Research

Which is Cheaper? OC versus CCR

Posted on

e71a85_0bb4ad836beb44febec4c30344bfca26_mv2

 

A few times a year, Add Helium gives a presentation titled ‘Why would I want a rebreather’ to groups of people interested in learning about the sport.

Cost of a rebreather is one of the most common reasons people steer away from them. The steep price tag often keeps people from seriously considering buying a rebreather, and is now a topic we address in our presentations by showing  a cost breakdown and comparison to diving open circuit.

It’s widely known that the cost of doing a closed-circuit helium dive is less than the cost of an open circuit dive, but to a layman, the actual difference isn’t that obvious. I mean, a rebreather is expensive, right? Plus the costs of cell replacement, and maintenance, CO2 absorbent? These are all things which add to the cost of a rebreather’s annual expenses. So what’s the bottom line? At the end of the year is it more or less expensive to dive a rebreather? Let’s take a look.

For those unaware, the cost of diving a rebreather does not vary based on depth, so a 30-foot dive costs the same as a dive to 400 feet. How can that be? Well the way the rebreather works, we only need our diluent gas to compensate for changes in the loop volume when descending. Once we’ve reached our target depth, the only gas that needs to be added is oxygen – which trickles in at the same rate our bodies metabolize it. On average this is very little – about 3/4 of a liter per minute, or .02 cubic feet per minute. So really the amount of diluent a rebreather diver needs, is the amount needed to get to their target depth, with a little bit of a safety reserve. For this comparison we’ll use a standard tanks size in rebreather diving – the 3-liter. 3-liter tanks come in a few different types, (steel or aluminum, low pressure or high pressure) but generally hold between 19 and 23 cubic feet of gas – more than enough to get to 330 feet and beyond.

The cost of diving a rebreather is broken down by the hour. 1 helium diluent fill, and 1 oxygen fill cost $10, and will last the diver about 3 hours. The amount of absorbent a rebreather uses will vary depending on make and model, but they are all generally the same. For example on a rEvo Rebreather, 1 absorbent cassette holds 3lbs of absorbent, and will give the user 3 hours of dive time. Sofnolime absorbent costs $129 for a 44lb jug, which breaks down to about $3.25 per hour. For those following with their calculators, We’ve accounted 4lbs for waste, and spillage. This means most rebreathers cost about $7 per hour to dive.

Now we all know that the cost of diving open circuit however, does vary based on depth. The deeper we dive, the more gas we need to carry, and the more helium we need to use in the mix. Also the more decompression gas we need to bring.

So let’s do a few comparisons shall we?

Here’s a 1-hour dive on a 60-foot reef:
Open Circuit Closed Circuit
36% Nitrox Fill – $7 Air diluent & O2 fill – $10 (3 hour duration)
Cost per dive – $7 Absorbent fill – $10 (3 hour duration)
Cost per dive – $7

Not much savings here. Although we could argue that we could save $4 or $5 by using an air fill in our rebreather diluent bottle, we won’t. Let’s take a look at something deeper, and do a 1-hour dive on our favorite South Florida shipwreck, the Hydro Atlantic. Thy Hydro sits in 172 feet, and 1-hour dives are common on her.
Open Circuit Closed Circuit
21/35 trimix fill – $56 Helium diluent & O2 fill – $10 (3 hour duration)
50% deco gas fill – $14 Absorbent fill – $10 (3 hour duration)
Cost per dive – $70 Cost per dive – $7

Wow, that really kicked it up a notch. Why? Because the amount of helium being used. Helium is an expensive gas, currently selling retail for between $1-2 per cubic foot, and in some areas much more. In our comparison, the open circuit diver is using around 70 cubic feet of helium, while our rebreather diver is using about 4 cubic feet.

Let’s look at another example. Here is a 90-minute dive on the Lowrance, another SoFla favorite, which sits in 209 feet of water.

Open Circuit Closed Circuit
18/45 trimix fill – $90 Helium diluent & O2 fill – $10 (3 hour duration)
50% deco gas fill – $14 Absorbent fill – $10 (3 hour duration)
Oxygen fill – $14 Cost per dive – $10.00
Cost per dive – $118

Even more money. The deeper the dive, the more helium is used, both in volume, and percentage of the gas mix, and makes for expensive an expensive dive for our open circuit diver.

Finally, let’s take a look at the RBJ – one of our premiere wreck sites here. The RBJ sits in 270 feet of water, and runtimes of 90 minutes are common.

Open Circuit Closed Circuit

15/55 trimix fill – $110 Helium diluent & O2 fill – $10 (3 hour duration)
32/20 travel gas fill – $12 Absorbent fill – $10 (3 hour duration)
50% deco gas fill – $14 Cost per dive – $10.00
Oxygen fill – $14
Cost per dive – $150

Here in South Florida, we are in the water pretty much every weekend. We grabbed the logbook of one of our divers (yes, he keeps a meticulous logbook) to get a more in-depth look. In the past 12 months of diving, this diver performed 126 dives on these wrecks, or dive sites in similar depth ranges. Of those dives:

36 were above 130 feet
44 were 130-200 feet
36 were 200-250 feet
10 were deeper than 250 feet

When we figure out the cost of these dives over the course of a year, we get the following:

Open Circuit Closed Circuit

$11,484 $2,638.44

That’s a pretty big difference! That’s almost enough your entire rebreather investment in the first year!

But wait, that figure doesn’t account for all the maintenance expenses you incur on a rebreather. Ah ha! So when we examine this diver’s logbook over the past 12 months, we see he purchased 3 oxygen cells, and a pair of mushroom valves, totaling $362.82. Hmm, that’s not a lot.

So nay-sayers, or people critical of this article will make arguments that it’s biased towards the rebreather, that the amount of gas you’d actually have to purchase to conduct these dives on open-circuit would be less. I once heard the argument, that the open-circut diver never drained gas, he had 8 sets of doubles, and 5 or 6 stage bottles he kept in his garage with various mixes. I pointed out, the value of the bottles he kept alone, was close to the cost of a rebreather, and that he still was paying more per dive.

1-2The numbers can be skewed in many ways – you could use less gas, or dive less, but the simple fact remains that when doing helium based dives, the cost savings of diving a rebreather is significant.

One could also point out the figures used for rebreather diving were artificially high, and many rebreather divers pump their own diluent and oxygen.

For those who make regular adventures into the ‘chipmunk voice club,’ the savings is substantial. How substantial is ultimately up to you, and how much helium diving you plan to do.

Keep in mind, these cost figures are also based on 2011 helium prices, and as anyone in the market will tell you, the cost of helium has risen dramatically since then. These figures if anything, are very conservative for the open circuit diver.

So be sure to show this page to your significant other when considering a rebreather purchase.

Safe diving.