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By the end of this topic, you should be able to answer this question: 1 What is a bubble check and when do you do it? 2 What is a descent check and when do you do it? 3 When should you check your gas supply underwater? 4 How do sidemount divers manage gas supply? In the Water Inwater checks look for things that are more easily visible in the water, especially small leaks. During entry something may have happened such as a hose that seemed secure before the dive disconnecting from an inflator. Inwater checks also may spot things that were accidentally missed during predive checks. Bubble Checks Bubble checks look for leaks so that they can be resolved before the dive starts. Divers scootering their DPVs. Bubble checks are conducted at the surface or in shallow water, depending on conditions and local protocols. The team checks each other’s equipment for bubbles/leaks at all first stages, cylinder valves, cylinder necks, manifolds, second stages, hoses, SPGs, inflators (including dry suit), inflation systems and any other equipment. Although you may be able to see some or all of your own equipment, always have a team mate check you because sometimes bubbles track away from you, making it hard to see for yourself. If you made your checks at the surface, take a moment to re-check underwater, if possible. The dive doesn’t continue until all bubbles/leaks, even small ones, are handled. Descent Checks When feasible, after starting your descent, the team should pause at about 6 metres/20 feet and check for: Loose gear – for example, dry suit hose detached from inflator, crotch strap not fastened, etc. Correct configuration – for example, look for hoses that are tangled, cylinders clipped to each other instead of a D-ring, etc. No bubbles – confirm bubble check, if done at the surface. Correct gas – everyone is breathing the correct gas and not accidentally breathing a deco cylinder, for example. Deco/stage cylinders are pressurized but turned off. Dive computer/timer(s) set and giving the correct information. In some instances, the descent check may not be feasible until reaching the bottom, due to current, logistics, conditions etc. The bubble and descent checks may be combined. In all cases, if a problem is found it must either be resolved or the dive aborted. Checking Gas Supply You know that the main tool for managing your gas during a dive is checking your SPG (or dive computer if it’s gas integrated), but there are some new techniques to learn as well. When you first start using double cylinders your gas may seem to last a long time, but don’t forget that as you progress your dives are likely to be deeper and longer so check your gas frequently and carefully. You’ll plan your gas requirements and reserve on the surface and confirm your minimum turn/ascent pressure again during your checks. We’ll discuss how to plan your gas requirements, including a reserve, in the Gas Planning section. When you reach your target depth it’s a good idea to recheck your SPG(s) just in case you’ve used more gas than anticipated during the descent. Throughout the dive check your SPG frequently making a note of how much gas you’ve got and how close you are to your turn/minimum ascent pressure. As you get closer to the limit, you should check more frequently to avoid exceeding your limit. It's better to start your ascent earlier and have more gas left when you surface than risk using more than you planned. As you’ve learned, tec divers always have at least two fully independent regulators with at least one SPG. If you carry one or more deco cylinders, you'll have additional regulators and SPGs. Each time you check your SPG confirm that it is the correct SPG. This is usually done by having each SPG clipped in a specific place on your equipment. If you notice that your SPG reading has not changed, check that you're looking at the correct SPG and that your isolator valve (if applicable) is fully open. Sidemount Gas Management Sidemount divers switch breathing from one cylinder to breathing from the other cylinder throughout the dive. This ensures that if they lose gas in one cylinder they still have sufficient gas in the other cylinder to abort the dive. How It's Done You will switch cylinders after a specific amount of gas has been used in each cylinder. Choosing how often to switch gas is a balance between optimal gas and trim management and not becoming task-loaded. For example To minimize pressure differences between cylinders, you may choose to switch up to every 30 bar/500 psi. To do this you: Breathe 30 bar/500 psi from your right bottom gas cylinder. Switch to your left bottom gas cylinder and breathe 30 bar/500 psi. Continue until you reach turn/minimum ascent pressure, or for no stop diving, so you end the dive with at least 50 bar/500 psi in each cylinder. Tec divers usually do not breathe more than one-third of each cylinder before switching: Use one-third of your right bottom gas cylinder then switch. Use two-thirds of your left bottom gas cylinder (some divers prefer to switch every third). Return to your right bottom gas cylinder. Using reserves based on the Rule of Thirds (more on methods of calculating gas reserves later), assuming you had no emergencies or delays, your dive should finish (or you should switch to a decompression cylinder on a decompression dive) before you have used the second third. This method reduces the number of gas switches. There will be a difference in gas pressure between the cylinder of up to one-third of your gas. At higher levels of tec diving many divers prefer to keep their gas pressures closer. The maximum buoyancy difference between one side (at switch pressure) is about 1- 1.5 kg/2-3 lbs, which most divers find manageable.