Yesterday I taught another Hog Repair class. As usual, it was fun for both of us. However, it was impressed upon me yet once more how little divers comprehend how o-rings work. I actually learned all about o-rings while working in Dr. Weltner's p-chem lab at UF, helping perform spectroscopy of hydrogen isotopes with, at the time, the world's largest superconducting magnet. Don't be too impressed: I was merely the 'plumber' and I worked in the realm of vacuums up to 10-4 torr (space is more like 10-12 torr). As a caveat, I was intensely interested in creating and maintaining a vacuum as opposed to pressure. But o-rings are o-rings and while the seal is in 'reverse', a seal is a seal.
Composition. The best place to start is a discussion about the various compositions of 0-rings. The first o-rings were made out of leather, later they were composed of rubber and today there are a veritable plethora of materials to choose from. For Scuba, the SAE sized o-rings come usually come in Buna (Nitrile), EPDM or Viton (Fluorocarbon). Each of these compounds has different flavors which make the o-ring tougher in regards to being able to handle pressure, abrasion resistance and/or resistance to exposure to oxygen. Buna is the cheapest and does rather well for most Scuba applications. It's not rated as "chemically resistant", but for the most part, it does fine. Is it "oxygen clean"? While it won't combust when exposed to pure oxygen, even up to 300 bar, it does suffer more over time. The Parker O-Ring Handbook rates Viton and EPDM as a "1" or acceptable for "cold oxygen". Buna has a "2" rating which means it should be fine in a static application. Above 200F, Viton drops to "2" while Buna and EPDM go all the way down to "4" which is unacceptable. Aggressive tank filling can create hot spots in excess of 300F due to adiabatic forces. No, you won't feel those hot spots by touching the tank, but they are there. It's no wonder why most reg repair techs prefer Viton. You should also know that Viton comes in a variety of compounds. Class A, B, F, Extreme, Clean Room and Food Grade are all options from Dupont. The class is determined by the amount of Fluorocarbon used in the manufacturing process. Most of the commercially available Viton is going to be Class A and that's perfectly fine for Scuba purposes.
Color. Natural Viton is brown in color. Black Viton has carbon black in it which makes it a bit more abrasion resistant but a tad more susceptible to oxygen and Green Viton simply has green dye. All classes of Viton are available in all these colors. Buna is most often black and EPDM comes in a rainbow of colors. Don't be fooled by the color. If you don't know the composition of an o-ring, simply assume it is Buna.
Durometer. This is the measurement of hardness and most scuba o-rings have a durometer of 70 to 75. The higher the value, the harder it is to deform or extrude and it's usually also more abrasion resistant. Some DIN o-rings have a durometer of 90.
Lubricants. The amount and type of lubricant depends on the kind of seal and whether the seal is dynamic (moves while in service) or static. Dynamic seals include the barrel o-ring in the hose connecting to your second stage. The o-ring on the other end though (first stage) is static. Dynamic seals require more lubrication than static seals. In addition, it's important that you use a lubricant designed for the type of seal. As a general rule, lubricants designed for dynamic situations are fine for static seals, but the opposite is not true. For regulators then, always use a dynamic lubricant such as Chrytolube or Tribolube. Which one is best? The one on your bench. Although much cheaper, I avoid the use of other lubricants for the same reason I use Viton. I don't think it's a place to "cheap out".
In addition to dynamic/static, it's important to understand if the seal is axial or radial. That barrel o-ring that connects your hose to the second stage is radial as it seals on the outside and inside of the o-ring. It's also one of the few "captured" o-rings in your regulator since it sits (captured) in a groove. With a radial seal, it doesn't matter how tight that nut is, the o-ring seals just by being inserted into the tube. The o-ring in either a DIN or yoke as it attaches to the tanks are examples of axial or compression seals. These seal on the face of the o-ring. The tighter you make it, the better the seal. Since either o-ring can fall out, they are not examples of a captured o-ring.
Sizing. Most regulators and tanks use o-rings that comply with SAE sizing. The first number defines the thickness of the o-ring. A "0" is thinner than a "1" or a "2". The most common o-ring used in a regulator is an 010. Yes, there are "metric" o-rings and they have their own sizing strategy. Normally simply 'matching' an o-ring is sufficient if you aren't sure. Here's a short list of the most popular SAE sizes used in Scuba:
SPG spool: 003
LP Ports: 010
Tank Valve Stem: 010
HP Ports: 011
Yoke: 014 or 015
DIN 111 or 112
HP tank neck: 116
LP tank neck: 214
O-ring Picks. These come in a variety of materials and shapes. Brass is soft and less likely to damage sealing surfaces. Stainless steel is my choice because I can get them very thin and sharp allowing me to slip it beneath an o-ring. I like the ones from www.McMaster.com with the loop handles. In any event, it's important not to scratch the sealing surfaces, so take the appropriate care. Lots of techniques here and it's impossible to describe all the strategies in this post. Hey, that's what the Hog Reg class is all about!
