Now let’s look at Joe’s situation. His SAC (worked out several years ago) is 0.5 cubic feet per minute. Referring to his logbook he sees that Saturday’s dive is going to have a maximum depth of about 149 feet (yes, Joe has dove the site many times before). His task to work out the ambient pressure is a little more complex than Vlada’s. Working in imperial requires depth to be divided by 33 and then one to be added to bring the figure to an absolute value. Working this out Joe gets (149/33) + 1 = 5.5 atmospheres… that seems to check out with his buddy’s calculation.

On previous dives, Joe has used a DF of 1.8 so he settles on an RMV of 0.5 x 5.5 x 1.8 = 4.95 cubic feet per minute. He does a quick conversion from feet to litres by multiplying his RMV by 28 (the number of litres in a cubic foot) and arrives at a metric equivalent RMV of 138.6 litres, which he rounds up to 140. So far, so good because his figures seem to corroborate Vlada’s, and he would expect his consumption to be higher than hers even though he is diving in familiar territory.

The next step would be to plan around the apex dive based on the available volume of gas in the smallest capacity cylinders in the team. Actually, the guideline for this step reads that the diver carrying the least gas becomes the control for the rest of the team. This is critical and is called gas matching.

A common error in gas planning is for the team to ignore this step. Matching a “gas pig” wearing the biggest tanks made with a light breather carrying much smaller capacity tanks is nuts if no allowance for gas matching is made. If “Big Ted” can empty a set of “regular-sized” cylinders in ten minutes, the temptation is for him to buy the biggest tanks he can carry. That’s fine if everyone has the same sized cylinders but creates a horrible situation if Big Ted’s buddy has tiny tanks (because they are “good on air”) and something goes wrong on a dive. Well, actually, if what goes wrong is that Ted’s buddy needs air, they may be OK. But the spirits that control things going pear-shaped on dives are mischievous little buggers and work the worse possible options. Conceivably, if Big Ted’s massive cylinders go for a Burton, his buddy wearing tiny tanks will not be carrying enough gas to get the two of them safety to the first gas switch… and beyond.

Well, Vlada and Joe have their act together and are not going to make that mistake. They will use Vlada as the controlling diver and work with her gas volume to plan the dive for them both.

Vlada wears the smaller cylinders (double 14 litre) and her local dive shop usually pumps them to a little more than 200 bar. This means when filled she will have 14 litres X 2 (she wears doubles) X 200 bar = 5600 litres. So now Vlada knows that her fully charged double 14 litre cylinders hold about 5600 litres of gas. Since her gas planning skills are basic, she follows basic gas planning rules. The Rule of Thirds is a standard in technical diving and states that one third of starting volume of gas is for the first half of the dive, one third or less for the second half, and one third for your buddy or for you in an emergency.

The Rule of Thirds is a simple way to reserve contingency gas. For hard overhead environments, such as cave diving, it guarantees the body recovery crew will find both divers near the cave entrance. I’ll explain this rather grim conclusion in the chapter dealing with special environments, but for now let’s just say that the rule of thirds should be modified for cave diving. For soft overheads such as decompression dives, especially when there is a three-person team, it works just fine as it is. But Vlada has been correctly trained and knows with a two-person team, she needs to modify the rule of thirds by tucking away some extra reserve gas, just in case.

The simple rule she was taught is for dives with two buddies to 45 metres or less, is to put 400 litres of gas aside before calculating thirds and round down to make the starting volume easily divisible by three.

Following this rule, Vlada’s 5600 starting volume becomes 5200, but 5200 does not divide by three into round figures… but 5100 does! One third of 5100 is 1700 litres which she takes as one third of her starting gas.

Two thirds therefore will be 3400 litres and this is her usable gas volume (UGV). Her total reserve is what would be left of her starting volume after she has consumed her UGV. In this case, 5600 litres minus the 3400 litres the Rule of Thirds allows her to use. This gives her a healthy 2200 litres as a reserve… this is the last third (1700 litres) plus the additional reserve (500 litres) and for the record, belongs to her buddy Joe.

Since Vlada has 3400 litres of useable gas and her estimated RMV for the dive is 121 litres per minute, she can calculate the absolute maximum bottom time she can do in her tanks by dividing one into the other (3400/121). This indicates how many minutes her gas will last at depth. The answer is a maximum of 28 minutes.

