This is factored into the total time spent underwater. When planning your gas calculations, factoring in the rule of thirds includes all time spent underwater. The last third should be the amount you have after you exit the water.
A two part question here (1-) are you saying that the total dive should only use 1/3 of supply and return to boat having 2/3 's reserve-OR- the diver could (if favorable conditions and training allow) use 2/3 total and always have 1/3 reserve upon return to boat ?? (2-) Since the reciprocal of 2/3 rds = 1.5 so if a dive will require 80 cu/ft usable air, we could multiply 1.5 X 80 cu/ft =120 cu/ft to determine tank size (120) for the dive requiring 80 cu/ft. with a 1/3 reserve- or- we could take our tank size(120 cu/ft) and divide by 1.5 , 120/1.5=80 cu/ft to determine what 2/3 rds of our tank would be...and not exceed that limit...is that correct ? As you may surmise from my comment questions I haven't been diving yet and need to get some of the the mental debris out of the way first.
Great question. The rule of thirds is based off a diver descending with a full tank and making a dive. Once he or she has used up the first third, he or she will turn and head back to the exit. Finally ending the dive still with a third in reserve. There are several things a dive needs to know before you can calculate how much air you need. First is no decompression limit and next is air consumption. With these two variables, a diver can plan his gas management very easily. This of course is planning with tables and not a computer. Although the computer will compensate for air consumption and no decompression time changes as you ascend, it's built in algorithm is more advanced than the math we can do on paper. Diving the rule of thirds is always the safest way to dive, however, most follow their computers and don't follow quite as a conservative dive plan. Just for an all around fun dive, especially on a dive charter, having enough air to reach one's no decompression limit is not necessarily needed, as most dive charters will give you a time limit much less than your no decompression limit. Thus, you will need to be out of the water before you ever run out of air. If you are planning dives that are not on a commercial charter, where you set your own rules, then taking enough gas with you to reach your no decompression limit and still have enough air to be able to plan and dive by the rule of thirds, comes more into play. The easiest way to figure up the rule of thirds is to take the starting pressure and divide it by 3. Example: 3000psi / 3= 1000psi. 3000psi--1000psi= 2000psi of usable gas. Start with 3000psi, turn at 2000psi, end with a 1000psi. This also applies to high pressure cylinders as well. 3442psi / 3 = 1147psi. 3442psi - 1147psi = 2295psi usuable gas. Start with 3442psi, turn at 2295psi, end with 1147psi. When diving with doubles that are manifolded together, the math is the same. In independent doubles such as a sidemount situation, you can either watch both gauges and apply the same principle to each tank, or add the total air supply together, and divide it by 3. Example for Sidemount Independent tanks. Two cylinders both at 3000psi. Total air supply 6000psi / 3 = 2000psi. 6000psi - 2000psi = 4000psi usable gas. Start with 6000psi, turn at 4000psi, end with 2000psi. Or simply watch both gauges and when both hit 2000psi turn, and end when both are at 1000psi. The second way seems easier to me, simply because there is no math involved while at depth. You have to remember, knowing how much psi you need is not the same as how much cubic feet you need. After calculation your SAC rate, you will need to convert that into your RMV rate. This will help you decide which tank or tanks will be best for any given dive. As a new diver knowing all this is very important, but so is having fun. Get certified, have fun, and don't make things more technical than they need to be. After you have several dives under your belt and you start to take other continuing education courses, the math will fall into place and help you extend your knowledge in diving. Have fun and safe diving.
Thanks for the videos. If you need 17.6cf for the entire 20minute dive. With the rule of thirds why is the required total 52.8cf and not (17.6 * 1.33) 22.88cf? 22.88 gives 2/3 for the planned 20 minute dive, and 1/3 extra.
