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DETERMINING SURFACE AIR CONSUMPTION RATES and GAS REQUIREMENTS By: David G. Vetter PADI #MSDT-93036ANDI #957 Every diver needs to know his or her own rate at which his or her breathing gas is consumed. This information is important both from the standpoint of selecting an appropriate sized cylinder when diving with others having a different rate, as well as to be able to predict how much gas will be required to make a dive to a known depth for a known period of time. Knowing this gas consumption rate is absolutely critical when entering the arena of “technical diving” where running short on the available gas supply can have dire consequences. A.C.R. There are several ways of both determining a divers rate of gas consumption, as well as several ways of expressing what this rate is. One means, which is used and taught to entry level divers, expresses air consumption in terms of PSI consumed per minute, and commonly called the Air Consumption Rate or A.C.R. To compute you’re A.C.R you simply take the number of PSI’s consumed during a dive and divide that by the time that you spent at depth; then multiply this number by 33 if diving in saltwater or 34 for freshwater. This result is then divided by your actual depth plus 33 if saltwater of 34 for freshwater. (The reason for the 33 or 34 gyration is simply to convert “depth” in feet, which is pressure, to another expression of “pressure”, being ATA or “Atmospheres Absolute”. In other words, while at the surface, 1 Atmosphere of pressure = 14.7 PSI. When descending in sea water to a depth of 33’ (34’ feet if diving in fresh water) we duplicate or double the total pressure being induced on our bodies, compared to that of the surface pressure, giving us a total (or Absolute) pressure of 29.4 PSI or 2 ATA.) The mathematical formula for determining A.C.R. looks like this: (PSI Consumed/Time at Depth) X 33 = Air Consumption Rate DEPTH + 33 Example: Let’s assume that a diver was to start his or her dive with a cylinder filled to 3000 PSI. During a dive to a depth of 40 fsw (Feet Sea Water) the diver consumed 1500 PSI over the course of a 40 minute dive. Following, find these numbers plugged into the formula: 1500/40 X 33 = 1,237.50 = 16.95 PSI per minute 40 + 33 73 “Normal” air consumption rates for most divers usually fall between 20 and 25 PSI per minute when diving on aluminum 80’s. The A.C.R will be higher for smaller tanks and lower for larger tanks. This method is meaningful and perfectly acceptable so long as the diver or divers using this calculation are always using the exact same cylinder, such as the tried and proven, ever popular and trusty aluminum 80. However, the A.C.R. becomes quite meaningless if a diver is constantly switching the size of their air tanks or comparing their A.C.R. to another diving using a different size tank.
S.A.C. RATES Probably the most versatile and meaningful way of determining the rate at which a diver consumes gas, is that of relating the quantity of gas consumed in terms of cubic feet per minute at normal atmospheric pressure. This is commonly referred to as a divers Surface Air Consumption (S.A.C.) rate. (Note that we will use the term “gas” in place of “Air” throughout this explanation because of the popularity of NITROX) Unlike the A.C.R. method, which is fine so long as the diver ALWAYS dives the exact same cylinder, the method shown below will allow a diver to switch from one cylinder size to another with the “rate” of consumption remaining unchanged. Once this SAC rate is known, it is an easy process to then calculate and reasonably predict what that same diver will consume at any depth and with any size cylinder (or cylinders). It should be noted that an almost endless variety of factors would affect a divers gas consumption rate (temperature, current, stress, gear configuration, etc.). For this reason it is recommended that a diver conduct the SAC rate determination procedure geared out exactly as they will be diving, and as closely as possible simulating the actual diving conditions normally encountered. (Example: To get the highest degree of accuracy, a diver should be wearing the same BCD, using the same regulator, wearing the same wet/dry suit, and if possible in the same range of water temperature at which they expect to be “normally” diving when conducting the test.) Probably the single largest factor affecting a divers rate of gas consumption is his or her level of exertion. Obviously, a diver will consume much more gas while swimming against a current than will that same diver use while holding perfectly still making an extended decompression stop. For this reason, most “techies” will determine both a “working” SAC rate and an “at rest” SAC rate in order to properly plan for dives. Typically, the ”at rest” rate will involve at least a 20-25% reduction in gas consumed for the obvious reasons. MAKING A “TEST DIVE”: In order to accurately determine their SAC rate, a diver will need to be equipped with a complete set of properly functioning gear. As a bare minimum the diver must have an underwater timing device, an SPG (Submersible Pressure Gauge), an accurate depth gauge, and an underwater slate to record data. (Note: A good air integrated dive computer will make combine all of these into one.) Also, a fair degree competency in the area of buoyancy control will be a tremendous help if conducting this test in open water.
