Ensuring an adequate air supply during a dive requires the use of a submersible pressure gage for monitoring air supply. Guestimating one’s air consumption and the time left at the diving depth are also imperative.
Running out of air is a sure way to ruin a great dive. The easiest way to avoid running out of air is to monitor your air supply, factor in your air consumption and predict how much time you will have underwater at a given depth and work level. Those three steps can be rough guestimates or more precise mathematical calculations.
Table of Contents
Monitoring Air Supply
A submersible pressure gauge is the only way to constantly know the available air pressure in a tank. Modern gauges are reliable, easy to read and durable. Gauges in the United States display data in psi (pounds per square inch). This is more accurately called psig, for pounds per square inch gauge but is shortened to psi by most divers.
The only tricks to operating a submersible pressure gauge are making sure you can accurately read the gauge face (perhaps without your glasses) and remembering to look at it periodically. Some gauges have audio warnings that beep when pressure is reduced to a preset level but most are passive – i.e., you must look at them.
Air Consumption Calculations
Air consumption and subsequent time available at depth can be guestimated, figured mathematically or predicted from a wheel style calculator. Which one you use depends upon the situation. A casual sport dive to see the sights needs far less precision than a deep wreck dive.
Let’s review some very basic diving physics to help us understand what happens to the amount of air available to a diver at depth.
Total Amount of Available Air (Boyle’s Law)
Surface (1 atm) 100% available
33 feet (2 atm) 50% available
66 feet (3 atm) 33% available
99 feet (4 atm) 25% available
Another factor in predicting how long air will last at depth is the activity. A very relaxed diver lying on the sand watching reef critters may breathe about 0.5 cubic feet per minute (cfm). The same diver casually kicking along may use 1 cfm. But, when this diver is swimming against a current and working hard, it would not be unusual to consume 2 to 3 cfm.
Look at the effect these consumption rates have on the time you can plan to stay at depth. These are calculated using a 72 cubic foot tank filled to 2,475 psi and not allowing any reserve air at the end of a dive.
Resting Easy Swim Hard Swim
0.5 cfm 1 cfm 2 cfm
33 ft. 72 minutes 36 minutes 18 minutes
66 ft. 48 minutes 24 minutes 12 minutes
99 ft. 36 minutes 18 minutes 9 minutes
Note that all of these times are within the no decompression limits except the 36 minutes at 99 feet. The U.S. Navy Tables no decompression limit for 100 feet is 25 minutes.
The examples given are good averages for a typical sport diver. If you fall into an average range and are calculating your air consumption for a recreational dive, this guestimate based on averages may be all you need. If this is the level of air consumption planning you are comfortable using, then make notes on several dives to see if the estimated cfm rates are accurate for you. Make a trial dive to 33 feet and spend nearly all of it quietly observing the environment. You’ll want to allow 300 to 500 psi for safely returning to the surface but track your bottom time and see how close you are to the 72 minutes. Repeat the process at 33 feet and casually kick around the reef. See if your time is reduced to the 36 minute range.
Why You May Not Be An Average Air User
A wide range of factors will change your air consumption on a given dive or explain the difference between you and your buddy.
NOVICE DIVERS USE MORE AIR THAN EXPERIENCED DIVERS: Beginners tend to be more anxious, stressed or nervous underwater, which increases their breathing rate.
COLDER WATER INCREASES BREATHING RATES: Protective layers, whether it is a T-shirt in 80 oF water or a drysuit in 50 oF water, will reduce your air consumption. Water colder than your body temperature conducts heat away many times faster than air. When you lose heat, you lose energy; your breathing rate and circulation increase to try and rewarm the body.
UNCOMFORTABLE SITUATIONS CAUSE STRESS, WHICH INCREASES BREATHING: It doesn’t matter whether you are irritated by a buddy, low visibility in murky water, equipment that doesn’t fit right or are just tired from standing fully geared up on a beach – any uncomfortable situation causes anxiety, which will increase your breathing rate and air consumption.
