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?

 

[cerich]

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.

yup, they even had a special tool for bending it

 

[rsingler]

I think we need to give the OP's instructor some credit. Keep in mind that one third of the attendees at most seminars have never ever opened a reg before their "certification", and that there is zero to minimal time to teach theory.
With those constraints, his/her instructions are a great way to keep "Remove and Replace" shop techs from send a reg out the door with a dangerously low lever. If you don't know that screwing in the orifice also increases spring tension, that's one less thing the tech can do that might hurt the diver.
Look at the number of old manuals that talk about adjusting the orifice in 1/4 turns, when the actual adjustment is in twelfths of a turn or less, and you'll see the danger. Open up a reg and compare lever heights after adding two "quarter turns", and you'll quickly see how dangerous it is.
Adding orifice adjustment to the tuning process is one way to prove that "a little knowledge is a dangerous thing".

It ain't rocket science, but it sure is precise!

 

[garywong007]

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.

Thank you axxel57!! You have explained very clearly on the topic! Yes, I guess we should distinguish between a balanced and unbalanced 2nd-stage when we talk about this subject. On a 2nd stage with a nyloc nut and no adjustment knob, I would adjust the CE by the depth of the orifice and lever height by the nyloc nut.

 

[garywong007]

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

Thank you cerish! That is interesting to know... bending the lever.... hmm.

 

[garywong007]

yup, they even had a special tool for bending it

 

[garywong007]

I think we need to give the OP's instructor some credit. Keep in mind that one third of the attendees at most seminars have never ever opened a reg before their "certification", and that there is zero to minimal time to teach theory.
With those constraints, his/her instructions are a great way to keep "Remove and Replace" shop techs from send a reg out the door with a dangerously low lever. If you don't know that screwing in the orifice also increases spring tension, that's one less thing the tech can do that might hurt the diver.
Look at the number of old manuals that talk about adjusting the orifice in 1/4 turns, when the actual adjustment is in twelfths of a turn or less, and you'll see the danger. Open up a reg and compare lever heights after adding two "quarter turns", and you'll quickly see how dangerous it is.
Adding orifice adjustment to the tuning process is one way to prove that "a little knowledge is a dangerous thing".

It ain't rocket science, but it sure is precise!

Hello rsingler, I fully agree with you. I am sure the lecturer is well-intended and knowledgable and probably he was just like what you said - trying not to make things more complicated than necessary.

Cheers

 

[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 gratefulmjc !! Got it!