# Breaking the Rule of Thirds: Gas Management for Sidemount

Third in, third out, third for reserve. Simple enough, but what about when your gas supply is split into separate cylinders? This takes a little more attention, but it’s easy once you get used to it.

Ok, so we’re not “breaking” the rule of thirds, more like adding another element to it. The main difference in gas management on sidemount vs back mounted doubles is the fact that you have completely independent gas supplies to manage, and you need to be sure to maintain an adequate reserve in both cylinders for an emergency. Imagine you have a failure in one of your tanks; it would be costly to realize you may not have enough gas in the other to make it to the exit. Also, in order to maintain proper trim, you need to keep the cylinders’ buoyancy characteristics similar (especially when diving aluminum bottles). In order to accomplish this, we need to focus on keeping our gas supplies balanced.

For an easy example, we will look at cylinders with a working pressure of 205bar/3000psi. Following the rule of thirds, we need to be back on the surface with 70bar/1000psi remaining in each cylinder. In order to keep the cylinders balanced, we like to keep them within 35 bar/500psi of each other. To do this with the fewest regulator changes as possible we first breathe cylinder “A” down to 175bar/2500psi, then switch and breathe cylinder “B” down to 140bar/2000psi. At this point, switch back to cylinder “A” and breathe it down to 140bar/2000psi. We are now at our turn pressure of 140bar/2000psi on both cylinders, so we turn the dive remaining on cylinder “A.” Now, at the furthest point from the entrance, we should still have enough gas in either cylinder to make it to the surface in the event of a catastrophic failure. Stay on cylinder “A” until you reach 105bar/1500psi, switch to “B” and breathe it down to 70bar/1000psi. You should be close to your exit now and able to switch back to “A” to finish the remaining portion of the dive, surfacing with at least 70bar/1000psi in each cylinder. We have effectively maintained our 1/3 reserve for ourselves as well as a teammate in the event of a failure at any point in the dive and also kept our cylinders balanced and in proper trim and only had to switch second stages 4 times.

As you can see, sidemount gas management is essentially the same as using back mounted doubles; it just requires a little more attention. Keeping the cylinders within 35bar/500psi of each other and following the rule of thirds will mean you should always have enough gas remaining in EITHER cylinder alone to get you to the exit. With a touch of additional focus, this is easy to maintain.

Below are a few of the TDI sidemount/cave courses offered.
TDI Sidemount Diver >
TDI Cavern Diver >
TDI Intro to Cave Diver >
TDI Full Cave Diver >

Contact SDI TDI and ERDI

Tel: 888.778.9073 | 207.729.4201

Email: Worldhq@tdisdi.com

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