Nitrox vs regular service kit - Scubapro Mk2+

[elmo]

I’m toying with the idea of servicing my regs myself. The simplest first stage I own would be my deco reg, a Scubapro Mk2+ “nitrox”, with a green cover. Apart from the colour and it having be sold as O2 clean, I believe it’s the same as any other Mk2+ inside. I see there is a regular Mk2/Mk2+ service kit available and a “nitrox” version, which comes with a little tube of lube. This reg will see up to 100% O2. I don’t care for the green sticker, and I already have O2 safe lubricant. Is there any reason to pay double the price for the nitrox service kit?

 

[happy-diver]

When I started with nitrox I listened but did not like viton, so went straight to epdm
Given that O2 specific components are chosen as they will give off less toxins in fire

So then my choice is to use what I want to use and I choose not to have fire instead

There certainly ain't no o2 kit though here

This was my between and post dive regeneration source of oxygen for decades

 

[Angelo Farina]

If you decide to source the O-rings and grease yourself, I warmly recommend to only use materials approved for high pressure pure oxygen.
When young I had a bad accident employing a rubber gasket (instead of Teflon) on a pure oxygen tank...
Nitrox up to 40% is not very dangerous, but 100% oxygen is another world...

 

[Tanks A Lot]

Name	Auto Ignition Temperature °C	Heat Of Combustion J/g	O2 Index %
EPDM	150–200	47,200	25.5
Nylon 6/6 (polyamide)	178	32,200	36
Zytel 42 (polyamide)	192	37,000
Buna-N (Nitrile rubber)	173	35,600	22
Hydrocarbon Greases	160–220	33,500–41,900	21
Polyurethane	181	21,800–27,200	22
PEEK	305	32,500	35
FKM (Viton)	300–325	12,600–20,900	56–100
PTFE (Teflon)	>427	4,200–7,100	95–100
Krytox (Perfluorinated lubricant)	>427	3,800–4,200	DNI

From ASTM: STP 1522

I just want to point out that EPDM is only marginally better for O2 service than NBR is. I said it before, an O2 accident never happens because one thing goes wrong, but several things chain together. Unfortunately EPDM O-Rings would be one of those things in such a chain. But EPDM O-Rings alone in a high O2 environment would never cause a catastrophic incident.
I dislike FKM (Viton) probably more than the next guy for it's horrendous mechanical properties. Yet it is one of the better materials with regards to O2 service.

As none of the O-Rings in an MK2+ will see a high ppO2, it matters very little what O-Rings you use. The seat does see a high ppO2, so it is of utmost importance to have a material that can withstand it. I do not know exactly what ScubaPro uses for seat materials, but I suspect that they use a polyamide like Nylon 6/6 on their normal kits. These polyamides are not suitable at all for a high ppO2 environment.
In their Nitrox kits they likely use a material like Kel-F or PTFE (Teflon) for the seats. These are suitable.

FKMs and PTFEs are of course no guarantee that an O2 fire can't happen. They do greatly decrease the risk, by lowering the auto ignition temperatures and heat given off in case of a combustion. As happy mentioned above, FKMs and PTFEs are a bitch once they do catch fire. In fact most materials with fluor in them are. They are extremly toxic once this happens.

TL.DR: If you can get your hand on a O2 compatible seat, don't bother with the Nitrox kit. If you can't, do not attempt high O2 service with a normal seat.

 

[rsingler]

I would be very surprised if you'll be able to find a Kel-F seat outside of NASA.

Furthermore, while the spontaneous ignition temps of PEEK and FKM are double that of EPDM (or Nitrile), they're only a hundred degrees apart.
Meanwhile, the theoretical max temperature due to adiabatic compression is three times ALL of the spontaneous ignition temperatures, yet we rarely see fires.

In other words, there are LOTS of factors involved, especially not slam opening tanks. Folks have played with oxygen for decades, and mishaps are rare. I agree with @Tanks A Lot - FKM is a lousy oring. Virtually the only places I use it are the tank valve, the Yoke/DIN bolt and the piston shaft. And frankly, after thirty years I've concluded that nitrile is a better seal than EPDM, and much easier to find in harder durometer. The shorter lifetime of nitrile is irrelevant to me, as I've rebuilt my reg long before I have to worry about that.

 

[Tanks A Lot]

I would be very surprised if you'll be able to find a Kel-F seat outside of NASA.

You are absolutely right, Kel-f is a bit too specialized for the SCUBA industry, that was a poor example.

Furthermore, while the spontaneous ignition temps of PEEK and FKM are double that of EPDM (or Nitrile), they're only a hundred degrees apart.
Meanwhile, the theoretical max temperature due to adiabatic compression is three times ALL of the spontaneous ignition temperatures, yet we rarely see fires.

The auto ignition is only part of the story I believe. I know of one study that specifically looked at VIRPs (Valve integrated pressure regulators) in the medical industry. While not everything from it is transferable to SCUBA, a lot is. If I remember correctly, one of the key takeaways was not only the auto ignition temperature, for the reasons you mentioned. A big impact seemed to have the heat of the resulting combustion. In essence, how hot something was burning was at least equally important as to when it starts burning.
The higher combustion temperatures greatly increased the risk of the surrounding materials, like the metals catching fire.

Pair a high temperature burning EPDM O-Ring with an aluminium body for example. Even in a high O2 environment you did not create a fire just yet. Add heat from adiabatic heating, particle impact or similar and you start to burn that EPDM. As it burns so hot, it will start to burn the aluminium, consuming the whole system.

If that same system would have been pair with brass, nickel, copper, monel, etc. the EPDM would have burned just the same. But the brass or similar has a much better resistance to O2 fires than the aluminium or stainless steel did.
Sidenote: Monel burns rapidly if in a mesh form or similar, otherwise it exhibits great O2 resistance.

Conversely, you may have gotten away pairing that very same aluminium body with a FKM O-Ring, as the heat generated would be much lower.

....In other words, there are LOTS of factors involved...

I could not agree more! An oxygen fire sort of lies on a continues line. A plethora of things have to go wrong to result in an accident. That starts with the basic mechanical layout, continues to material selection and stops with the user. All must have made some poor choices for something to happen.

 

[elmo]

Thanks for all the replies providing a lot of great information. Given Mk2 regs have been around and serviced for decades, does anyone know what’s actually in the service kits? And in particular, what’s different between the regular and nitrox kits? If the nitrox kit is safer, as opposed to just more colourful, I’ll definitely go for that.

 

[rsingler]

Next week, I'll be back at my shop and will photograph and post Nitrox vs. standard Mk2 kits.
We'll see if there's any apparent visual difference between the seats in the two kits, and I can test a couple of o-rings.
The Nitrox kit's wholesale cost is 100% higher, so there may indeed be a fancy seat in it.

 

[Tanks A Lot]

I just wanted to clarify what I have written above about the O-Rings on the MK2+ was wrong.

On a yoke MK2+ there is of course one O-Ring exposed to high ppO2, the one on the HP outlet. If it is a DIN version, there are three - The HP outlet and the two on the DIN retainer. Incidentally these are the O-Rings which are made up of FKM in the Nitrox kit. The upper small O-Ring on the piston is also made up of FKM, which surprises me. The lower big one on the piston is the standard NBR.
On paper, ScubaPro does not have a Nitrox compatible yoke version, just the DIN version. That is not really relevant though.

Why that upper O-Ring is made up of FKM is a bit puzzling to me.

 

[rsingler]

Why that upper O-Ring is made up of FKM is a bit puzzling to me.

It IS exposed to tank pressure on one side.
And lawyers.