Q for reg techs- pressures

[donclaypool]

I'm checking my cracking pressure and I/P on my old MK5 Scubapro's. 1.5" cracking and 110# DYNAMIC I/P. Just sitting there it runs up to 140-150 psi quickly but holds 110# on deep inhalation. I thought I should be holding 125 psi with nothing more than a small bounce in pressure on inhalation. Do I need to get the hp seat replaced and pressure adjusted? Would like to do it myself. (Yes I'm competent) Comments? This "old" reg doesn't have a High Flow port for your primary second , does it?
Thanks,
Don

 

[cortez]

I'm checking my cracking pressure and I/P on my old MK5 Scubapro's. 1.5" cracking and 110# DYNAMIC I/P. Just sitting there it runs up to 140-150 psi quickly but holds 110# on deep inhalation. I thought I should be holding 125 psi with nothing more than a small bounce in pressure on inhalation. Do I need to get the hp seat replaced and pressure adjusted? Would like to do it myself. (Yes I'm competent) Comments? This "old" reg doesn't have a High Flow port for your primary second , does it?
Thanks,
Don

With many Scubapro piston regulators you will see some IP drift. What you are describing sounds more severe than that. I have never worked on a Mk5, but checking HP seat would be a good place to start. I don't know that the seat is necessarily bad, it may just be positioned wrong. You may also want to check any spring positioning and tension. Beyond that trying taking a look at the HP orifice to see if there is any maring or damage that could account for the IP swing.

Don't want to preach, but its always best to have it serviced by a certified tech. They have knowledge of product/part improvements that may not have leaked in to the public domain, so to speak.

 

[redacted]

IP spec is 125 to 145. You can't control what it will drop to when you inhale and I assume that is the 110 figure. IP set higher than 145 is unnecessary and can lead to freeflows. A common HP seat failure mode is the IP creaps uncontrolably above 150 until the 2nd starts to freeflow around 160 psi. Withy a new seat, the IP should lock up steady quickly when you stop inhaling. With a used seat, you might experience a 5 to 10 psi creap after the initial recovery before it locks up. Lowering the IP requires the ambient chamber to be opened, remove the HP seat retainer & seat, piston removed and the removal of one or more washers (5 psi per washer). Some special tools are required but they can be fabricated and purchased. Newer style seat adjusts the IP by changing the height of the seat installed (kit come with 3 different seats).

 

[donclaypool]

Yep, Awap, that's just what she does. Just a little creep but stops at 150. I''ll keep an eye on it.
Thanks for the replies.
don

 

[DA Aquamaster]

The Mk 5 has a knife edge seating surface on the piston stem and should lock up with no creep once a seating groove is established in the HP seat.

Some very minor creep is normal until the seat is broken in. Once the seat is broken in (25-50 cycles) and the purge is released or the inhalation is stopped, the needle on the IP guage will swing rapidly to the IP and lock up. ANY creep at all is excessive for this reg once the seating groove is established in the seat.

The IP range for a MK 5 is 125-145 psi and it should remain within that range with supply pressures of 300 to 3000 psi. A 4-5 psi drop in IP is normal as tank pressure falls from 3000 to 300 psi.

The Mk 5 and Mk 10 seem to have problems with IP increasing as the main springs age. In the past, once all the shims were remoived there was not much else to do other than replace the mainspring. But now new seats of differring heights are included in the rebuild kits and a taller seat can be installed to lower the IP.

The IP swing on inhalation depends on a couple of factors with the flow rate fo the second stage being the key factor. A high performance second stage will flow more air and create more swing than a low performance second stage.

 

[rescuediver009]

I think that it is likely the hp seat seating issue but one thing that can affect this is your supply pressure, or the tank not on all the way. That much of a drop just shouldn't be. I have to agree with cortez though....

Don't want to preach, but its always best to have it serviced by a certified tech. They have knowledge of product/part improvements that may not have leaked in to the public domain, so to speak.

 

[DA Aquamaster]

I have a new old stock Mk 5 Adjustable that I picked up from a dive shop that we bought out when the owner retired after 30 years in business. I completely serviced it late this spring, upgraded the "Adjustable" second stage to the latest "Balanced Adjustable" configuration and have not used it since and just ran some tests on it this evening.

At 1000 psi supply pressure I have an IP of 142 psi and a swing all the way down to 112 psi on a heavy rapid inhalation. When fully depressing the purge, the IP drops all the way to 105 psi. This is with an essentially new Mk 5 Balanced Adjustable that is in as good a condition now as it was when it left the factory in the late 70's.

The IP drop Don is reporting is entirely normal for the Mk 5. The creep however is not, as there should be no creep at all in a Mk 5 with a properly functioning HP seat.

 

[rescuediver009]

I would think that this is because of the really high flow rate that is higher than the recovery rate of the reg...

 

[DA Aquamaster]

I would think that this is because of the really high flow rate that is higher than the recovery rate of the reg...

That is the logical conclusion but I wondered how much was too much and if a current configuration BA was excessive compared to other second stages it would have been paired with. So I did some more testing of the Mk 5 with a variety of other older second stages that it would have potentially been paired with during it's career including:

Scubapro High Performance (080)
Scubapro Air 1
Scubapro D300
Scubapro D400
(I'd have loved to try a Pilot second stage, but I don't own one.)

I also tested some other second stages of the same general vintage:

US Divers Aquarius
US Divers Conshelf XII
Sherwood second stage

As well as a couple others I just had laying around:

Dacor Viper Octo
Dacor Pacer Aero

During a full sustained depression of the purge button most of them showed a pressure drop from the normal IP of 142 to 110 with the High Performance dropping to 115 psi and the D300 and D400 dropping to 105 psi.

The Dacor regs stood out as the Viper only showed a pressure drop to 125 psi and had noticeably less airflow. (not my first choice for a second stage as it is apparently a real dog compared to everything else made in the last 30 years)

However the relatively inexpensive late 80's vintage Pacer Aero showed a drop to 105 psi for the first second or so then fell sharply to 90 psi. It apparently has some interesting venturi effects occurring that really open up the valve once things get cooking.

But depending on the Scubapro second stage attached, an IP drop of 30-40 psi was encountered with the Mk 5 and I would consequently conclude that this was pretty normal for the Mk 5. All tests were done at a supply pressure of 1000 psi.

SP used to publish a variety of test data in their catalogs and I remember comparing the specs of the Mk 5 and Mk 10 when I bought my first reg in 1983. I recall the Mk 10 having fairly even flow rates at high and low pressures with 110 SCFM at 3000 psi and 90 SCFM at 500 psi while the Mk 5 fell from 105 SCFM at 3000 psi to 64 SCFM at 500 psi.

The Mk 5 had a stellar reputation as a high performance workhorse at the time (and still does) and the Mk 10 was relatively new and still not fully trusted. But the MK 10 had much better performance at lower tank pressures. It was enough to sell me on the Mk 10.