How to calculate my breathing rate underwater while Scuba diving

Many divers don't worry about air consumption. When they begin to run low on air they simply surface and end the dive. But wouldn't it be fun to stay down longer - to surface because you are nearing the no-decompression limit instead of because you are running low on air? Wouldn't it be nice to be able to stay with the guided group for the entire dive, and not be the one who cuts the dive short because he has depleted his air supply? The vast majority of divers would prefer to stay down longer, if at all possible.
Well, it is possible. Taking control of air consumption should be a priority for every diver as soon as possible after becoming certified.
The first step in taking control of your air consumption is determining your surface consumption rate (SCR). SCR is how much air you breathe while resting on the surface.
There are two ways to determine SCR. As described earlier, you can sit on shore or on board a boat and breathe from the regulator for a few minutes. Note the exact tank pressure and time when you begin this exercise and note the same when you finish. If the tank pressure decreased by 200 psi in 10 minutes of breathing, your SCR is 20 psi per minute: 200 psi divided by 10 minutes = 20. You have room for improvement.
The other and more common way to determine SCR is to go diving. Descend to 33 feet (10 m) and note the beginning tank pressure and time on a dive slate. Dive normally at a consistent depth of 33 feet for 10 minutes. At the end of the timed period note the ending tank pressure and the duration of the exercise (10 minutes) on the slate.
Back on the surface following the dive, subtract the ending pressure noted from the beginning pressure to determine air used during the period. Now, divide the air used by the number of minutes of the test. The quotient is how much air you used per minute at a depth of 33 feet, not your SCR. To determine SCR, divide by two the consumption rate at 33 feet. The answer is your SCR.
If you use 500 psi in the 10-minute test, your SCR is 25 psi per minute (500 psi ÷ 10 minutes = 50 psi per minute at 33 feet ÷ 2 = 25 psi SCR).
Calculating SCR by sitting on the surface breathing from the scuba unit gives you a resting consumption rate. Monitoring your consumption at depth and then mathematically calculating what it would be at the surface gives a more realistic reading because it reflects the additional air used when actually involved in diving activities.
There is nothing magical about monitoring usage for 10 minutes. Any period of time will work, but 10 minutes or longer provides a more accurate representation, and using 10 minutes facilitates the math.
There also is nothing special about 33 feet other than 33 feet represents the depth of two atmospheres, where pressure is twice what it is on the surface. Performing the consumption rate monitoring at exact atmosphere depths - 33, 66, 99 feet (10, 20, 30 m) - also facilitates the math; it is easier to divide by a round number than to calculate decimals.
Regardless of the depth you choose for your air consumption test, it is important to maintain that depth throughout the timed monitoring period and swim at a leisurely pace.
You Know Your SCR, So What?
Few experienced divers worry about air consumption as long as they are diving a familiar environment and using their normal equipment. They have completed the learning curve and have fine-tuned their consumption rates in every way possible.
But to new, inexperienced and infrequent divers air consumption should be a concern. That is where your SCR comes in handy. Simply knowing your SCR, however, is of little use unless you apply it to a real-life diving situation.
When I led dives in Molokini Crater in Hawaii, during the dive briefing we would explain that the guide and group would follow a carefully planned route that took them from the anchor line, across a sand channel, past a few coral heads to a ledge where a family of eels normally hung out. Then we would proceed to another slightly deeper ledge where four out of five dives we would encounter several white tip sharks. At the shark ledge we would turn around and make our way back to the boat.
We also explained that when the first diver reached 1,000 psi remaining in his tank, the entire group would reverse course and head back toward the boat, even if we had not yet reached the eels or sharks. Following that statement we could immediately tell from their expressions which divers normally used a lot of air. The thought of being the one who would cause the entire group to return to the boat before reaching the eels and sharks cast concern across a few faces.
This is a situation in which knowing SCR comes in handy. The maximum depth of the Molokini dive was 60 feet (18 m) and the planned bottom time 45 minutes. One of those concerned divers could have quickly done the math and determined that, "Yes, I can do that profile and still have 500 psi remaining upon surfacing" or "Whoa! I will be low on air 20 minutes into the dive."
Here is how. If a diver knows his SCR he can estimate his consumption rate at depth. The maximum depth on this dive is 60 feet (18 m); that is just short of three atmospheres. At that depth a diver consumes three times more air than at the surface. Assuming a SCR of 30 psi, he multiplies his SCR by three and estimates that his consumption rate at depth will be about 90 psi per minute. So, how long can he stay down?
Assuming the tank he is using is the same size as used when he calculated his SCR, estimated bottom time is determined by dividing the tank pressure, 3,000 psi in this example, by his estimated consumption rate at depth of 90 psi per minute (3,000 ÷ 90 = 33). A rough approximation is that this diver will not be able to stay down beyond 33 minutes.
But wait. He doesn't want to breathe the tank dry, does he? Since he has been instructed to surface with at least 500 psi remaining in his tank, that margin of safety should be deducted from the available air supply, before calculating how long his tank will last at 60 feet (18 m). The calculation looks like this: 3,000 psi - 500 psi (safety margin) ÷ 90 = 27 minutes. This is how long he should be able to stay at 60 feet before reaching the 500-psi benchmark.
Using tank pressure (psi) to estimate how long the air supply will last is only valid if the tank being used is the same size (cubic feet) as the one used when calculating DCR and SCR. If it is a different size, estimated air time must be calculated using cubic feet per minute (CFM) (see sidebar).
We have determined that this diver will not be able to dive with the group. Or, based on the parameters presented in the briefing, he will cause everyone to reverse course and return to the boat when he reaches 1,000 psi, about 22 minutes into the dive. It must be noted, however, that this diver's estimated consumption rate is based on a constant depth of 60 feet; he will not be at 60 feet for the entire dive. The first few minutes will be spent descending, the last few ascending; he won't be breathing 90 psi per minute during those times. Even considering that, this diver faces an air consumption situation. Is there anything he can do to extend the life of his air supply?
With the concurrence of the dive guide, possibly this diver and his buddy could stay with the group but remain at a shallower depth. Once the group reaches the eels and sharks the shallower buddy team could drop down to 60 feet for a short period and then return to a shallower depth for the swim back to the boat.
If you find yourself in a situation where your estimated air consumption rate at the planned depth hints that your air supply will not last through the profile, be upfront about it. Discuss it with the divemaster. There may an alternative that will keep you both safer or at least he can keep a close eye on you during the dive.
The first time you calculate your SCR shouldn't be the last. Periodically calculate your SCR because as you gain experience and your comfort in the water increases, your air consumption will improve naturally. There are, however, a number of things you can do to accelerate the process.

