Is the mk25 superior to the mk17???

[DA Aquamaster]

IP is short for intermediate pressure, the 1st stage should deliver air at a certain pressure, 140 psi is normal, it can vary some, the second stage steps that down to current psi for your depth.

I am very much interested in hearing how a 4 port non swivel 1st stage routes hoses better than 5 port swivel 1st stage does, even willing to view diagrams... really

There are two situations where the Mk 17 hose routing is superior (in my opinion). With doubles you have two first stages, both are mounted on their sides with a high pressure port and 2 low pressure LP ports facing down.

From the reg on the left tank, the SPG will route down the left side of diver as will the dry suit inflator hose. The back up second stage will route down and make a 90 degree bend to the right to cross over the top of the right shoulder where it secures under the chin with a bungee around your neck.

From the reg on the right tank, the primary second stage (on a 7' hose) will route down the right side of the divers body, run under the light cannister on the right hip, curve up across the chest, over the left shoulder and around the back of the neck to the divers mouth. The inflator hose will routere straight down, make a 90 degree bend to the left and go over the left shoulder to the inflator.

I can probably dig up a picture later, but all the hose route straight down and/or cross over in front of and well below the manifold and stay very close to the body creating very ;ittle drag and minimal line traps.

The Mk 25 routing is similar, but cants the reg to a 45 degree angle to utilize the port on the end of the turret and one on the sides of the turret to ensure the LP hoses are aslo routed more or less down rather than out away from the diver. In my opinion, the Mk 17 hose routing is cleaner.

The Mk 17 is also superb for single tank hose routing if and only if you can limit the configuration to 2 LP hoses. For example on my travel reg, I will again set the first stage on it;s side and route the hoises straight down. I will route a 7' long hose down the right side of the body, the SPG down the left side and the inflator hosedown and then over the left shoulder to an AIR 2 to provide both octo and inflator. The resulting hose routing is exceptionally clean and the reduced number of hoses makes it very travel freindly.

Alternatively you can use a conventional octo and keep the Mk 17 on it's side by routing the back up reg as you would with doubles and then use a 90 degree degree elbow to route the inflator hose from the "top" of the Mk 17 forward to the inflator and still keep the hose routing clean even with 3 LP hoses.

In the event you use a regular octo and no elbow you need 3 LP hoses and the MK 17 then has to be turned vertically with hoses popping out all over to each side. In that case the Mk 25 with it's turret is superior.

 

[Belce]

My current config with the Mk25/A700 is up/down and standard high pressure ports up, lp ports down. Would it make sense to have the low pressure port on the bottom route to my regulator over my right shoulder, the hoses for my dry suit and air2 route from the two up ports on my left and have the wireless transmitter for my computer point down as well as it is attached to the high pressure port?

Thanks

 

[ScubaInChicago]

From the reg on the left tank, the SPG will route down the left side of diver as will the dry suit inflator hose. The back up second stage will route down and make a 90 degree bend to the right to cross over the top of the right shoulder where it secures under the chin with a bungee around your neck.QUOTE]

I'm using twin Mk25's on 45 degree angles and the L/ tank post I have a 24" LP hose off the bottom port to the bungeed reg. My question is; Do you still use the same 24" hose or go to a 26"/28" hose to make up for the travel distance on the Mk17? I've tried the 22" hose on the current setup and felt like I needed to twist my head to the right to be comfortable.

Thanks,
Dan

 

[gmudiver]

This thread has helped out a lot. Thanks again everyone.

by the way I also started to read about the mk19. its frustrating that the US gets cut out of a lot of things, it use to just annoy me when Europe got cars we didn't now I see that it extends to the scuba industry as well

 

[Belce]

The job of the 1st stage in a 2 stage regulator system is provide enough air to all the things that need air such as the bcd and/or drysuit and provide enough air at a consistent pressure to the second stage reg. At a normal intermediate pressure of 140 psi that is amost 10 atmospheres. At a depth of 130 feet or 40 meters that is an absolute pressure of 5 atmospheres. As long as your 1st stage can provide volume, you will find it easy to draw on as long as the pressure is less than intermediate.

A balanced 1st stage provides air with tank pressure applied to both sides of the diaphragm or piston, an unbalanced has tank pressure applied to only one side. When you dive an unbalanced 1st stage it has a 3000 psi assist to your breathing when full and a 500 psi assist when at reserve and therefore an unbalanced 1st stage will become harder to breathe on when it gets low. Regardless of depth dived the 1st stage will provide a consistent intermediate pressure and enough volume for your needs. As the intermediate pressure in the all the hoses from your low pressure ports greatly exceeds the absolute pressure you dive in there is no reason for it to vary with depth. In fact your second stage is able to work better if provided with enough volume at a consistent pressure that exceeds ambient.

 

[Brian Sharpe]

ScubaPro seems the market the mk25 as a higher preforming reg, with the mk17 coming in second. And it seems a lot of people if they have both use it as their first choice.

