DROPPING INTO THE FRIGID WATERS of a freshwater lake in winter is something I’ve never relished. The reality of living in the UK is that the local lake is often our only option, but I am the guy who first tried out the Thermalution heated undervest in the some-would-say balmy waters of Grenada in the Caribbean.
Am I a man or a mouse “Squeak!”
The undervest worked well then, under my wetsuit. It’s designed to do so, and the maker claims that the electrics stay watertight to 100m deep, although I never tested that claim.
At the time I mused that it would be more appropriate under a drysuit, but of course there was the problem of routeing the controller in such a way that the drysuit stayed dry inside.
Well, the Taiwanese manufacturer has come up with a solution in the Yellow Grade Advanced Undervest. You wear it in exactly the same way, but the controller is wirelessly linked to the vest, and you can mount it on your wrist.
Before you get visions of being fried alive, I should point out that when I tried it without a wetsuit it was a dismal failure, as all the heat it produced was conducted away by the water.
It needs to operate within the micro-climate produced either by the thin layer of water that gets trapped in your wetsuit, or the thin layer of air trapped in a drysuit.

How It Works
We all know by now that it is the air in a drysuit that insulates us from the cold. The drysuit forms a barrier to the water, and the undersuit simply holds that barrier away from your skin. It is the air within the undersuit that does the “keeping you warm” bit, and the Thermalution heated undervest merely has to warm that small amount of air.
But it’s more clever than that. It uses what is called “Far Infrared technology” to radiate long wavelengths of energy into your body.
Sounds a bit dangerous, doesn’t it The maker describes it as therapeutic. All I can tell you is that the vest does appear to heat up very quickly when you’re wearing it.
The vest is of a flexible man-made fibre like Lycra, and has a non-metallic heating panel built into the back. Two pockets take the twin battery-packs, which are the size of a fag-packet.
The batteries are factory-sealed lithium-polymer and designed to be watertight under a wetsuit at 100m deep, so I guess they’ll offer no problem under the leakiest drysuit, and my O’Three fits me like a glove and doesn’t let in a drop of water.
The battery-packs are connected to the vest via cables with wet-connectors. Strangely, the designers have put the receiver unit for the wireless connection in the small of the back.
I’d have thought it would have been more reliable through proximity to the controller at the front but, hey, what do I know
So you wear the vest under your undersuit, and the controller straps onto a forearm. There are three progressive settings and an LED indicator that goes from green through orange to red according to the setting.
To confirm that the receiver is working, it vibrates for a second, twice for the mid setting, and three times for the highest heat setting.
It gives a single long vibration to confirm that it’s shutting down. It’s a bit like having a small mouse inside your suit with you.
Some might say it feels like having a small fart in your drysuit, but I wouldn’t know about that.

Charging
The battery life of the controller is around 18 months so I guess that will be an ongoing expense when the time comes to replace it, because it too is factory-sealed.
The main batteries are simply disconnected from the suit and reconnected to a multi-voltage intelligent charger. They take around eight hours to fully charge, and are claimed to be good for around 500 recharges.
A fully charged pair of batteries are said to last for between three and four hours, so I guess that should be enough to suit even the most adventurous diver.

In The Water
I used the Thermalution undervest for a leisurely swim round Wraysbury Lake last January. I wore it over a Merino wool base layer and under a Bare Mid Layer undersuit.
The vest heated up within a few minutes, and at times I felt really hot. More to the point,
I never felt really cold.
The water on my hands and face felt icy at first, but because the rest of me was able to get really warm within my drysuit my circulation was soon ensuring that I was comfortably warm at my extremities as well as my core. Even my inhaled gas failed to chill me.
The remote-control switch looked a bit fiddly, but even using the heaviest dry-gloves I could find I was still able to manipulate it easily.
I fiddled with the heat settings and enjoyed the sensation of the “mouse” telling me it was doing something in response to the radio signal.
At times I actually got too hot even at the lowest setting, and resorted to turning the vest off for a few minutes just to cool down a bit.
I think this undervest is something you would use throughout a dive rather than merely turning it on during a long hang because you’re beginning to get chilled, but you might well switch it off altogether for periods.
At around a kilo, you can pack the whole kit for travelling with impunity and get a lot more use out of it by combining it perhaps with a thinner wetsuit than you would otherwise take when travelling to a warmer climate.
I thought of this as I gazed at the frigid water in Wraysbury Lake and reflected that my dad would never have a heater in his car “in case he got used to it”. Well, I’m getting used to this.

COMPARABLE PRODUCTS (BUT NOT INTENDED FOR USE WITH WETSUITS):
O’Three HUV: £710
Typhoon Icebreaker: £499
Typhoon Heated Waistbelt: £119
Silent Planet Heated Vest: £354 plus battery-pack

SPECS
PRICE £499
SIZES S/M, XL/XXL
DEPTH RATING 100m
DURATION 3-4hr
CONTROL three settings, wireless
BATTERY 2 x 7.4V li-polymer 4400mAH
RECHARGE TIME 8hr
CONTACT www.pdeuk.com
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