Combining an UNBALANCED 1st stage with a BALANCED 2nd-stage, what would I experience

[scubafanatic]

140 is fine for the MK2; it can drop almost 20 psi over the supply range, so if you want 125 with a near empty tank, you would set it at 145-150 at 3000 PSI. Pairing a MK2 with a 109 is one of the most bulletproof, abuse-tolerant regulators you could ever get. As far as depth goes, I've had my MK2 down to around 130 feet and it worked fine. If you want to check out the difference between a balanced and unbalanced 2nd stage over the supply range on the MK2, just go on a dive with one converted 109 and one left unbalanced, both tuned for best performance at the top range of the IP. With a full tank you wouldn't tell any difference between the two 2nd stages, but as the tank empties you'd find a subtle but increasing difference on direct comparison. My experience with the MK2 is that you really notice the breathing getting stiffer under 500PSI but not really before. And it still provides plenty of air, just the cracking effort increases.

If I were to only use my BA upgraded 2nds on the MK 5, how do I tune the 2nds to match the possible 20 psi drop in IP as tank pressure falls ? Not really an issue with my balanced MK 5's as their IP is stable throughout the tank psi range but the MK 2 may be a different story.

 

[halocline]

If your 2nd stages are converted to balanced, they'll work fine with IP from 125-150. I suspect you understand that being balanced means they divert IP air to push back against the poppet along with the small mechanical spring. So as IP fluctuates, so does the opposing force on the seat, although not quite completely. You'll still feel a noticeable difference on direct comparison between 125 and 150 driving the same balanced 2nd stage, but nothing like the difference with unbalanced 2nds.

I set my MK5s low, usually anywhere from 125-130. But if you were using unbalanced 109s back and forth between the MK2 and the MK, you'd want to set the MK5 IP relatively high and adjust the 2nd stages accordingly.

BTW, you IP on the MK5 is not entirely stable through the supply range. It can be 7-10PSI higher at a full 3000 PSI than at 500. I think this is mostly due to friction in the HP o-ring, and I've found polyurethane 90 duro o-rings help mitigate that somewhat. On the other hand, I had a bunch of polyurethane o-rings go bad in storage, now I don't buy them anymore and put up with the slightly less extrusion-resistant EPDM o-rings. Those last forever.

There's also the fact that even in a dead on straight piston with a sharp knife edge, there is some small amount of surface area for HP air to push on, meaning there is a small downstream force on the piston, resulting in IP drop over supply range. I've read here that MK25 pistons are slightly flared to counteract that, but I've never really measured that so I don't know. The MK25 also has the bushings to prevent o-ring extrusion.

I guess the safest thing to do in all situations with piston 1st stages is to adjust your 2nd stages for best performance at a full tank.

 

[Luis H]

Be aware that if your MK-2 is true vintage, like from the 70’s, the typical Scubapro DIN adapter will not work.

The newer Mk-2 have a male threaded yoke retainer similar to the Scubapro balanced first stages and they share the same DIN adapter. The earlier Mk-2 had a yoke nut retainer and I don’t believe that there was ever a DIN adapter made for that style of body.

If you unscrew the yoke, you will see what I am talking about. You can also download the two different diagrams from VintageDoubleHose.com or I can email them to you.

At least in 1977 when I wrote a letter to Scubapro for a European threaded connection adapter (it was not called DIN back then), they didn’t have a clue of what I was talking about. I was specifically asking for an adapter for my Mk-5, but the same would apply to the Mk-2. In the 70's no one was really using DIN connections in the US. I still have a carbon copy of my letter and the letter from Scubapro.

---------- Post added March 3rd, 2015 at 07:42 PM ----------

If you do in fact have a DIN adapter for the old style Mk-2, I would like to see a picture of it. It would have to look like a Conshelf DIN adapter or similar. The regulator would stick out a bit. I would be interested in knowing if they ever made one.

Thanks

 

[scubafanatic]

Be aware that if your MK-2 is true vintage, like from the 70’s, the typical Scubapro DIN adapter will not work.

The newer Mk-2 have a male threaded yoke retainer similar to the Scubapro balanced first stages and they share the same DIN adapter. The earlier Mk-2 had a yoke nut retainer and I don’t believe that there was ever a DIN adapter made for that style of body.

If you unscrew the yoke, you will see what I am talking about. You can also download the two different diagrams from VintageDoubleHose.com or I can email them to you.

At least in 1977 when I wrote a letter to Scubapro for a European threaded connection adapter (it was not called DIN back then), they didn’t have a clue of what I was talking about. I was specifically asking for an adapter for my Mk-5, but the same would apply to the Mk-2. In the 70's no one was really using DIN connections in the US. I still have a carbon copy of my letter and the letter from Scubapro.

---------- Post added March 3rd, 2015 at 07:42 PM ----------

If you do in fact have a DIN adapter for the old style Mk-2, I would like to see a picture of it. It would have to look like a Conshelf DIN adapter or similar. The regulator would stick out a bit. I would be interested in knowing if they ever made one.

Thanks

While my 'new' DIN adapter hasn't arrived just yet, the SB part # = 10.043.030

The seller says it fits : Scubapro Mark 2, Mark 3, Mark 200 DIN Adapter Fits 200 BAR Valve New This multiple model Scubapro DIN Adapter is designed to convert the standard yoke system to the 200 BAR DIN system. This adapter fits the Mark 2, Mark 3, and Mark 200 First Stages. This DIN conversion kit is designed for use at a working pressure not to exceed 3300 psi (232 BAR) with the above first stages.

 

[Luis H]

Hopefully you have a newer MK-2.

Notice the difference between Items 13 and 14 in the vintage Mk-2 versus items 7 and 15 in the newer Mk-2.

I don’t know when they changed the design, but I think it was in the 80’s. Does anyone know?


This is what the vintage MK-2 looked like:

Here is a diagram of the vintage Mk-2:

This is what the new Mk-2 looks like:

This is a Mk-5 for reference:
Item 27 is similar to the modern yoke retainer on the newer Mk-2.

 

[scubafanatic]

Hi Luis, based on your photos, I definitely have a vintage MK 2, so at this point I don't know for sure if my DIN adapter would fit. I did just buy a spare vintage MK 2 'body' , and I was easily able to screw the DIN adapter into it tonight, and the parts seemed made for each other with zero effort involved, so from that perspective it appears I may well actually have a real vintage MK 2 DIN adapter.
I'll probably get the entire 'almost mint' MK 2 in the mail tomorrow (Dallas ice storm tonight permitting) so I can visually (from the outside at least) compare the entire MK 2 unit from one seller with the NIB separate DIN adapter and MK 2 'body' from a different seller to see if I have a rare part that shouldn't exist!