I don't speak bars and >>litres. [quoted text clipped - 23 lines] > Needless to say, he wont be going back out on that boat (name purposely, > instantly forgotten). That's why ya hoard those old SPGs - the Bourdon tube gets all stretched out and it never drops below 500. esg > "Start your ascent at 50 bar" is one of those easy-to-remember general > rules that provides a good margin of safety for OW diving, with very > few exceptions, I think. On any decent single gas dive in the Mediterranean with a deco obligation, I start my ascent after consumption of about one third of my gas. That gives me time for deep stops, a slow ascent, deliberate decompression time, and about 30-40 bar left. With a 105 cf bottle, and and a total time of about 70 minutes, regardless of depth. Matthias > > "Start your ascent at 50 bar" is one of those easy-to-remember general > > rules that provides a good margin of safety for OW diving, with very [quoted text clipped - 7 lines] > With a 105 cf bottle, and and a total time of about 70 > minutes, regardless of depth. Sounds good, Matthias. On any dives where a screw-up can hurt, I like using "rule of thumb" dive planning, too. The "rule of thirds" certainly makes sense for more advanced dives than OW. Most of my dives are pretty tame OW stuff, so I'm pretty loose about it. If I did have a rule of thumb for those kind of dives, it would be to exit with enough air to be able to clear the valve after the tank warms up. 8^) Dave C > Most of my dives are pretty tame OW stuff, so I'm pretty loose about > it. If I did have a rule of thumb for those kind of dives, it would be > to exit with enough air to be able to clear the valve after the tank > warms up. 8^) That leaves you with insufficient supply to power the beer-kegs for the after dive party, shouild they need so. Matthias Damn, nobody seemed to mention enough gas in reserve to save your buddy, as well as yourself, at any time during the dive. Makes one wonder about the mind sets of the group. ;-) Of course, if everyone's deco obligations have been met, and the surface is close enough and unobstructed as to be considered the source of emergency gas, 7 bar in my dubs is plenty to prevent water intrusion. That's called extra play time. Curtis > Damn, nobody seemed to mention enough gas in reserve to save your buddy, > as well as yourself, at any time during the dive. Makes one wonder about > the mind sets of the group. ;-) [quoted text clipped - 3 lines] > gas, 7 bar in my dubs is plenty to prevent water intrusion. That's called > extra play time. I did, rule of thirds. > I did, rule of thirds. Yes, you did in essence, rule of thirds is the bare minimum based on that presumption. I'm sometimes diving with far more reserve just in case double sh.t happens. Curtis > > I did, rule of thirds. > [quoted text clipped - 3 lines] > I'm sometimes diving with far more reserve just in case double sh.t > happens. In those situations, we dive on stages, with the dubs as bailout gas. As I am sure you do. -- >> I'm sometimes diving with far more reserve just in case double sh.t >> happens. > > In those situations, we dive on stages, with the dubs as bailout gas. > > As I am sure you do. Yes Curtis > El Stroko Guapo wote: > > For recreational diving at the OW level, > > "start your ascent at 50 bar" is all the gas planning that is *essential*, > > along with a knowledge that deeper = proportionately more air consumption > > Respectfully, I disagree. Heh, that'd be me who wrote that. Not sure if ESG would like to be linked to my heresy or not :) > However, if you teach someone "start your ascent at 50 bar" and they > are diving with a 6 liter or a 7 liter cylinder they are going to come > horribly short. If you are diving with a 6 liter cylinder and you are > at 50 bar you only have 300 liters of gas left! You are absolutely correct in theory. However, I dont know of any OW diver who goes out with 6 or 7 liter tanks. > I think people are cleverer than what we (as instructors) give them > credit for - giving students a simple rule to remember which doesn't > hold true in all instances is also performing a disservice! Here, it is my turn to disagree. We train a few hundred OW students every season, and get a lot of people with 20 dives or less, and it is amazing how little they know or remember about theory. It is also amazing how many eyes start to glaze over and how hard it can be for some people to figure out the pressure at 30m. My standard formula for OW now is K.I.S.S. I now stick to the "50 bar and up you go" adage, with the usual riders (different tank sizes, currents, greater depth, etc). I do a quick and dirty example on more detailed calculations, and if people are interested, I happily go into all of this in more detail but that happens maybe once every fifteen classes. Vandit |
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