Cleaning. A stiff nylon or soft, soft stainless steel brush is best. Brass is OK but has a hard time with verdigris (green crud). Ultrasonic cleaners are best and while you can use a commercial solution, I opt for white vinegar. I avoid overly aggressive cleaners such as Simple Green or the Purple stuff. Many regulator manufacturers caution against those as well.
Failures. Most failures are caused by dirt or corrosion. Doing a great job of cleaning will almost always resolve these issues. Lubricants seem to attract dirt, so use them sparingly. Nicking or cutting o-rings is another sure fire way to cause a failure. Trying to tighten an o-ring under pressure often results in cutting it. While over tightening rarely causes a 0-ring problem, it can cause cracking or the stretching of brass fittings, which might look strong because they've been chrome plated to reduce verdigris but are actually still rather fragile. Not tightening an axial seal sufficiently will allow the o-ring to extrude or creep through the crack.
Threads. all Scuba regulators, even those from overseas, use NPS or National Pipe Straight threads for the port plugs. Most the metal plumbing in your house uses NPT or tapered threads. How do you know? If it seals with an o-ring, it's a straight pipe thread. If it's being sealed with teflon tape or pipe dope, it's tapered and is very rare nowadays in Scuba. The common sizes (as I remember them)
1/4"x 18 NPS
3/8"x 18 NPS
US tanks (Non-US tanks can be different)
3/4"x 14 NPS
7/8"x 14 NF (Not a pipe thread)
DIN
5/8"x 14 BSP parallel (Different thread from NPS)
So, there you have it. If you have any questions or comments please feel free to post them. Disagree? I don't have a problem with that, but I would ask (in advance) that you provide documentation of some sort. There are a lot of myths about o-rings out there and just because you believe it to be true, doesn't make it so. GTS is a great way to start, but don't be afraid to cite off-line references as well. I spent four years in the p-chem lab as well as the Chemistry Machine shop much of that dealing with o-rings. In addition, I spent about 30 years in the automotive industry where again, I had to use o-rings and diagnose failures, especially in AC systems.
Composition. The best place to start is a discussion about the various compositions of 0-rings. The first o-rings were made out of leather, later they were composed of rubber and today there are a veritable plethora of materials to choose from. For Scuba, the SAE sized o-rings come usually come in Buna (Nitrile), EPDM or Viton (Fluorocarbon). Each of these compounds has different flavors which make the o-ring tougher in regards to being able to handle pressure, abrasion resistance and/or resistance to exposure to oxygen. Buna is the cheapest and does rather well for most Scuba applications. It's not rated as "chemically resistant", but for the most part, it does fine. Is it "oxygen clean"? While it won't combust when exposed to pure oxygen, even up to 300 bar, it does suffer more over time. The Parker O-Ring Handbook rates Viton and EPDM as a "1" or acceptable for "cold oxygen". Buna has a "2" rating which means it should be fine in a static application. Above 200F, Viton drops to "2" while Buna and EPDM go all the way down to "4" which is unacceptable. Aggressive tank filling can create hot spots in excess of 300F due to adiabatic forces. No, you won't feel those hot spots by touching the tank, but they are there. It's no wonder why most reg repair techs prefer Viton. You should also know that Viton comes in a variety of compounds. Class A, B, F, Extreme, Clean Room and Food Grade are all options from Dupont. The class is determined by the amount of Fluorocarbon used in the manufacturing process. Most of the commercially available Viton is going to be Class A and that's perfectly fine for Scuba purposes.
Color. Natural Viton is brown in color. Black Viton has carbon black in it which makes it a bit more abrasion resistant but a tad more susceptible to oxygen and Green Viton simply has green dye. All classes of Viton are available in all these colors. Buna is most often black and EPDM comes in a rainbow of colors. Don't be fooled by the color. If you don't know the composition of an o-ring, simply assume it is Buna.
Durometer. This is the measurement of hardness and most scuba o-rings have a durometer of 70 to 75. The higher the value, the harder it is to deform or extrude and it's usually also more abrasion resistant. Some DIN o-rings have a durometer of 90.
