There seems to be a tendency for newer sidemount divers to think all, or at least most, cylinders are created equal. This isn’t really the case. Cylinders, whether sidemounted or backmounted, will have some very similar results when it comes to different volumes. However, when it comes to buoyancy characteristics, cylinders will vary.
Let’s look choosing your set of sidemount cylinders. Just as in backmounted, manifolded doubles, you must pair match your cylinders. It not a matter of making sure they are the same length – a 1/4 inch difference in length won’t make much difference in sidemount. Rather, it’s the buoyancy characteristics of the cylinders. I had a student that bought a pair of cylinders but ended up buying one cylinder from one location and the other from a different location. With a quick glance, the cylinders appeared identical. The differences did not become evident until during the first dive.
The first dive of my sidemount courses consists of having my students get in the water, getting neutrally buoyant, bending their knees, and relaxing. This way I can look at how their bodies hover and cant and make adjustments to the cylinder rigging so they end up horizontal. This particular student was consistently lower on one side. The rigging was identical on both sides. After we got out of the water, we examined his sidemount rig and the shoulder straps were evenly spaced on the waist strap. Nothing was off kilter and he shouldn’t have been off kilter either. Then I looked at his cylinders. One had a much more defined transition from the body of the cylinder to the neck while the other was a much more gradual transition. I looked at the manufacture dates and they were different. We placed the cylinders in the water with both having very similar pressures and they even sat differently in the water. Their buoyancy characteristics were different enough that they caused quite a significant difference in trim in the water. Lesson learned – pair match your cylinders.
Let’s look at the next mistake often made by newer divers – bigger cylinders will mean more bottom time. This isn’t necessarily true. Often times, bigger cylinders simply mean more drag in the water, which in turn means more effort to move through the water. More effort equates to higher gas consumption. Bigger cylinders could actually mean less bottom time due to this. When I first went back to backmount after a year of strictly diving sidemount I grabbed a set of LP108s. A couple days later I decided to try a different set of cylinders, a set of LP85s. I reached my turn pressure in each set of cylinders at the same location in the cave. That means I used almost 20 cubic feet less on the same dive using the 85s than I did using the 108s. The additional drag and subsequent effort of moving the larger and heavier cylinders resulted in a much higher gas consumption.
The cylinders I prefer for a majority of my diving are LP85s, HP100s, and LP95s. They are all on the smaller sider but provide sufficient gas to do decent length dives. However, even these cylinders are not created equally. Cylinders will have different buoyancy characteristics dependent on the manufacturer. This is the case whether they are the smaller cylinders listed here or the larger cylinders such as LP108s and LP 120s.
Let’s look at the LP95s first. I dive Faber LP95s. I’ve handled PST LP 95s and they are too heavy for my liking. While they are the same dimensions as the Fabers, they do weigh in heavier out of the water and have more negative buoyancy in the water. This makes them a little more difficult to handle, especially when having to remove a cylinder to pass through a restriction. The Faber LP95s are less negative and much easier to handle in the water. LP95s are also shorter than LP85s and HP100s, 2 inches shorter. I bring this up because it does make a difference. The Faber cylinders, because they are neutral when empty, have a tendency to have the bottoms roll up (see photos below) as their pressures decrease. With the Faber LP95s, this doesn’t become noticeable until they reach around 1600 psi. By this time most cave divers are already doing decompression. I did make the mistake recently of staying on my LP95s on the way out of a cave instead of switching back to my stage cylinder. At the end of the dive my cylinder pressures were at 1300 psi and I could definitely feel the cylinders rolling up. I will also take this time to say I do not use Fabers for any type of trimix diving. I did that once and found myself looking for additional weight at the end of the dive. The lower weight of the helium along with heavier undergarments made a difference I wasn’t expecting.
For LP85s and HP100s, I prefer Worthington cylinders. Because these cylinders are 3/4 of an inch less in diameter, they are easier to handle in the water than their LP95 counterparts. The Worthington LP85s and HP100s still end up being slightly more negative than even the Faber LP95s, but it’s a minimal difference that the smaller diameter almost negates. The Faber LP85s and HP100s are even less negative. The issue with this is when they get to about 2200 psi these cylinder begin to roll up along their attachment points. By the time they are at 1600 psi, they are extremely out of trim which in turn makes the diver extremely out of trim. The extra 2 inches in length makes enough of a difference to create this change in them. I’ve had people tell me this isn’t true. They are correct…if they are not using the loop bungee system to secure the valves of their cylinders in their armpits. Every Faber LP85 and HP100 I’ve seen in sidemount configuration does what you see in the photos below when used with the loop bungees. Why not use a different bungee system? I haven’t been able to find one that works well for trimming out cylinders properly and being easy to set up. Ring bungees need to be too far back making them difficult to pull up and attach to the cylinders. Wrap around bungees tend to let the valves fall too much below the diver. So for me the obvious choice is to use Worthington LP85s or HP100s for the majority of my dives in which I want the smaller cylinders or use Faber LP85s or HP100s when I know my pressure at the end of the dive will be no less than 2000 psi. What I choose isn’t necessarily the best choice for everyone. But if you’re in the market for new cylinders my advice is to try before you buy. Get in the water with the cylinders. Take them on a real dive and to various pressures. And get some photos of yourself from various angles so you can see for yourself what those cylinders are doing.
I hope this information helps you in your quest for buying cylinders. And if you have bought a set of cylinders that aren’t working for you, contact me. Even if they’re Faber LP85s I may just buy them because they do make the best backmounted cylinders for me.
Full Faber LP85s trimmed decently
Same diver later in the dive with Faber LP85s with about 1600 psi
Faber LP85s with about 2200 psi with roll up just beginning – notice how the cylinder is positioned in relation to the cord
Faber LP85s with about 1600 psi showing a significant roll up – notice the difference in the cylinder position compared to the previous photo