The Actual Test Dive: 1.) Enter the water and allow the cylinder to “acclimate” to the temperature of the water being dived. (The pressure in the tank will change, sometimes dramatically, depending upon the difference between the air temperature and the water temperature.) While the actual size of the cylinder is not important for the purposes of this test, what IS important to know is both the exact size of the cylinder in Cubic Feet and the rated pressure for that cylinder. (EXAMPLE: Let’s assume you have chosen an aluminum 63 CF tank, rated for 3000 PSI for our first example.) Note: This data will always be displayed on the shoulder of the tank, but may be a bit cryptic to the untrained eye. 2.) When ready, descend to whatever depth you have decided to conduct the test. (The actual depth does not matter, but make sure to stay precisely at your selected depth for the entire test!) 3.) When at depth and when ready to proceed, note the EXACT time (Or start the chronometer if so using.) and also the EXACT tank pressure from the SPG or A/I Dive computer. Enter this the start time and the tank pressure reading onto your dive slate under “STARTING PRESSURE”. Now simply begin swimming at a normal rate through the water to determine the “working” SAC rate (or just relax and stay put if determining the “at rest” SAC rate.) 4.) While the duration of the test dive is not critical, the longer the dive, the more accurate the results will tend to be. A minimum of 10 minutes is recommended, but whatever duration is chosen, be sure to stop EXACTLY at an easy to determine point (EXAMPLE: Conclude the test at precisely 15 minutes, not 15 minutes and 25 seconds). 5.) At the conclusion of the time, note the EXACT reading from the SPG or A/I dive computer and enter the results onto the dive slate under “ENDING PRESSURE”, along with the ending time or just exactly what the duration of the dive was. 6.) Upon exiting the water, simply subtract your “ENDING PRESSURE” figure from the “STARTING PRESSURE” to determine your actual “PSI USED” during the test dive.
Now you have all of the data required to determine the SAC rate. Apply the following formula: (psi used ÷ working pressure) X tot. cyl. cap. Cubic feet per min SAC = ____________________________________ ÷ min. (depth in feet + 33) ÷ 33
EXAMPLE #1: You used 1,430 psi from a 63 cubic foot cylinder, which has a rated pressure of 3000 psi. During your test dive, you swam at exactly 30 feet of depth for exactly 15 minutes.
(1,430 ÷ 3000) X 63 30.03 _________________ ÷ 15 = ______ ÷ 15 = 1.048 cubic feet per minute ( 30 + 33 ) ÷ 33 1.91 SAC Rate
EXAMPLE #2: You used 1,650 psi from an aluminum 80 cubic foot cylinder (Actually rated at 77.8 cubic feet), which has a rated pressure of 3000 psi. During the test dive, you swam at a depth of exactly 50 feet for exactly 16 minutes.
(1,650 ÷ 3000) X 77.8 42.79 _________________ ÷ 16 = ______ ÷ 16 = 1.063 cubic feet per minute ( 50 + 33 ) ÷ 33 2.515 SAC Rate
Note: If you are a newer diver who is calculating your SAC rate for the first time, don’t be concerned if your calculated SAC rate is markedly higher than that of a more experienced diver. It is normal for a divers SAC rate to reduce dramatically between their first few dives and the time that they log 25 dives or so. This is largely due to the more relaxed state that only experience and time spent underwater can bring.
DETERMINING GAS REQUIREMENTS: Now that the SAC rate has been established, it is a simple matter to determine what the volume of gas will be for a given dive to a known depth. Here is the formula: Cubic feet required = (min X SAC rate) X ((depth in feet + 33) ÷ 33) EXAMPLE: If you have determined your working SAC rate to be .80 cubic feet per minute, how much breathing gas will be required for a dive to 110 feet for 10 minutes?
Cubic feet required = (10 X .80) X ((110 + 33) ÷ 33) Or Cubic feet required = 8 X 4.3 Or Cubic feet required = 34.4
(Note: Obviously, this example does not include the additional gas required for the actual descent to our depth of 110’, nor does it include the necessary gas required for our ascent, for a safety stop, nor the gas which also be carried for a margin of safety. The use of a fully redundant “pony” bottle is highly recommended if diving below 100’ of depth with a single tank system. Enroll in a reputable TECHNICAL DIVING training course for answers to these and many other issues required for safe deep diving.) Safe Diving! |
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