MK2 overhaul question

[dumpsterDiver]

This is an honest question.. I don't know the answer..but... does 2 or 5 or 8 psi make any discernable difference in the performance of the regulator with respect to WOB? Obviously, changing the IP may require an adjustment in the second stage seat, but does a few psi in the intermediate pressure matter?

So the concensus is no?

 

[axxel57]

So the concensus is no?

Depends how you look at it.


If you dive an unbalanced or balanced 1st together with an unbalanced 2nd and you adjust your 1st to an IP of 8psi lower than the top value of the range, that means that the cracking effort (CE) of your 2nd will increase by about 0,2 to 0,3 inch/h2o which is not much but you can feel it.


Diving under very strenuous conditions I would like to avoid that, especially when diving an unbalanced 1st which will decrease the IP over the course of a dive another 15 -20+psi and increase by that the cracking effort and WOB according to that.


If you use a balanced 2nd the cracking effort and with it the WOB would increase by only 0,1 - 0,2 inch/h2o in the same situation.


For normal dive conditions I would say a 0,3-0,6 inch/h2o increase of the CE does not really matter if you stay in the end with a CE below 2,0inch/h2o.


But at higher flow (greater depths) the dynamic IP lowers quite more and increases the CE until the Venturi neutralizes that. When the Venturi kicks in depends form model to model.

In short, 2-8psi IP adjustment lower than the max IP for a model is generally no problem.

 

[redacted]

I don't see any relationship between the size of the piston head and the size of the orifice. Consider the Mk10 and the No 5. The size of the piston head ditates the force of the spring and does relate to the effects of piston oring friction in the overall forces budget. The smaller piston inflates the effect of that friction which has an impact on IP stability. That is why I retired my 10s in favor of my 5s.

Within spec range, changing IP should have no noticable effect on performance as long as the 2nd stages are adjusted for the selected IP setting. I set my adjustable 2nds for a slight leak so their performance is less acceptable to small IP changes. The pilot and D-series 2nds use quite a bit less mechanical spring force so vthey are even less acceptable to such changes than the balanced barrel poppet 2nds.

The performance effect on unbalanced 2nd may be much more noticable but even they should remain quite usable with IP within 20 psi of the tuned setting. Slight leaks or small increases in breathing effort should be it.

 

[halocline]

I don't see any relationship between the size of the piston head and the size of the orifice. Consider the Mk10 and the No 5. The size of the piston head ditates the force of the spring and does relate to the effects of piston oring friction in the overall forces budget. The smaller piston inflates the effect of that friction which has an impact on IP stability. That is why I retired my 10s in favor of my 5s.

I'm not sure if you're referring to my post, but...
As you know, for the MK2 design, there's downstream force on the seat through the orifice. As the tank empties, this force is reduced and that's why the IP drops. The amount of drop due to this is dependent on the ratio of area inside the orifice to the area of the piston head. Larger orifice with same size piston, more drop over the supply range. So the only way to get a bigger orifice in the MK2 without increasing the IP drop is to also make the piston bigger.

For the MK5/10 it doesn't matter because there's no downstream force on the seat that IP has to overcome. I'm with you on the friction increase in the MK10 but I tolerate it because I have DIN connectors and spec boots for my MK10s, which I use for my cave set.

 

[axxel57]

I don't see any relationship between the size of the piston head and the size of the orifice. Consider the Mk10 and the No 5. The size of the piston head ditates the force of the spring and does relate to the effects of piston oring friction in the overall forces budget. The smaller piston inflates the effect of that friction which has an impact on IP stability. That is why I retired my 10s in favor of my 5s.

Within spec range, changing IP should have no noticable effect on performance as long as the 2nd stages are adjusted for the selected IP setting. I set my adjustable 2nds for a slight leak so their performance is less acceptable to small IP changes. The pilot and D-series 2nds use quite a bit less mechanical spring force so vthey are even less acceptable to such changes than the balanced barrel poppet 2nds.

The performance effect on unbalanced 2nd may be much more noticable but even they should remain quite usable with IP within 20 psi of the tuned setting. Slight leaks or small increases in breathing effort should be it.

Awap, we were discussing the effects of a bigger orifice for the IP in an unbalanced flow by piston, not a balanced flow through piston.

The downstream force of the tank pressure on the soft seat of the piston is increased when you alter the size of the orifice (if you make it bigger to produce a higher air flow rate).