The first time you calculate your SCR shouldn't be the last.
Periodically calculate your SCR because as you gain experience and your comfort in the water increases, your air consumption will improve naturally.

Terms to Remember
DCR Depth Consumption Rate; the air a diver uses at a specific depth, expressed in psi per minute.
SCR Surface Consumption Rate; the air a diver uses at the surface, expressed in psi per minute.
EDCR Estimated Depth Consumption Rate; an estimate of the air a diver will use at a specific depth, expressed in psi per minute.
EAT Estimated Air Time; the duration a diver's air supply (with a 500-psi safety margin remaining) will last at a specific depth, expressed in minutes; based on psi/minute or cubic feet/minute.
ATM Atmospheres
CFM Cubic Feet per Minute; the cubic feet of air a diver uses.
ECFM Estimated Cubic Feet per Minute; an estimate of the cubic feet of air a diver will use at a specific depth.

Calculating Air Consumption
Hands-On Examples of Air Consumption Formulas
The examples in this column show how to calculate your DCR, SCR, EDCR and EAT using an 80 cubic foot tank filled to 3,000 psi.
DCR = air used ÷ time at a constant depth
Example for a dive at 66 ft (3 ATM):
DCR = 1,000 psi used ÷ 10 minutes = 100 psi/minute
SCR = DCR ÷ atmospheres of constant depth
Example using the diver's previous DCR:
SCR = 100 psi/minute ÷ 3 ATM = 33 psi/minute
EDCR = SCR x ATM of planned depth
Example for dive planned to 99 feet (4 ATM)
using the diver's previous SCR:
EDCR = 33 psi per minute x 4 ATM =
132 psi/minute @ 99 feet
EAT = (Tank pressure - safety margin) ÷ EDCR
Example:
EAT = (3,000 - 500) ÷ 132 = 19 minutes