WORKING TOO HARD IN THE WATER USES LOTS OF AIR: Kicking like crazy, swimming against a current, treading water on the surface without inflating your BC or fighting surf requires lots of air. Reducing these high exertion activities reduces your air consumption and increases your bottom time. Diving is a slow, deliberate activity, not a high speed aerobics class at depth.
POOR EQUIPMENT PLACEMENT OR USE CREATES DRAG: This takes energy to overcome. More energy requires more air. Work on your buoyancy control – reduce lead, reduce air in your BC, reduce the size of the breaths you take. When your BC has little air inside, it is more streamlined. Tuck in your gauge hoses, move your knife to the inside of your leg, eliminate things hanging from your body. Compare the profiles of a fish and a diver. See the difference? Aim for a sleek, fast moving “tuna” look, not a cumbersome, “inflated porcupine fish” look. Continuously clearing a poorly fitting mask can be a big air user. If you use your buoyancy compensator like an elevator, frequently power inflating and deflating, you will waste a fair amount of air.
POOR PHYSICAL CONDITION INCREASES AIR CONSUMPTION: Being overweight, tired, a smoker or generally out of shape will increase your air consumption. If you keep your body in good shape through regular exercise you will use air more efficiently.
If you need or enjoy using math to determine how long a scuba cylinder will last, here are some examples of applying Boyle’s Law.
Problem: How long can a diver swim at surface pressure on a fully charged tank (71.2 cu. ft. at 2,475 psig) with 300 psi reserve at the end of the dive? From an available volume air chart you will find that 300 psi equals 8.2 cf of air (or you could compute it).
Therefore the diver has available:
71.2 – 8.2 cu. ft./1 cu. ft./minute = 63 cu. ft./1 cu. ft./minute = 63 min.
Problem: How long could the diver be at 33 feet in seawater? Since the rate of air consumption is constant, we can rewrite Boyle’s Law as:
P1 T1 = P2 T2 where P1 = 1 atmosphere, T1 = time available at the surface, P2 = depth pressure, T2 = time at depth.
Calculate P2 substituting
1 + 33/33 = 2 atm
We then have 1 x 63 = 2 x T2 or 1 x 63/2 = T2 = 31.5 minutes
Rounding off, T2 = 31 minutes.
Wheel Style Calculator
If you want the precision of the mathematical example but don’t want the math, consider using a graphic circular slide rule based on computer generated logarithmic scales, such as the NAUI SAC Rate calculator. SAC stands for Surface Air Consumption. Knowing your SAC Rate is like knowing the EPA mileage guide on a car – it tells how efficiently you are operating underwater. Using the SAC Rate calculator is as easy as spinning three plastic dials and reading the result.
1. Determine the psi you used on your dive by subtracting the psi remaining after your dive from the psi at the start.
2. Locate this number on the outermost Total PSI Used dial.
3. Locate the average depth of your dive on the middle Average Depth dial. Line this number up with the Total PSI Used number found in step 2.
4. While holding these two dials in the above set position, locate your total down time on the innermost Down Time dial. Line that number up with the Set Time Here marker.
5. Read your Surface Air Consumption Rate in the SAC Rate window.
Typical SAC rates are: student diver, 40 to 60; beginning diver, 30 to 40; advanced diver, 20 to 30; experienced diver, 15 to 20.
Air consumption numbers are important information to the extent they help you plan a safe dive. Some purists will want to calculate to the last molecule, others will be happy to come out within five minutes. Either way is fine – if it safely meets your diving needs.
If you use the simple rule of thumb for 33, 66 and 99 feet, you can split the difference for intermediate depths. If half your dive is resting and half is swimming at a 1 cfm consumption rate, you can factor that in with round numbers and probably come pretty close to a mathematical solution.
It can really be pretty simple – if you start with 2,475 psi and want to end the dive with about 500 pounds of air, when you get down to 1,500 psi on your gauge, your dive is half over. Check your watch and note the number of minutes of down time so far – that’s how much time you have left for the second half of the dive. This is practical, pretty simple and pretty effective. No diver’s slate or graduate degree is required!