What determines the cracking point on a 2nd-stage?

[garywong007]

Hi, recently I attended a regulator service training of a known brand and I am not sure if I completely agree with what the lecturer said that the only thing that can adjust the cracking effort on a 2nd-stage is by the adjustment knob - which by turning clockwise or CCW will change the tension of the spring connected to the poppet. In my own experience, by turning in the orifice on the other end of the inlet tube (to the point where it is not just barely touching the soft seat on the poppet but actually pushing back the poppet) can also increase the tension on the spring and thus increase cracking effort. Although this is not the proper way to do it (because the lever will drop too much as the orifice is pushed in further) but just for the sake of argument, I think this can also vary the cracking effort, is my understanding correct?

 

[Mike1967]

The knob is for fine adjustment to stop a freeflow if you have it tuned hot, the actual adjustment is inside the fitting where the hose screws on.

 

[happy-diver]

On the bench I adjust mine by ear (another reason for the tinnitus)
and then I shake the reg so when there's no rattle it's ready to dive

I have read about current setting regs off, which I also tune against
although I've never experienced it

 

[garywong007]

On the bench I adjust mine by ear (another reason for the tinnitus)
and then I shake the reg so when there's no rattle it's ready to dive

I have read about current setting regs off, which I also tune against
although I've never experienced it

Thank you but that doesn't my question. What I would like to know is whether the adjustment knob is the ONLY way to change the cracking effort on a 2nd-stage. As far as I know, the cracking effort is a function of the tension of the spring on the poppet but the tension of the spring can also be increased by turning the orifice more into the inlet tube (thus push harder on the poppet and compress the spring behind it). Therefore I do not agree with what the lecturer said.

 

[gratefulmjc]

In my own experience, by turning in the orifice on the other end of the inlet tube (to the point where it is not just barely touching the soft seat on the poppet but actually pushing back the poppet) can also increase the tension on the spring and thus increase cracking effort. Although this is not the proper way to do it (because the lever will drop too much as the orifice is pushed in further) but just for the sake of argument, I think this can also vary the cracking effort, is my understanding correct?

To address your question, as you said, "for the sake of argument", you are correct...adjustments that compress the poppet spring, such as turning in the orifice, will increase cracking effort.

And from a practical perspective, you are also correct (for the reason you mention...lever height can get too low to fully open the valve), that increasing cracking effort by turning in the orifice "is not the proper way to do it". (Though I read it implicitly in "not the proper way", I'll add that turning in the orifice to the point of dropping the lever too far to fully open the valve is not only improper but is asking for trouble.)

 

[garywong007]

To address your question, as you said, "for the sake of argument", you are correct...adjustments that compress the poppet spring, such as turning in the orifice, will increase cracking effort.

And from a practical perspective, you are also correct (for the reason you mention...lever height can get too low to fully open the valve), that increasing cracking effort by turning in the orifice "is not the proper way to do it". (Though I read it implicitly in "not the proper way", I'll add that turning in the orifice to the point of dropping the lever too far to fully open the valve is not only improper but is asking for trouble.)

Thank you for your input! So, I guess what the lecturer was trying to say is that using the adjustment knob is the only PREFERABLE way to vary the tension spring (and thus the cracking effort) and not really the ONLY WAY.

 

[axxel57]

What the lecturer probably wanted to point out, was to explain the basic procedural difference between adjusting an unbalanced downstream 2nd and adjusting a balanced barrel 2nd.
Adjusting the Cracking Effort (CE) of an unbalanced 2nd like the R-Series of SP is done basically by turning the orifice in one or the other direction, the lever height is adjusted with the nyloc nut.
Of course one could argue that adjusting the lever height to the point that the 2nd starts to leak or that the lever drops to a point where you can't press the purge button anymore ( means no free flow can be activated) is adjusting the CE, but leads in the wrong direction.
Same with a barrel type balanced 2nd. Here the orifice is mainly responsible for the correct lever height, and the Adjustment Knob is responsible for the CE.
Also here you can argue that the orifice adjustment can effect the CE, that's somehow right, but it is, from the technician point of view, I think, not anything a lecturer should point out.

 

[cerich]

Thank you but that doesn't my question. What I would like to know is whether the adjustment knob is the ONLY way to change the cracking effort on a 2nd-stage. As far as I know, the cracking effort is a function of the tension of the spring on the poppet but the tension of the spring can also be increased by turning the orifice more into the inlet tube (thus push harder on the poppet and compress the spring behind it). Therefore I do not agree with what the lecturer said.

I don't agree with him.

Regardless of design, the cracking pressure changes depending on IP, even within the manuf recommended IP range. Not by a lot, but certainly does and you can easily test and prove it.

Yes, orifice will also impact cracking effort, but much more on an unbalanced 2nd than a balanced within reason. Some second stages have micro adjustment valve, also impacts.

Back in the stone ages, one big brand allowed the tech to bend the demand lever a wee bit to adjust cracking pressure.. haven't seen anyone recommend that in a LONG time.. LOL

It sounds like the instructor was either keeping it simple per how they(manuf) wanted them serviced, or had a simple understanding

 

[keyshunter]

I seem to recall that Sherwood recommended bending the demand lever. But, to their credit. the old Sherwoods were about as simple and reliable as a brick.

 

[lexvil]

Hi, recently I attended a regulator service training of a known brand and I am not sure if I completely agree with what the lecturer said that the only thing that can adjust the cracking effort on a 2nd-stage is by the adjustment knob - which by turning clockwise or CCW will change the tension of the spring connected to the poppet. In my own experience, by turning in the orifice on the other end of the inlet tube (to the point where it is not just barely touching the soft seat on the poppet but actually pushing back the poppet) can also increase the tension on the spring and thus increase cracking effort. Although this is not the proper way to do it (because the lever will drop too much as the orifice is pushed in further) but just for the sake of argument, I think this can also vary the cracking effort, is my understanding correct?

This can possibly be a case of semantics, from a manufacturer training point of view, if you add “the only thing A DIVER can DO TO adjust” its correct from their point of view.

everything else is tuning and those adjustments are “secret”