Scubapro D300 upgrade to S600 or A700 Query

[ams511]

Although haven't tried the 620ti I'd agree 100% with the rest and I'll add a G250 and 156 to the list. Just finished a dive with my D300, beautiful, effortless, even with a pesky slow leak. I'll throw out one minor negative about the D series, occasional bubble interference, certainly I notice it more than the G250/G260/156.

There was some guy on eBay selling a D400 with a D350 a MK-20 UL (brown version) and I think an ancient computer. I had on my watch list and then it showed up as sold for under $100. I have no idea how I missed it. The guy must have lowered the buy it now price.

 

[halocline]

All of the high performance SP 2nd stages I've used (pilot, converted pilot, air1, 109, balanced/adjustable,G250, G250HP, S600, D300, D400) have their own characteristics. It's really a matter of opinion as to which is 'better'.

The pilot feels very different from the rest, and the center-balanced valves (converted pilot through D400) have a different feel from the conventional 2nd stages. Overall IMO the best performing one I own, in terms of easy and natural breathing, is the converted pilot. But if I had to choose one to own and get rid of all the rest, it would be balanced/adjustable due to ease of service and bulletproof reliability, and the breathing performance is almost as good as the center balanced valves.

I use the D300 quite a bit in my cave set because it can be disassembled in the water without tools (the value of which is really questionable, but I still do it) and I really like the way they breathe. I also have lots of parts for them and it's very easy for me to keep them maintained.

 

[ams511]

The pilot feels very different from the rest, and the center-balanced valves (converted pilot through D400) have a different feel from the conventional 2nd stages. Overall IMO the best performing one I own, in terms of easy and natural breathing, is the converted pilot.

Why would a converted pilot be breathe better than the Air 1?

 

[halocline]

Why would a converted pilot be breathe better than the Air 1?

I think it has to do with the metal case, although that's a guess. I'm also talking about a very small sample size; 2 converted pilots and maybe 4 air1s. There's a certain smoothness and stability with the converted pilot that I find more enjoyable than the air1.

I notice a similar difference in feel between a well-tuned balanced/adjustable and a G250.

 

[craydive]

Thanks for all the advice.
Have looked at the secondhand market and scored a mk10 + D300 to match my existing mk10+D300. The second D300 reg is in great order, looks like it has been stored for a long time, and very little use when it was used.
Also scored for a backup reg, an unused mk17+s600. Will need an contents gauge for this one. Between the three regs, I reckon that will keep me going for a long time now. Have also purchased some neoprene rubber glue, and will try to seal my original D300 exahust valve rubber, where it was occasionally resting on my chin and leaving a black mark.

 

[craydive]

My mind tells me that the most problems I have had with regs is the second stage, either free flowing or being hard to breath. Both conditions are alignment issues with the second stage.

Questions
1] As it is my second stages that cause more issues, would it be a reasonable idea to have a couple of second stages aligned for the one first stage, so a quick change on site could be effected if required.

2] I am wondering, but need to be corrected, is that the Mk10 1st stage has a small plastic disc (shim) that sets the intermediate line pressure, by the shim thickness. I am wondering if the shim gets indented slightly over time (storage) and hence the intermediate line pressure drops a little over time, and potentially causing misalignment with the second stage.

3] If the shim does cause a small drop in line pressure, with the indenting, would it be better to use the mk17 that I have as my main 1st stage.

 

[halocline]

My mind tells me that the most problems I have had with regs is the second stage, either free flowing or being hard to breath. Both conditions are alignment issues with the second stage.

Questions
1] As it is my second stages that cause more issues, would it be a reasonable idea to have a couple of second stages aligned for the one first stage, so a quick change on site could be effected if required.

2] I am wondering, but need to be corrected, is that the Mk10 1st stage has a small plastic disc (shim) that sets the intermediate line pressure, by the shim thickness. I am wondering if the shim gets indented slightly over time (storage) and hence the intermediate line pressure drops a little over time, and potentially causing misalignment with the second stage.

3] If the shim does cause a small drop in line pressure, with the indenting, would it be better to use the mk17 that I have as my main 1st stage.

I also find that 2nd stages require more frequent adjustment than 1st stages, but I tune my 2nd stages very lightly so that they are close to free flow. This results in occasional small leaks that I can easily adjust out.

The MK10 1st stage shims don't typically compress and the IP stability is very good. I have routinely serviced MK10s, let them sit for years, and then used them. (I have several so they don't all get used often) The main spring in 1st stages is not under significant compression while the reg is not pressurized.

The MK17 is not fully balanced, and as such, the IP rises as tank pressure drops, actually a significant (8-10 PSI?) amount. Although the MK10 is a balanced design, there is always some friction in the high pressure o-ring and a small amount of downstream bias in the piston edge, so the MK10 IP drops somewhat as tank pressure drops. ( maybe 5 PSI)

So either way you will have more variance in IP over the course of a single dive than you would by leaving the reg in storage for years. Fortunately, the D series 2nd stages have only a slight downstream bias, which means they are very tolerant of changes in IP. So I wouldn't be too concerned about 'aligning' (we generally say 'tuning') each 2nd stage to a specific 1st stage. Just aim for a good middle ground with IP on the 1st stages, maybe 135 at full tank with the MK10 and 135 at near empty tank with the MK17, tune your D350s to that, and enjoy your diving!

 

[buddhasummer]

I have a couple of MK17s, one swings around 12psi. I have not noticed any difference from full to empty tank with a G250, 156, G260 or a D300. The ip swing does however irk me somewhat, it shouldn't but it does.

 

[craydive]

Thanks again. A answer that explains the question very well. I didn't realise the mk17 varied that much, and for that matter, the mk10 not varying that much.

It is also true that I hadn't serviced my original mk10/D300 for a number of years, and then used it in April this year, without any problems. I know the IP was down a little, but the D300 breathed a treat. I had measured the IP before I left to go diving overseas, and did think the shim might have been the cause of the lower IP, but to be honest, I don't actually know what the IP was a number of years ago. It could have been low then.

I will service my reg, and just go diving. I will also service the other mk10/D300 I have, and a third D400 I recently picked up (a week ago). The D400 is a bit of a mix with the D300 exhaust cover used. It is still in the post, so don't know the condition of it.

This site is expensive. It seems I have only really had one reg for decades, and now suddenly I have a fleet of them.

Thank you for being generous with your time when answering my questions

 

[buddhasummer]

Early D400s I believe, came with the same covers as the D300. I have two.