When all other factors (spring pressure, piston head size, friction etc.) stay the same, the IP would be slightly higher, what could be adjusted with the shim system.

But absolute would be the part of the piston resisting force, the downstream force of the tank pressure higher than with a smaller orifice.

If the resistance against the air pressure exerted on the piston head ( IP) in the MK2 cap consists basically of about 130psi spring pressure and about 15psi downstream force of the tank pressure on the soft seat on the piston(145psi),then at near empty tank there are those 10-15psi missing for the production of the IP, because the spring power is faster ‘overwhelmed’ and closes faster producing a significant lower IP (unbalanced 1st), you know that.

If with a slight bigger orifice the flow rate is increased, also the downstream force on the soft seat is increased, let’s say 17-18psi instead of the 15psi from before.

At near empty tank are now 12-17psi missing, making the 1st more unbalanced than it was before.

If you want to avoid this effect you have to bring the side in balance with the bigger orifice where the force is produced, against spring and air downstream force are resisting, this would be the area of the piston head.

If you increase in proportion as slightly the size of the piston head as the orifice, the downstream force of the tank pressure will be in proportion to the total IP the same as before.

I think that is what Halo meant and how I understand it.

Concerning the effect of a IP reduction on the performance of 2nds that’s what I wrote.

 

[redacted]

I don't believe the piston head size is related to orifice size, regardless of whether we are talking balanced or unbalanced piston. In the unbalanced case, look at the Mk2, Mk3, and Mk200. They have versions which share a common body (hard seat). They differ in the size of the piston head and the spring. They probably also have different flow through holes providing different flow rates. But they all share about the same IP drop at low tank pressures. And according to Wolfinger, that drop may be calculated based on the orifice size and the pressure differential - piston head size is not parameter in that calculation.

 

[bl6394]

So the concensus is no?

I spoke to our most experienced reg tech. He said that in past classes he had attended - the advice was given as "145 plus or minus 5 psi at 3000. Up to 3 shims / washers. If unable to bring within specs - then replace spring"

I'll shoot an email to Scubapro on Monday and see if they have any further direction or clarification.

 

[axxel57]

I don't believe the piston head size is related to orifice size, regardless of whether we are talking balanced or unbalanced piston. In the unbalanced case, look at the Mk2, Mk3, and Mk200. They have versions which share a common body (hard seat). They differ in the size of the piston head and the spring. They probably also have different flow through holes providing different flow rates. But they all share about the same IP drop at low tank pressures. And according to Wolfinger, that drop may be calculated based on the orifice size and the pressure differential - piston head size is not parameter in that calculation.

I read that differently.

When Wolfinger describes the Flow-by Piston Function he states that the sealing force of the piston is generated by the air pressure across the piston head.

'The amount of pressure required to accomplish this seal is called ´intermediate pressure`(IP)'.

This air pressure would not only overcome the resisting spring power, but also the supply pressure force on the soft seat, which would be with an orifice with a 0,080 diameter at 3000psi supply pressure 15psi and with 300psi 1,5psi.

This would mean that the IP 'would vary 13,5 lbs. throughout the 3000-300psi supply spectrum range (based upon a 1 square inch piston)'.

 

[redacted]

I read that differently.

When Wolfinger describes the Flow-by Piston Function he states that the sealing force of the piston is generated by the air pressure across the piston head.

'The amount of pressure required to accomplish this seal is called ´intermediate pressure`(IP)'.

This air pressure would not only overcome the resisting spring power, but also the supply pressure force on the soft seat, which would be with an orifice with a 0,080 diameter at 3000psi supply pressure 15psi and with 300psi 1,5psi.

This would mean that the IP 'would vary 13,5 lbs. throughout the 3000-300psi supply spectrum range (based upon a 1 square inch piston)'.

Correct, but it is also correct for a flow by piston with a .75 or 1.25 square inch piston head.

Sorry for edit.

 

[axxel57]

Correct, but it is also correct for a flow through piston with a .75 or 1.25 square inch piston head.

What is correct?

That the IP would vary 13,5lbs. or that the piston is producing a sealing force through air force over the piston head?

Remember, the supply pressure force of a flow through piston design would be very small ( maybe 2-5psi) because it could apply the downstream supply pressure only on the sharp edges of the flow through piston.

This is why the spring in this design has to be 'harder' than in the flow by design.