With this in mind, she suggests to Joe that they need to plan a dive with about 20 minutes of bottom time with 25 minutes as the outside contingency. A dive of 25 minutes is what is called an Apex dive for her at this site with her current cylinder / gear set up.

Joe dives double 130 high-pressure steel cylinders. These carry about 260 cubic feet when charged to their working pressure of 3440 psi. Joe now makes a quick calculation to find out what the usable volume of gas for his dive with Vlada is in psi. This is based on Vlada’s figures of 3400 litres because SHE is the control for the dive. This translates into approximately 121 cubic feet (there are about 28 litres in one cubic foot so 3400/28 gives the conversion).

If I allowed for simple examples in my lectures, Joe would start the dive with fully charged cylinders and his next step would be to convert the controlling volume derived from Vlada’s calculations (121 cubic feet) into psi and everything would be done and dusted. Indeed, Joe would finish the dive with a huge reserve, and there’s nothing wrong with ending a dive with a lot of spare gas. However, let’s make things more interesting

Joe’s doubles are currently filled to 2200 psi with a lean trimix (20/30) left over from a previous weekend’s dive. The mix is useable for the planned depth but he needs to know: “Do I have enough gas for the planned dive?”

For divers who insist in working with imperial units, the process of converting pressure to approximate volume (an inaccurate but ubiquitous norm used by the diving community) is more complex than for divers using metric units. One additional challenge is due to the “sizing” of dive cylinders. While metric use wet volume, US manufacturers quote cylinder sizes in standard cubic feet at the cylinder’s working pressure and specific temperature. Let’s look at the challenge this presents for Joe. At their working pressure of 3442 psi his double 130s hold approximately 260 cubic feet.

For this to be useful information, he needs to know things like how many cubic feet a pressure drop of 100 psi represents and conversely the pressure drop in psi when a cubic foot of gas has been breathed. To find the answer to the first question he has to divide rated volume by working pressure in units of 100 psi. Here’s the calculation: 260/34.42 = 7.5 cubic feet per 100 psi (approximately).

This 7.5 cubic foot figure is called the cylinder baseline. Joe has this written down in his dive notes for easy reference. In fact, Joe has baseline figures for various other sizes of cylinders – doubles and singles. (See the appendix for a comparison table of popular cylinder sizes.)

To answer the second question he flips the values in the equation: 3442/260 = 13.25, which determines the pressure drop for every cubic foot of gas he breathes is 13.25 psi. To sense-check both values, Joe can multiply 13.25 by the cylinder baseline and can expect the answer to be close to 100. It is actually 99.375, and that’s close enough for his purposes. (At this point Vlada is thinking how happy she is to be working in metric but she keeps drinking green tea and resists the temptation to tell her buddy about the superior feeling she’s experiencing.)

With these figures in hand, Joe can now work out how many cubic feet of trimix are in his doubles. He can see how many times 13.25 will divide into 2200 to give him the equivalent value for cubic feet (2200 / 13.25 = 166) or he could multiply 7.5 by 22 (the number of units of 100 psi in 2200 psi), which gives him a figure of 165 cubic feet. He can work with either approximation and either way, he comes up with the same answer: Since he needs 129 to 130 cubic feet PLUS a reserve of about 70 cubic feet to match Vlada’s gas plan, he needs a fill because at the moment he is more than 35 cubic feet short.

Of course, he and Vlada could plan their dive around Joe’s starting volume since it is usual to plan a dive around the buddy with the least volume of gas, but Joe feels that it would be better for him to top off his tanks. Rather than dilute his trimix content radically by blowing his cylinders up to 3442, he opts to add just a little more than required and keep his helium percentage as rich as possible… while still only paying for an air fill. Thirty five cubic feet is 35 X 13.25 psi (about 465 psi) will do the trick and so he arranges to top off his tanks to 2700 psi. When this is done, his doubles will contain a little more than the 200 cubic feet (5600 litres) the planned dive requires.

**Turn-Around Pressures**

Now that Vlada and Joe have worked out their gas needs and planned to have sufficient gas for their planned 20-minute dive, there’s one last gas matching chore for them to take care of: working out their respective turning pressure.

Turning pressure is literally the halfway point of a diver’s bottom time. It is in fact the reading that a diver’s SPG will show when one half of their usable gas volume (one third of their starting gas volume) has been used.

