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 One version of the new Divex AH5 freeflow helmet is designed for use with a drysuit, the other version with a traditional neck seal. When UCI Editor John Bevan test-dived them both, he was delighted with what he found.
The venue was Divex"s expanding headquarters at Westhill. By a strange coincidence, this is located on a hill to the west of Aberdeen. There is no Easthill. Unless you call that Norway. The dive tank is tastefully sited to provide an attractive feature in the Divex reception area. As a visitor you are often treated to a live underwater performance while the helpful receptionist sets up your meeting. It"s a sort of human aquarium, and much more atmospheric than the fishy aquariums you usually see at the dentist"s. Stealth I was quite familiar with the tank, having previously used it to test-dive the Divex Stealth rebreather. I knew my way around it, and up and down it. My first stop, however, was in the state-of-the-art Training Room where Managing Director Derek Clark summarised his view of Divex"s enviable position in the marketplace and the latest major expansion of the premises at Westhill. "We want to achieve the same reputation that Siebe Gorman had during their long existence," he added. This was followed by a detailed briefing by Sales Engineer Richard Bird on the development of the AH series of helmets. Nobody was better-qualified than Richard to explain the AH5, having been involved with the AH helmet since its first appearance in 1969 when he worked for Underwater Instrumentation. The helmet was actually launched at the first Oceanology Exhibition in Brighton. Now, 861 helmets later, Richard was delighted to explain how the design of the helmet had been progressively improved over the years. I also met Bill Elrick, the new Divex Training Officer. An ex-RN CPO Diver and HSE Part II Diver, Bill pointed out: "We have upgraded the Divex portfolio of training courses, and we now offer excellent training for "competent persons" and diving technicians". It was Bill"s job to supervise my dives, and it was comforting to know that I was in such capable hands. Once familiarised with the theory and design of the AH5, the next step was to see how it performed in the water. The constant-volume suit/ helmet arrangement was my first exercise. The helmet is locked to a neck ring which is integral to the suit. The air in the suit therefore communicates with the air in the helmet. This means that you can adjust the buoyancy of the suit by playing with the exhaust and/or the inlet valve on the helmet. For comfort and safety, as with any drysuit arrangement, the amount of air in the suit should normally be minimised. One of the nice aspects of the constant-volume arrangement, especially when you are first getting used to balancing the airflow through the helmet, is that any excessive use of the valves simply inflates or deflates the suit, without any appreciable change in internal pressure. Locked Once the helmet was locked on, I found it remarkably easy to adjust the inlet and outlet flows to give me a comfortable air flow. The position of the inlet valve had been slightly changed to offset it from the helmet body a little more. This made it even easier to use. This would be more important if you were wearing gloves. The streamlined aluminium-bronze neck-ring assembly with a robust safety interlock was a very nicely designed feature. The adjustment of the "jock-strap", or perhaps more correctly the "jock-harness", was important to keep the helmet down on the shoulders. If this was left loose, the small amount of air in the upper part of the suit could tend to lift the helmet off the shoulders and slightly reduce your downward vision. Once in the water, it was also easy to change the adjustment of the strap to optimise the helmet position. Under water, two things immediately struck me. First was the wonderful ease of breathing. The other was the huge field of view through the large face-plate. Comfortable It was so comfortable that within seconds I had virtually forgotten that I was wearing a helmet at all. It was as if I was wearing nothing on my head. I then tried all sorts of positions to see if the buoyancy distribution of the suit might cause any instability. I could find no problem at all. A knock valve on the inside of the helmet allowed me hands-off venting of air whenever I needed to make myself momentarily more negative. Because there is a small positive air pressure within the helmet, the possibility of water ingress is negligible. Richard Bird explained: "The bonus here is that the helmet is well suited to working in polluted water. The diver can remain perfectly dry, and there is no possibility of the outside water entering the breathing circuit. There is no oral-nasal mask, and the air vents away at the back of the helmet. "The polluted-water application is an extremely important feature of the con- stant-volume suit/helmet arrangement, not least because it is relatively inexpensive. The internal suit overpressure can be increased when the polluted water application is a primary requirement. This even makes the helmet suitable for nuclear diving". In the past, one of the criticisms of freeflow helmets has been the relatively high noise caused by the constant flow of gas, and this is one of the areas where Divex has made significant improvements. The air pathway within the AH5 helmet has been simplified, making it much quieter. This has involved moving the air inlet from the back of the helmet to a redesigned manifold block on the side. Air-flow When asked after the dive what I thought of the air-flow noise, I had to confess that I hadn"t noticed any "noise", and two-way voice communications with the surface were as clear as a bell. However, when a free-flow helmet is taken into deeper water, the additional air flow does slightly increase the noise, so in order to ensure compliance with governmental health requirements, Divex has provided sound-exposure tables in the helmet manual to advise on the maximum exposure times for various depths. Neck-seal My next dive was to try the same helmet but this time with the neck-seal facility. I couldn"t help thinking, as I pushed my head through the neck seal, that this is what it must feel like to be born. I decided, however, that it wasn"t healthy to pursue that line of thought .... The performance with the neck seal was predictably similar, but there were a couple of interesting differences. The first was the balancing of the inlet and outlet valves, for any excessive mis-balancing caused a slight increase or decrease of pressure in the helmet. This was slightly disconcerting initially, but there was no need for concern because it is not possible to over-pressurise the helmet. Even with the exhaust valve wound down completely, air still vents freely. You can feel the neck dam gently moving up or down if the balance is not quite right. So, after just 30 seconds or so playing with the valves, I was able to establish balanced flows at high or low rates at will. The other difference of course was, at least with a drysuit, that the adjustment of the volume of air in the suit was controlled using the suit"s own inlet and exhaust valves. Within moments of getting into the water, I again felt unaware of the presence of the helmet. It really gave a great feeling of freedom of head movement and ease of breathing. As I moved around again, trying different positions to test for any change in the helmet"s performance, the helmet carried on working with no noticeable difference, regardless of how my body was positioned. Ladder The dives over all too soon, I climbed back up the ladder, and Bob Thompson, Senior Technician, helped Bill Elrick with the de-kitting under the discerning eye of Richard Bird. We discussed the helmet"s performance, and, frankly, I could not fault it. It is robust, simple, comfortable, light-weight, and a dream to use. It has a superb pedigree, and the latest improvements in air flow, noise reduction and valve operation take it as close to perfection as you can get. Richard Bird added: "Very importantly for future sales in Europe, the AH5 has been awarded its CE mark". In my opinion the AH5 has to be the leader in its class. With a surprisingly low purchase price and low maintenance requirement, it is undoubtedly going to be at least as successful as its popular predecessors. Richard also pointed out that: "For the many current owners of AH3s and AH4s out there, Divex can easily upgrade their helmets to AH5s". Details - Divex Ltd, Enterprise Drive, Westhill, Aberdeen, AB32 6TQ, UK, Tel: +44 (0)1224 740145, Fax: +44 (0)1224 740172, E-mail: info@divex.co.uk, Website: www.divex.co.uk |
© 2002 Underwater World Publications Ltd.