DGX PRD (burst disk) compatible with Scubapro valve?

[Thunder Struck]

I have a Scubapro "pro" valve on an Scubapro/Faber LP95 from circa 1999 in need of a new burst disk. Scubapro service manual lists it as part # 01.024.011 which consists of a 3 piece kit, the bolt, the copper disk, and a plastic washer. Does anybody know if the burst disk assembly from DGX (a one piece burst assembly) will work as a direct replacement for the Scubapro unit?

What I thought curious was the pressure rating for the Scubapro burst disk is marked as 3400 for a 2400 psi cylinder, with a burst range listed at 3150-3500. Using the 5/3 (1.666) rating for the burst disk for service pressure (2400 psi) calls for a 4000 psi disk. DGX has exactly that, a 4000 psi rated disk assembly. IIRC the burst disks are supposed to blow at +0% to -10% of rating. The rating for the Scubapro disk is apparently outside of that. Scubapro shows part # 01.024.012 as a "4200" marked disk and rated to burst at 3960-4400 psi as their next step up.

The DGX assembly lists being a "typical" 3/8-24 UNF thread. I don't know what the Scubapro is threaded at and the manual doesn't state, although it appears to be 3/8" diameter barring being some metric variant. Was planning on stopping by the hardware store the check the thread pitch on their thread testing racks.

Scubapro lists their torque spec as 90-110 in/lbs and DGX lists theirs as 100-120 in/lbs.

DGX couldn't provide an answer, but, being the cool cats they are, stated I was welcome to order one and try it and return it if it didn't work. But, rather than risking striped threads or non-sealing surfaces, I thought I'd check with the brain-trust here first.

 

[rjack321]

I have a Scubapro "pro" valve on an Scubapro/Faber LP95 from circa 1999 in need of a new burst disk. Scubapro service manual lists it as part # 01.024.011 which consists of a 3 piece kit, the bolt, the copper disk, and a plastic washer. Does anybody know if the burst disk assembly from DGX (a one piece burst assembly) will work as a direct replacement for the Scubapro unit?

What I thought curious was the pressure rating for the Scubapro burst disk is marked as 3400 for a 2400 psi cylinder, with a burst range listed at 3150-3500. Using the 5/3 (1.666) rating for the burst disk for service pressure (2400 psi) calls for a 4000 psi disk. DGX has exactly that, a 4000 psi rated disk assembly. IIRC the burst disks are supposed to blow at +0% to -10% of rating. The rating for the Scubapro disk is apparently outside of that. Scubapro shows part # 01.024.012 as a "4200" marked disk and rated to burst at 3960-4400 psi as their next step up.

The DGX assembly lists being a "typical" 3/8-24 UNF thread. I don't know what the Scubapro is threaded at and the manual doesn't state, although it appears to be 3/8" diameter barring being some metric variant. Was planning on stopping by the hardware store the check the thread pitch on their thread testing racks.

Scubapro lists their torque spec as 90-110 in/lbs and DGX lists theirs as 100-120 in/lbs.

DGX couldn't provide an answer, but, being the cool cats they are, stated I was welcome to order one and try it and return it if it didn't work. But, rather than risking striped threads or non-sealing surfaces, I thought I'd check with the brain-trust here first.

The SP and DGX assemblies are the same threading and work interchangeably in my lone SP valve. I don't know when that SP was made though and it's in theory possible your threads are different than mine.

The SP disk is going off the metric bar pressure rating which is 182 bar * 5/3rds = 305 bar = 4423 psi. Hence the 4400 psi high end minus 10% burst. Plus or minus some minor rounding. Why they do that I dunno, to address the Canadian market where that tank is TC rated to 182 bar (2640) instead of the US market where its a 2400+ service pressure? Who knows. 3400psi is for sure a (catalog?) error as even 2250psi lp72s get a 3750 burst disk. Although those are near impossible to find so most folks slap a 4K in there as close enough.

100 in/lbs is a lot less than you'd think

 

[Thunder Struck]

The SP and DGX assemblies are the same threading and work interchangeably in my lone SP valve. I don't know when that SP was made though and it's in theory possible your threads are different than mine.