Great question. This is based off the total dive itself being 20 minutes long (not the time spent at the deepest depth), including descent, time at bottom, and ascent, excluding safety stop time, since this is a no decompression dive. Even though by this calculation a diver would not be anywhere close to their no decompression limit, we are still doing the calculations based off planning the dive with tables instead of a computer. Tables only allow for a square profiles, not a multi level profiles. This of course does not hold true if one uses the old PADI Wheel or ERDP to plan a multi level dive. If a diver was to use a computer, not only would it compensate for added no decompression time as he made his ascent, it would also compensate for added air time, if it was an air integrated model. Most would agree that a 53 cubic feet cylinder would not be the best choice for this dive, but would be the bare minimum one would need to use. Once again, this is based off the rule of thirds being one third to dive on, one third to get back on, and one third for an unexpected emergency. Though planning with the tables is a quick and simple way to plan dives, they are still based off a square profiles, not a multi level profiles.
I was wondering the same thing. Your answer cleared it up for me too. Since the rule of thirds is not just 2/3 of time spend at bottom, then multiplying by 3 and not 3/2 makes sense now! Thanks!
Hello elias scholegl, in theory it may be possible, however, since we all have different tissues in our bodies (both fast and slow tissues) it would be difficult to be exact. The on gassing and off gassing of nitrogen will occur at different rates for different divers. This is why the tables and our dive computers are conservative by nature. Great question though.
Based off the formulas you provided, in order to figure the size required, you would at least have to have history of a dive at that particular depth. If the depth is new to you though, how could you figure it? Would you say you could use a similar depth that you have been to before without it being the exact depth?
Calculating a SAC rate or RMV rate is based off an average. SAC rate changes depending on what Cylinder you use, however our Respitory Minute Volume will never change. My suggestion is to log every dive and make notation of you air consumption. After a series of dives, (you pick the number), calculate your SAC and RMV, and then average them out. This is the best and most accurate way to do it.
How do you determine how much you'll need if you want to include a safety stop and ascent rate?
This is factored into the total time spent underwater. When planning your gas calculations, factoring in the rule of thirds includes all time spent underwater. The last third should be the amount you have after you exit the water.
I need a172cf tank...lol
I would be too short for a cylinder that large. That would be a cascade cylinder.
A two part question here (1-) are you saying that the total dive should only use 1/3 of supply and return to boat having 2/3 's reserve-OR- the diver could (if favorable conditions and training allow) use 2/3 total and always have 1/3 reserve upon return to boat ?? (2-) Since the reciprocal of 2/3 rds = 1.5 so if a dive will require 80 cu/ft usable air, we could multiply 1.5 X 80 cu/ft =120 cu/ft to determine tank size (120) for the dive requiring 80 cu/ft. with a 1/3 reserve- or- we could take our tank size(120 cu/ft) and divide by 1.5 , 120/1.5=80 cu/ft to determine what 2/3 rds of our tank would be...and not exceed that limit...is that correct ? As you may surmise from my comment questions I haven't been diving yet and need to get some of the the mental debris out of the way first.
