The idea that no buoyancy/volume compensating air should get trapped inside the undersuit seems to have been abandoned now.
We took to using thinsulates and other high-tech fibre garments to maintain that space, with a layer of insulating air between our warm bodies and the cold outer wall of the drysuit.
That is the function of the undersuit; to maintain an air gap between you and the drysuit. That layer of air does the insulating.
However, anyone who has been on a PADI Instructor Development Course will know that kneeling stationary under water for long periods as required results in all this insulating air migrating to the upper shoulders, leaving your legs to freeze. When I did an IDC in Malta in February many years ago, I swapped from a drysuit to a thick semi-dry for this very reason.
If only there was some way of keeping the air evenly spread inside the drysuit.

A Thoughtful Solution
The boys at Fourth Element have given this problem some thought. They considered the diver swimming horizontally (some would insist that this is the only way to dive) and realised that, because air rises to the highest point, precious little would remain on the lower side, on the chest and stomach, for example.
Heat is then lost through conduction through the fibres of the undersuit and the outer wall of the drysuit.
So they designed an undersuit that had a thick layer of tough insulation at what would be the front when standing up and the lower surface when swimming horizontally in a drysuit.
The “Spacetek” material on the chest and thighs of this undersuit is made from a material that is 8mm thick and, while flexible, is not easily compressed.
The fact that it doesn’t compress during a dive reduces the conductive heat-loss, and keeps the diver warmer for longer. The undersuit is called the Halo 3D.

Real-World Diving
In my world, divers are not always horizontal, so this feature doesn’t really help those kneeling or otherwise upright for long time, such as divers who prefer to assume that position during long waits on shotlines. For this reason, the suit’s insulation is further enhanced in other areas.
I don’t have the advantage that some others do in carrying a lot of natural bioprene. My skinny arms suffer from the cold, as does the rest of my body.
So I was pleased to discover that the inside of the forearms has an extra layer of thermal fleece, this being one area that experiences blood-flow close to the skin surface. Blood flowing out towards the hands is less likely to be cooled if this part of the body is better insulated.
The back of the Halo 3D has additional fleece layers. with a further extra layer across the kidneys, another site of high blood-flow close to the surface of the body.

Extra Features
The suit has a long, double-headed zip with external cover, and two P-valve holes, one in each thigh, so that a diver may decide which side is the more convenient for routeing the tube.
These holes are not fully opened, however, so those who don’t use them won’t have gaping holes on the inner thigh.
A sleeve pocket on the right arm will take cash, a key or other valuables, though you’d need to be confident of the watertightness of your drysuit before stowing an electronic car key there.
The manufacturer recommends that the machine-washable Halo 3D is worn with a base layer such as its Xerotherm, Drybase or something similar. I used my trusty Icebreaker Merino wool base layer.

In the Water
If I hoped that the muscle-bound profile of the undersuit would show through a membrane drysuit and make me look especially manly, I was disappointed.
And because the padding is so incompressible, one important problem became immediately apparent.
I needed three or four kilos more lead than normal to get under the water, and a body of my dimensions already displaces a lot of water when wearing a conventional squashed undersuit.
The Halo 3D certainly did its job and kept me very warm. However, I got exceptionally sweaty once fully equipped to dive simply in the process of getting to the water – thanks to all that extra lead I was carrying!

COMPARABLE UNDERSUITS TO CONSIDER:
Weezle Extreme, £172
O’Three PBB Extreme, £170
Scubapro Vancouver, £140

SPECS
PRICE £300
SIZES 11 sizes from S to XXXL
CONTACT DIVER GUIDE width=100% width=100% width=100% width=100% width=100% width=100% width=100% width=100% width=100% width=100%