The SP disk is going off the metric bar pressure rating which is 182 bar * 5/3rds = 305 bar = 4423 psi. Hence the 4400 psi high end minus 10% burst. Plus or minus some minor rounding. Why they do that I dunno, to address the Canadian market where that tank is TC rated to 182 bar (2640) instead of the US market where its a 2400+ service pressure? Who knows. 3400psi is for sure a (catalog?) error as even 2250psi lp72s get a 3750 burst disk. Although those are near impossible to find so most folks slap a 4K in there as close enough.

100 in/lbs is a lot less than you'd think

Thank you!

I'll double check the threads at the hardware store tomorrow.

SP did have some higher pressure cylinders sold under their name as I recall, hence the need for the higher rated assembly. On that note, they also have a lower rated assembly, PN# 01.024.010 a 3000 psi disk for cylinders up to 2015 psi.

From the 1997 version of the Scubapro valve service manual (41.900.004)

I know 100 in/lb is pretty light. Funny story, when I was rebuilding a valve I went ahead and put a new burst disk assembly in it. I had the click style torque wrench set to 100 in/lb and was tightening it up. Well, what I didn't realize at first was the torque wrench was broke in that it didn't click when it hit the target. So just as I start to think, "Seems like this is bit high I wonder if there's a problem with the wre---" the head of the PRD broke off. Lovely. So, I wander down to China (Harbor) Freight to get a stuck screw extractor. Pretty easy process considering the PRD bolt is already hollow in the middle. Hah! As soon as the extractor bit started to bite into the remnants of the shaft, the extractor bit snapped, flush with the bottom of the PRD. Hauntings of my old Jr High shop teacher started to echo in my head: "Good tools ain't cheap and cheap tools ain't good." Some suggestions were provided from friends on things I could do, but, given that DGX sells a basic valve for $55, it isn't worth putting any more of my time into it.

The hunt for the new PRD assembly came about from last week I had my bottle filled at my local shop. 2640 bottle (2400 w/ + hydro) was at 2300. The new kid, wanting to be helpful offered to top it off. Why not, he can do it while I carry out and load other bottles into my truck. He asks again what it should be, I tell him 2600, which I have "2640 PSI" written in bold paint pen on the top of the cylinder just below the neck. He sets the regulator on the fill station to 2600 psi and starts filling it as I start to carry out my other cylinders. As I walk out I hear a rush of air consistent with a fill whip being left open. As I walk in there's still the rush of air going on. The owner is holding a handful of rags on the valve of my cylinder trying to muffle the sound and the kid has a real sheepish look on his face. He immediately starts to appologize. Apparently he knew that he needed to set the fill regulator to 2600 but he set to 3600 instead. He connected the whip, started to fill it nice and slow the way we like, and a couple minutes later there was the rush of air and he realized something went wrong.

I was not least bit upset. In all honesty my inner instructor was actually happy the incident happened the way it did and to turn it into a teachable moment. There were no injuries (except his pride) and there was no property damage other than the copper burst disk deciding it was time to explore another plane of existance. I took the time to explain what happened, reviewed how burst disks work, disassembled and showed him the internals, how it failed exactly as designed to, showed how the containing bolt has multiple holes to help prevent the cyldinder from taking off in a particular direction, and that this worked out exactly how it was engineered to do. Like the old carpenter's adage of "measure twice, cut once," he learned the importance of double checking his readings. I simply swung by my house, picked up a new Thermo modular valve I had sitting on my bench and I was back in business within the hour. I must admit too, the modular valve on a single cylinder sure makes it easy on the hand to carry.

He impressed me when he immediately took credit for his mistake, telling me how he made the mistake and made no effort to blame my equipment or come up with some excuse. He was adamant to reimburse me, I told him not to worry about it. He still insisted on doing something to make it right. I just told him to throw an extra beer in his post dive beverage cooler if we plan a dive together (I think he's old enough to purchase beer at least). To me, this moment was worth far more than the $8 burst disk which I'm happy to cover so this kid was able to safely experience what he did in additrion to his display of integrity.

Funny thing, the owner commented to me the next day how he was telling her how he felt so bad about the incident. Not knowing what I had gone over with him, she told him not to worry as if there was anybody that wouldn't be upset and instead turn this incident into a positive teachable moment it would be me.

Now then, had SP used a 4000 psi burst disk instead of a 3500 psi disk under the 5/3 rating method, this whole thing never would have happened. But then, given the manner in which it did happen, I'm still rather glad it did happen.

Anyway, longer reply than I expected to put here. Time for another gin and tonic and off to bed!