Great question. The rule of thirds is based off a diver descending with a full tank and making a dive. Once he or she has used up the first third, he or she will turn and head back to the exit. Finally ending the dive still with a third in reserve. There are several things a dive needs to know before you can calculate how much air you need. First is no decompression limit and next is air consumption. With these two variables, a diver can plan his gas management very easily. This of course is planning with tables and not a computer. Although the computer will compensate for air consumption and no decompression time changes as you ascend, it's built in algorithm is more advanced than the math we can do on paper. Diving the rule of thirds is always the safest way to dive, however, most follow their computers and don't follow quite as a conservative dive plan. Just for an all around fun dive, especially on a dive charter, having enough air to reach one's no decompression limit is not necessarily needed, as most dive charters will give you a time limit much less than your no decompression limit. Thus, you will need to be out of the water before you ever run out of air. If you are planning dives that are not on a commercial charter, where you set your own rules, then taking enough gas with you to reach your no decompression limit and still have enough air to be able to plan and dive by the rule of thirds, comes more into play. The easiest way to figure up the rule of thirds is to take the starting pressure and divide it by 3. Example: 3000psi / 3= 1000psi. 3000psi--1000psi= 2000psi of usable gas. Start with 3000psi, turn at 2000psi, end with a 1000psi. This also applies to high pressure cylinders as well. 3442psi / 3 = 1147psi. 3442psi - 1147psi = 2295psi usuable gas. Start with 3442psi, turn at 2295psi, end with 1147psi. When diving with doubles that are manifolded together, the math is the same. In independent doubles such as a sidemount situation, you can either watch both gauges and apply the same principle to each tank, or add the total air supply together, and divide it by 3. Example for Sidemount Independent tanks. Two cylinders both at 3000psi. Total air supply 6000psi / 3 = 2000psi. 6000psi - 2000psi = 4000psi usable gas. Start with 6000psi, turn at 4000psi, end with 2000psi. Or simply watch both gauges and when both hit 2000psi turn, and end when both are at 1000psi. The second way seems easier to me, simply because there is no math involved while at depth. You have to remember, knowing how much psi you need is not the same as how much cubic feet you need. After calculation your SAC rate, you will need to convert that into your RMV rate. This will help you decide which tank or tanks will be best for any given dive. As a new diver knowing all this is very important, but so is having fun. Get certified, have fun, and don't make things more technical than they need to be. After you have several dives under your belt and you start to take other continuing education courses, the math will fall into place and help you extend your knowledge in diving. Have fun and safe diving.
Thanks man! Appreciate the detail and the suggestion to keep-it-simple.
Thanks for the videos.
If you need 17.6cf for the entire 20minute dive. With the rule of thirds why is the required total 52.8cf and not (17.6 * 1.33) 22.88cf?
22.88 gives 2/3 for the planned 20 minute dive, and 1/3 extra.
Great question. This is based off the total dive itself being 20 minutes long (not the time spent at the deepest depth), including descent, time at bottom, and ascent, excluding safety stop time, since this is a no decompression dive. Even though by this calculation a diver would not be anywhere close to their no decompression limit, we are still doing the calculations based off planning the dive with tables instead of a computer. Tables only allow for a square profiles, not a multi level profiles. This of course does not hold true if one uses the old PADI Wheel or ERDP to plan a multi level dive. If a diver was to use a computer, not only would it compensate for added no decompression time as he made his ascent, it would also compensate for added air time, if it was an air integrated model. Most would agree that a 53 cubic feet cylinder would not be the best choice for this dive, but would be the bare minimum one would need to use. Once again, this is based off the rule of thirds being one third to dive on, one third to get back on, and one third for an unexpected emergency. Though planning with the tables is a quick and simple way to plan dives, they are still based off a square profiles, not a multi level profiles.
I was wondering the same thing. Your answer cleared it up for me too. Since the rule of thirds is not just 2/3 of time spend at bottom, then multiplying by 3 and not 3/2 makes sense now! Thanks!
Great video! Can you use your SAC rate to calculate your nitrogen uptake at any depth after a certain time period ?
Hello elias scholegl, in theory it may be possible, however, since we all have different tissues in our bodies (both fast and slow tissues) it would be difficult to be exact. The on gassing and off gassing of nitrogen will occur at different rates for different divers. This is why the tables and our dive computers are conservative by nature. Great question though.
Based off the formulas you provided, in order to figure the size required, you would at least have to have history of a dive at that particular depth. If the depth is new to you though, how could you figure it? Would you say you could use a similar depth that you have been to before without it being the exact depth?
Calculating a SAC rate or RMV rate is based off an average. SAC rate changes depending on what Cylinder you use, however our Respitory Minute Volume will never change. My suggestion is to log every dive and make notation of you air consumption. After a series of dives, (you pick the number), calculate your SAC and RMV, and then average them out. This is the best and most accurate way to do it.