Sherwood Maximus

[wetbehindtheear]

While I have always liked the design feature of the constant bleed system, the performance of the first stage has always left a bit to be desired. While I was serving as the Science Editor for Rodale's Scuba Diving Magazine, we tested many, many regulators on the ANSTI breathing simulator. The Sherwood first stage could never provide enough gas flow to meet the minimum standards for a US Navy Class A classification. The overall flow rate was, if my fading memory serves me right, at about 3,500 liters per minute. This is plenty of flow for the vast majority of recreational dives, but when you crank up the machine to 2.5 liters per breath and a rate of 25 breaths per minute it just can't keep up. The intermediate pressure can't rebound quickly enough to compensate when the depth gets down past recreational levels. Just for comparison, a Mares MR12 provides about 4,000 lpm with a Mares V16 going at 4,400 lpm. I would estimate that a ScubaPro MK25 with that huge flow-through piston could easily deliver 5,000 lpm...

Greg

I went out yesterday to a wreck at 100' in 34F water. 2 Divers with Sherwood Blizzards (virtually identical to the Maximus), and one with a Mares. Within 10 minutes the Mares first stage started to freeze up and his octo started to freeflow. Diver 1 assisted Diver 3 with shutting down his tank and provided him his SKO octo to breath. The sherwood first stage provided plenty of air at 100' for 2 "airhog" divers, and did NOT ice up. The Mares iced up with only one.

Of course the SKO freeflowed as soon as Diver 3 released it - a known problem with these octos when they are released face-up. The SKO is being replaced soon.

Moral of the story - The Maximus/Blizzard provides enough air for 2 air hogs at 100' and won't freeze.

And SKO octos shouldn't be trusted.

 

[Greg Barlow]

I personally now have more than 300 dives in waters that range from 34-40 degrees and have never had even the faintest hint of a problem. My regular dive buddy, who happens to be an ice diving instructor, has never had his Mares regs ice over, and his dives are often in moving water that even dips down to 30 degrees. I am a firm believer in following cold water protocol, as many free flows are caused by not doing so. As I mentioned, the Sherwood constant bleed system prevents water from entering the balancing chamber. This is a very good feature that helps to a great degree. If Sherwood engineers the design to have a greater gas flow then they will improve the performance.

I am not questioning the validity of your claims, but am making the objective point that any and ALL regulators can go into a free flow siutation. Apeks, Poseidon, Sherwood, Mares, etc., none are totally immune. Create a flow situation where the adiabatic cooling exceeds what the water temp can compensate for and all regulators can ice up. I personally never place a reg in my mouth until I am submerged when the water or air temp is below 45 degrees. I also never use more than one low pressure port at one time. When I use a LP hose for inflation purposes, it is done so in short 1-2 second burst. I also refuse to use a tank pressure in excess or 3,000psi and a IP of over 140psi to reduce the adiabatic cooling effect. The little things add up very quickly and can greatly alleviate problems. Prevention is the name of the game. I must be doing something right, or I would have had at least one problem in more than 29 years of diving.

Greg

 

[wetbehindtheear]

My main point was that the gas volume supplied by the sherwood first stage was more than enough for two divers within recreational limits, regardless of conditions.

Your cold-water diving precautions are standard and well-recognized as appropriate. One potential contributing factor in this case was that the Mares and one of the Sherwoods were on 3442 psi E8s, and neither had lowered IPs (both around 153 last service).

I was pleased with the way the two divers involved dealt with the situation - took the appropriate steps to shut off the offending reg and share gas, completing the requisite 3 minute safety stop and had even resolved the freeflow before the end of the dive. I just wish I'd remembered to keep filming. Woulda been some great footage.

 

[Greg Barlow]

I am not one to bash gear or manufacturers, it is just that when purely scientifice testing is completed the results are very interesting. A diver breathing at a fairly high effort of 37.5 liters per minute at 130' or so is at a point that basically any manufacturer's low end model will easily meet. Now, increase that to 75.0 lpm and the typical regs start falling off rather quickly. BTW, a human (even a well conditioned athelete) can't maintain that type of cardiovascular stress for more than a minute or so. That is in the range of anaroebic stress, which is equivalent of running full speed for 100 meters. Rodale's testing is done with incoming air pressure that is about half of the US Navy limit. They then increase the chamber pressure to an equivalent depth of 198' with the effort cranked up to 75.0 lpm. Some say this is an unrealistic test....Yes, it is. But, it does test a regulator to the point of where it exceeds a human's ability. To me, that is pretty comforting....

One other point, hundreds of tests have shown that a person can't discern between cracking effort on regs until that difference exceeds 0.4" of water. I want to chuckle when shop employees have a prospective buyer "check" a reg by inhaling through it in the shop. That tells you absolutely nothing except that the employee needs some further undersanding of regulator mechanics.

A high IP allows the reg to have a lower WOB, but at a price as you know. If I only made dives in tropical waters, then I would set my regs at the high end of the scale. If I made only very cold water dives, then it goes to the low end. I set at the midrange to even out the performance/reliability curve.

I can understand why you are pleased with the outcome. It sounds like you have some well trained and level headed friends. I made a dive with a guy who panicked once at a small problem. Yes, I made one dive with the guy....

Thanks for your great discussion. Safe diving!

Greg