Lubricants. The amount and type of lubricant depends on the kind of seal and whether the seal is dynamic (moves while in service) or static. Dynamic seals include the barrel o-ring in the hose connecting to your second stage. The o-ring on the other end though (first stage) is static. Dynamic seals require more lubrication than static seals. In addition, it's important that you use a lubricant designed for the type of seal. As a general rule, lubricants designed for dynamic situations are fine for static seals, but the opposite is not true. For regulators then, always use a dynamic lubricant such as Chrytolube or Tribolube. Which one is best? The one on your bench. Although much cheaper, I avoid the use of other lubricants for the same reason I use Viton. I don't think it's a place to "cheap out".
In addition to dynamic/static, it's important to understand if the seal is axial or radial. That barrel o-ring that connects your hose to the second stage is radial as it seals on the outside and inside of the o-ring. It's also one of the few "captured" o-rings in your regulator since it sits (captured) in a groove. With a radial seal, it doesn't matter how tight that nut is, the o-ring seals just by being inserted into the tube. The o-ring in either a DIN or yoke as it attaches to the tanks are examples of axial or compression seals. These seal on the face of the o-ring. The tighter you make it, the better the seal. Since either o-ring can fall out, they are not examples of a captured o-ring.
Sizing. Most regulators and tanks use o-rings that comply with SAE sizing. The first number defines the thickness of the o-ring. A "0" is thinner than a "1" or a "2". The most common o-ring used in a regulator is an 010. Yes, there are "metric" o-rings and they have their own sizing strategy. Normally simply 'matching' an o-ring is sufficient if you aren't sure. Here's a short list of the most popular SAE sizes used in Scuba:
SPG spool: 003
LP Ports: 010
Tank Valve Stem: 010
HP Ports: 011
Yoke: 014 or 015
DIN 111 or 112
HP tank neck: 116
LP tank neck: 214
O-ring Picks. These come in a variety of materials and shapes. Brass is soft and less likely to damage sealing surfaces. Stainless steel is my choice because I can get them very thin and sharp allowing me to slip it beneath an o-ring. I like the ones from www.McMaster.com with the loop handles. In any event, it's important not to scratch the sealing surfaces, so take the appropriate care. Lots of techniques here and it's impossible to describe all the strategies in this post. Hey, that's what the Hog Reg class is all about!
Cleaning. A stiff nylon or soft, soft stainless steel brush is best. Brass is OK but has a hard time with verdigris (green crud). Ultrasonic cleaners are best and while you can use a commercial solution, I opt for white vinegar. I avoid overly aggressive cleaners such as Simple Green or the Purple stuff. Many regulator manufacturers caution against those as well.
Failures. Most failures are caused by dirt or corrosion. Doing a great job of cleaning will almost always resolve these issues. Lubricants seem to attract dirt, so use them sparingly. Nicking or cutting o-rings is another sure fire way to cause a failure. Trying to tighten an o-ring under pressure often results in cutting it. While over tightening rarely causes a 0-ring problem, it can cause cracking or the stretching of brass fittings, which might look strong because they've been chrome plated to reduce verdigris but are actually still rather fragile. Not tightening an axial seal sufficiently will allow the o-ring to extrude or creep through the crack.
Threads. all Scuba regulators, even those from overseas, use NPS or National Pipe Straight threads for the port plugs. Most the metal plumbing in your house uses NPT or tapered threads. How do you know? If it seals with an o-ring, it's a straight pipe thread. If it's being sealed with teflon tape or pipe dope, it's tapered and is very rare nowadays in Scuba. The common sizes (as I remember them)
1/4"x 18 NPS
3/8"x 18 NPS
US tanks (Non-US tanks can be different)
3/4"x 14 NPS
7/8"x 14 NF (Not a pipe thread)
DIN
5/8"x 14 BSP parallel (Different thread from NPS)
So, there you have it. If you have any questions or comments please feel free to post them. Disagree? I don't have a problem with that, but I would ask (in advance) that you provide documentation of some sort. There are a lot of myths about o-rings out there and just because you believe it to be true, doesn't make it so. GTS is a great way to start, but don't be afraid to cite off-line references as well. I spent four years in the p-chem lab as well as the Chemistry Machine shop much of that dealing with o-rings. In addition, I spent about 30 years in the automotive industry where again, I had to use o-rings and diagnose failures, especially in AC systems.