When a team of divers are all using the same size cylinders charged to exactly the same pressure, working out turn pressures for each member of the team is very straightforward, since everyone has the same starting volume of gas and readings for pressure drop for everyone will be exactly the same. So, if everyone dives exactly the same tanks charged to 220 bar (about 3200 psi) a turn pressure of about 150 bar (roughly 2200 psi) would be universal for all members of the team. Just in case this is all new to you, I arrived at that turn pressure by dividing 220 bar by three… well, I cheated a little because I picked a figure lower than 220 that three went into an whole number of times (no fractions) and opted for 210, and that netted me 70. Then I subtracted 70 from the starting pressure (220), and that gave me 150 bar.

Mandating that everyone will dive similar tanks filled to the same pressure is a great way to simplify team planning and logistics. However, it does not reflect the real-world situation entirely. In the real world divers invest their money in gear that suits them and that fits in with their plans. Sometimes this corresponds exactly with what their dive buddies use but more often, cylinder sizes, brands and capacities vary. No worries. This just means that working out turn pressures is not as simple: even when fill pressures are the same, because different size cylinders return totally different volumes of gas for the same pressure drop.

For example, a five bar pressure drop in a set of double 8 litre cylinders signifies a volume of 80 litres, but in a set of double 16s, five bar represents 160 litres. Of course the same is true in the imperial world. One hundred psi in a set of aluminum 80s is about 5.2 cubic feet, but in a set of 130s, we’ve already calculated that the same pressure drop represents 7.5 cubic feet. In both cases the difference is significant enough to cause a whole heap of grief to the ill informed.

To calculate correct turning pressures for their weekend dive, Vlada and Joe do some simple maths. Both need to know – and note in their dive notebooks – how many bar or psi their spg will show when they have used one third of their starting volume of gas.

Vlada goes first. Her usable gas volume is 3400 litres. This represents about two thirds of her starting volume minus a few extra litres for additional security. Since 3400 is what she can use for her whole bottom time, 1700 litres is what she can use for half her bottom time, which is that same as saying 1700 litres is one third. To find her turning pressure, she simply works out what the pressure drop is for 1700 litres by dividing it by her cylinders baseline… or wet volume in the metric world. This is 1700 litres / 28 litres which is equal to a little more than 60 bar. Her turning pressure then is her starting pressure (200 bar) minus thirds (60 bar) which is 140bar.

Now Joe has two options. He can convert 1800 litres to cubic feet and work out his drop pressure from that converted number or he can do his calculations in pure imperial and sense-check when finished. He opts to work in imperial. From his earlier calculation Joe knows that his usable gas volume is 128.5 cubic feet. He rounds this down to 128. That’s two thirds of his starting volume and all the gas he can use for the whole bottom time. Therefore the turning point of the dive will arrive when he has used 64 cubic feet of gas. The drop pressure in his cylinders for 64 cubic feet will be 64 / 7.5 (tank baseline) X 100 psi, which equals 853 psi. This means that Joe’s turning pressure is 2700 psi minus 853 psi or 1847 psi.

Both Joe and Vlada write their respective turn pressures (and those of their buddy) in their waterproof note books. For them, the gas matching exercise is done.

In essence, this example is way more complex than need be. For instance, buddies rarely work in both metric and imperial and Joe’s calcs for topping off his bigger cylinders was a device to get you thinking correctly about matching starting gas volumes. But the basic steps are the same for all dives using the Rule of Thirds or Modified Rule of Thirds.

• Know your SAC and know your buddy’s SAC

• Guesstimate the RMV for the planned dive

• Find the controlling volume for the whole team by finding out who is starting with the least gas (volume NOT PSI or BAR)

• Put aside a sensible reserve before calculating thirds (Modified Thirds)

• Divide remaining volume by three (into thirds)

• Two thirds are for the dive and the remainder is contingency gas

• Work out how many minutes the UGV will last according to the RMV calculations. Use this number to cut tables, plan ascent times and so on

• Work out turning pressure for each team member based on one third of controlling volume

• Write down turn pressure for self and buddy

• Make note of gas volumes actually used on the dive. This will inform you how accurate your Dive Factor (DF) estimates were.

• Adjust for future dives

Now let’s talk about gas choices and drill down a little further into why Joe topped off his trimix with air!