I know the mk25 is piston, and the mk17 is a diaphragm and environmentally sealed.

To me the mk17 seems to have a wider temperature range in which you can dive it in, and also because its sealed would be more reliable due to keeping out contaminates...right?

Exactly the conversation I was having with my LDS yesterday, based on conditions here in Eastern Ontario his recommendation was the Mk17 (cold water, lots of silt & crud in the water) and that's what he uses for his rentals.

 

[tregrrr]

Simply not correct at all.

Yes the Mk2 is unbalanced BUT it is still depth compensating. Its IP wil be 140psi +- above ambient regardless of depth.
***snip***

Simply not correct at all.

Yes the Mk2 is unbalanced BUT it is still depth compensating. Its IP wil be 140psi +- above ambient regardless of depth.

The difference between a balanced and unbalanced 1st stage is that a balanced 1st stage will give a constant IP regardless of tank pressure while with a unbalanced 1st stage (piston) the Ip will drop as the tank pressure drops.


Thank you, Packhorse. You saved me some typing. Some people do not seem to understand this concept.

well its also unfortunate that the people who "don't understand this concept" be the ones teaching gear servicing... My bad assuming a CD was right.

The part that confounds me still is when in a pinch while working a warm water island, with no access to my brand's distribution / service, I had my preferred 1st stage crap out after 400 dives so I swapped in a freshly serviced Mk2 from the rental fleet. I know first hand it gets thicker to breathe at 150' off a mk2 piston than a off my usual 'balanced diaphragm.'

Care to explain how at 3300psi starting tank pressures there can be such disparity in inhalation effort?? The 150' was always at the start of dive when affixing a descent line for guests to follow to a wreck. I always started with a fresh tank and the depth was always the same (+/- 16 inches due to tides) as was water temp and most every other variable I can think of EXCEPT the first stage reg. This is what gave me an "ah-ha" moment, and cemented the apparently flawed teaching into my mind; the information I originally posted seemed to support and make perfect sense in light of actual dive experience and in my mind gave the theory credibility enough to post in the first place.

 

[rigdiver]

Is one of your 1st stages a "flow through" piston vs the other not being such?

 

[tregrrr]

Is one of your 1st stages a "flow through" piston vs the other not being such?

SP Mk2... maybe a 2000-2004 vintage at the oldest.

as for being a "flow through" piston, I assume that is the official term. I am now having to doubt a bunch of my so called training since apparently I was taught wrong on a number of issues.

bleh

 

[DA Aquamaster]

The Mk 2 is a "flow by" piston design as the seat is located on the bottom of the piston stem and the orifice sits under it. When you inhale, a small bleed hole in the side of the piston stem just above the seat allows the drop in air pressure in the IP secrtion of the reg due to the inhalation to be passed to the chamber above the piston head and that drop in pressure lifts the piston stem/seat off the orifice and the air then "flows by" the seat - essentially squeezing past the gap between the orifice and seat. It is not the most responsive design as the air pressure change is indirectly transferred to the compression chamber above the piston head.

Also, since it is an unbalanced design, the downstream pressure from the tank acts directly on the seat providing more opening assistance with a full tank, and as a result, a higher intermediate pressure. When combined with an unbalanced second stage, this change in IP means the second stage can be adjusted to breathe very easy at full tank pressures but will breathe slightly harder as the tank pressure falls. The decreased tank pressure reduces the asssitance in opening and in turn causes the IP to decrease. This reduced Ip then in turn reduces the downstream force providing assistance to the second stage and the inhalation effort rises.

More importantly in terms of brute performance, the size of the orifice is limited. The larger the area of the orifice, the greater the change in downstream assist and the greater the IP change. A larger piston head can reduce the change in Ip, but there are practical limits. As the pressure drops from 3300 psi to 300 psi the IP will drop as much as 25 psi and it ill create a noticeable increase in inhalation effort.

At depth, it is the smaller orifice and lower flow rate that create the increase in inhalation effort. Even with a fairly good SAC of .6, you are breathing 3.3 cfm at 150'. But you are not inhaling evenly over 60 seconds, rather in four or five 2-3 second inhalations per minute, so that 3 cfm is flowing in just 8 to 15 seconds creating flow rates up to 25 SCFM and potentially even higher if you are breathing hard. The higher flow rate begins to make itself felt in higher IP drop on inhalation in the first stage but also in the second stage as the max performance of the second stage begins to be reached. In most cases the second stage on an unbalanced first stage is lower performing than on a balanced first stage, and 30 SCFM is pretty close to maximum performance. A high perfromance balanced second stage on the other hand can deliver anywhere from 50 to 75 SCFM.

So...at depths below 130' you can quickly begin out breathing many unbalanced first and second stage combinations where a balanced first and second stage would still deliver lots of gas.