The Mk18 is basically a Mk16 with a swivel turret. Both of these regulators have now been discontinued by Scubapro, hence the discounted price. The current equivalent regulators are the Mk19, with swivel turret, and Mk17 without. All of the above regulators are of the balanced diaphragm design. The current models, Mk19 and Mk17, having finned casings, are claimed to be better suited to cold water than the discontinued models.

The Mk25 is a balanced piston design which, theoretically, is capable of providing higher air delivery than any of the above diaphragm designs. However, in practice, air delivery to the diver is limited by the 2nd stage performance, which is invariably much less than the 1st stage. Scubapro quote the air delivery of the Mk16 and Mk17 1st stages as being > 5000 l/min; the Mk18 and Mk19 should be the same, the top of the range Mk25 is given as > 8500 l/min. By comparison, the top performing X650 2nd stage has a quoted air delivery of > 2000 l/min, the slightly cheaper S600 >1850 l/min and the bog standard R295 > 1400 l/min

I would guess that the Mk18 should easily meet all your needs unless you are planning to dive in really cold water, in which case you may be better off considering the current Scubapro Mk17 and Mk19 models or looking elsewhere, Apeks perhaps.

 

For a twinset the Mk17 or Mk25 offer very good hose routing.

I believe that the Mk19 has a port at the end of the turret like the Mk25. If so, it would offer similar routing ability but would stick up a bit more and be at more risk of getting clattered on those tight squeezes.

If you want a diaphragm reg, stick with the Mk17 (or Apeks DS4)

 

thanks for the info! I was originally going to buy a mk17 already until i saw the prices for mk18s.

I might still get the mk18 to start with since the price of a mk18+s600 is the same as a S600! so pretty much paying for the reg and getting the mk18 on top.

Since I was going to get second s600 to replace my R390 as well, as I find a very significant difference in level of comfort in breathing between the two.

 

this may or may not be of interest, but i was considering some new regs recently and on balance, if i'd gone scubapro would have gone for a Mk11. can see no reason why not. they're smaller then the mk17 and although, not designed to be used at extremely low temperatures, they are fine down to 4 degrees C. cheap too

 

Since I was going to get second s600 to replace my R390 as well, as I find a very significant difference in level of comfort in breathing between the two.

The S600 is a balanced 2nd stage regulator, there is a narrow passage down the centre of the poppet which feeds air into a small balance chamber. This reduces the level of spring force required to keep the valve closed. The R390 is a conventional downstream type valve without any balance chamber and thus requires a higher spring load. Theoretically the S600 will require less breathing effort. However, with its lighter spring the S600 can become more susceptible to free flow. The simpler less fiddly design of the R390 is often preferred, especially as an octo, and it can be fixed just about anywhere. I do agree with you though that the R390 does not have a particularly smooth delivery, but it is very robust.

I have a Scubapro Mk16 with R390 2nd stage & R295 octo which I use for diving abroad (read warm). Nice and light as well. I have 2 x Apeks DS4 and XTX 40 for UK waters.