Scuba Pro Mark 5 seat question

[pasadenatim]

I've been servicing my Mark 5, and recently acquired an NOS parts kit that contained 3 HP seats. The dimensions of the cone indentations varied on all the seats. I read that this was another means to adjust the IP? Currently, I'm using 3 shims to attain an IP of 135. Does anyone know the use of these seats and how I may be able to raise the IP with one? Thanks for any help.

 

[Couv]

You are correct that the 3 different seats can be used as a quick method of adjusting the IP. Measure the seats with a dial caliper or place the seats on a table with the cave cone up and compare their heights. The shortest one will produce the greatest IP while the longest will produce the lowest IP. The reason for this is that the piston must travel farther for the shorter one, thus compressing the spring more. You can expect approximately 5 psi change between each seat graduation.

Simply remove the seat retainer from the first stage body; remove the old seat and replace it with one of your new seats. Tip: Lubricate the new seat to make it easier to replace if the IP is not where you want it.

BTW: 135 psi is right on target for the IP.

 

[pasadenatim]

Thanks for the reply couv. That makes sense now. I'll give that a try. Like I said, I had to use the maximum of 3 shims to get the IP up to 135. Will try the new seats on the next service without the shims to see where the IP falls. Thanks again.

 

[DA Aquamaster]

The piston springs in the Mk 5 (and Mk 10) get slightly stiffer with age, so in addition to adding the concave surface for improved flow rate, Scubapro came out with a set of three seats, two of which are taller, to enable technicians to lower the IP on older regulators without the need for new piston springs.

 

[redacted]

I would be thinking of getting a decade or more of service for your Mk5 from those 3 seats. First, I would not mess with your current seat until IP heads north of 145. Then I would use the tallest seat (lowest IP) with no shims. I would be happy with 125 and add shims if lower than that. I would not hesitate to use 4 shims if necessary. Once you install and remove a seat from the carrier, it is unlikely to provide satisfactory service again.

 

[Couv]

The piston springs in the Mk 5 (and Mk 10) get slightly stiffer with age, so in addition to adding the concave surface for improved flow rate, Scubapro came out with a set of three seats, two of which are taller, to enable technicians to lower the IP on older regulators without the need for new piston springs.

Thanks Larry, good to see you again in the DIY forum. Perhaps you can clarify the meaning of the markings on the back of the seats. To me, they look like (-II) for the lowest IP, (-I) for the next highest, and (+) for the highest IP. However, others have posted that the orientation of the seats as I view them is incorrect (for instance -II is actually an equal sign turned 90 degrees.) The order and interpretation I use is based on the size, but I only have one kit and I'm comparing them while still in the package.

At any rate, Pasadenatim, AWAP hit the nail on the head. No reason to start adjusting the IP until you have reason to do so. SP gives a pretty wide range (125-145 psi) at low and high tank pressure so conserve your parts until needed.

 

[pasadenatim]

All, Thanks for the replies. Sounds good. I'm happy with the IP and performance and will take the advice of servicing when necessary and the mixed use of the shims and different seats to get the desired IP. Great info.

 

[Zung]

I remember there were some "colorful" discussions between DA and another guy. I tried searching for it but couldn't find it; I guess it's got deleted. Instead, I found some measurements I did, once upon a time:

• Trident A242: 10.4 mm

• SP "-II": 10.3 mm

• SP "-I": 10.0 mm

• SP "+": 9.6 mm

In addition, there's also the option of using a shim from a MK20/25 kit and put it on the seat retainer for a few PSI more. It's slightly bigger and fits well there.

 

[DA Aquamaster]

Thanks Larry, good to see you again in the DIY forum. Perhaps you can clarify the meaning of the markings on the back of the seats. To me, they look like (-II) for the lowest IP, (-I) for the next highest, and (+) for the highest IP. However, others have posted that the orientation of the seats as I view them is incorrect (for instance -II is actually an equal sign turned 90 degrees.) The order and interpretation I use is based on the size, but I only have one kit and I'm comparing them while still in the package.

At any rate, Pasadenatim, AWAP hit the nail on the head. No reason to start adjusting the IP until you have reason to do so. SP gives a pretty wide range (125-145 psi) at low and high tank pressure so conserve your parts until needed.

I look it the markings on the seat like you were stacking something under the seat.

+ is the "normal" low height.
-I is slightly higher, like the seat was sitting on a plate, and
-II is higher still, like the seat was sitting on a pair of plates.

In this case the "plates" are each about 0.3 mm thick.

Higher seats = lower IP.

In addition, there's also the option of using a shim from a MK20/25 kit and put it on the seat retainer for a few PSI more. It's slightly bigger and fits well there.

Yes, one thick shim or two thin ones will (sorta) fit, but I don't really recommend it as it results in the o-ring being less than fully captured. I have one on the seat retainer of a Mk 8 that I'm using to run a potato cannon to get the Ip a bit higher - but if the o-ring fails in that application no one has to bail out to an alternate gas supply.

 

[redacted]

I have had some success adding a shim between the body and the seat retainer to get another 5 psi of IP. But it can compromise the seal on the seat retainer. If it seals, it will work. I have also cut a disk from shim material to insert under the hp seat to drop IP about 5 psi with less disassembly. That works too but you must be careful to get the seat reinstalled in the retainer in the same position so as not to cut another groove.

I started doing this crap because seats were hard to come by. Now I do it just beause it works.

Scubapro USA sucks.

Love the EU consumer protection laws (and ebay).