The simplistic idea is that as BC stands for 'buoyancy compensator', that is what should be used. Air in the suit to compensate for squeeze and air in the BC for buoyancy control, runs the argument. It is as if its advocates are unaware of why the BC was invented in the first place. People are incompressible. It doesn't matter how deep you go, you keep the same dimensions. The same cannot be said of your neoprene wetsuit. As you go deeper, the integral gas-cells within it get compressed and so displace less water. The effect is to make you progressively heavier. So if you use a wetsuit (or semi-dry), you add air to a BC to make up the difference. If your suit could be kept at constant volume, this effect would not occur. Putting air into your drysuit as you go deeper keeps it at constant volume, so your buoyancy is controlled. The BC then becomes redundant and for use only as additional surface support or as an emergency buoyancy device. This assumes that you're diving within a reasonable margin of neutral buoyancy when you enter the water. A lot of people use far too much lead to 'get under'. If you organise your weights so that your eyes break the surface when you inhale and you sink when you exhale, you will need only to exhale in order to start a descent. 'Ah,' some of you say, 'I need a lot of lead because I use a neoprene drysuit.' Well the Typhoon TCS is a neoprene drysuit and I found that, although there is definite compression at depth, down to 40m it is not enough to need to call the BC into play. That's because it's made from 3mm neoprene that has already been compressed.
Thinner undersuit The good thing about neoprene is that it has some insulating properties of its own, so you can use a thinner undersuit. Use it in the Red Sea in winter and you probably need only a T-shirt. It keeps you as warm as a wetsuit but, of course, you are still dry when you climb out of it. At the same time, it is much more sleek than a membrane drysuit. I found I could swim as unimpeded as in a normal semi-dry. At Wraysbury Lake I used the full Weezle undersuit, which squeezes down to fit the available space. Securely zipped up, I strode out purposefully. My will faltered only when I looked at the icy water and decided that a dip was beyond the call of duty, however good the suit was going to be! The Typhoon TCS is a pretty standard design. It has a cross-shoulder zip hidden under a neoprene flap, neoprene wrist seals and a latex neck seal protected from the flush of cold water by a neoprene collar. It has strong rubber boots lined with neoprene and tough rubber knee-pads. There are no-slip patches at the shoulders. A zipped thigh pocket is big enough for a small reel but not big enough to act as a sea-anchor. I have always been tall and skinny and my first drysuit enveloped me in folds of material, because to get the height I had to buy one that fitted someone with a 52in chest. Manufacturers have now got a handle on the new generation of people (I was obviously one of the first) and I now find I am a standard size, even though clothes I bought in the '70s still fit me (I'm waiting for the fashions to come back.) The TCS fitted perfectly.
State of equilibrium I used the suit with a 15 litre steel tank and minimal-size wing-style BC for emergency buoyancy and surface support. Did I put air in the BC for buoyancy control? No. I kept the suit neutral by inflating it via the Apex rotating inflation valve at the centre of the chest on descents, and vented it using the Apeks automatic constant-volume valve at the shoulder, being careful to keep this at the highest point on ascent. So how do you achieve this perfect state of equilibrium? It's simple. Get your buoyancy right before you start, and bear in mind the loss of weight due to the gas being exhaled during the dive. The average weight of breathing gas consumed during a dive with a standard single tank is about 2kg. If you use multiple tanks and expect to consume a fair weight of gas on the dive, you will need to put on extra lead to compensate for losing this weight. In that case, you might well need to add gas to the BC in addition to the suit. The Typhoon TCS comes in a wide range of eight off-the-peg sizes and costs£499.