Scubapro D400/D350 cracking effort questions

[ericpitar]

Hello all,

4 questions about the D350/D400.

1 A person states his D400 lp seat needs replacing already and that the reg is set at its "least sensitive, otherwise it would freeflow". Despite the fact he stated it's set at its "least sensitive", does the fact that the lp seat "needs replacement" *and* that it is "near freeflow" imply it is likely still set at a relatively (relative to the normal cracking efforts on D350/D400's) low crackin g effort (.5" to 1") given the condition of his lp seat?


2 My understanding is the cracking effort could be measured by submerging the reg in water while hooked up to a tank. With the D350/D400, what orientation would one submerge the unit to measure this? Does the mouthpiece face sideways (as if in the divers mouth looking straight ahead) or should the mouthpiece be pointing up? Do you take the measurement based on the depth between the part of the reg that is deepest in the water to the waterline? Is there another perhaps more "hitech" way to measure the cracking effort?

3) Using different terminology, is there any other way to request/communicate to a tech to set the reg to a "cracking effort of x inches" ?

4)Since the scubapro specs call for cracking efforts in the 1.0 to 1.4" range, I'm assuming the tech may press for this if I have him service it. If so, would a person likely perceive a difference between this 1-1.4" range and a cracking effort of .6 to .7"? If it's any indication of my sensitivity, I've dived my R109BA and R190 up to 95ft depth but couldn't tell the difference between them regardless of the adjustment I made in the R109.

thank you all.

e

 

[sphillips3]

I'm a certified ScubaPro tech currently working in a ScubaPro shop so I'll take a bit of a run at this.

If I was handed the regulator mentioned in item #1 with the explanation you provide I would take both the 1st and 2nd stages and put them on my service bench. I'd test the Interstage Pressure output of the 1st stage (which can affect cracking pressure/free flow of the 2nd stage) as well as the cracking pressure, lever height, and breathing effort of the 2nd stage. I'm not sure how the customer would know that the "LP seat needs replacement" since there is no "LP seat" in a second stage (certainly not in a D350/400). There is a poppet assembly, which in the D350/400 is a bit of a complicated arrangement, but nothing called a LP seat. Also, the bit about the regulator being set to it's "least sensitive" but having a "low cracking effort" does not quite line up so I'd be confused until I got the units and put them on the bench.

For the D-350/400 the diaphragm is under the lower portion of the reg - under the ridged cover. For the field test of cracking pressure you need to put the reg in the water with the diaphragm parallel to the water (mouthpiece pointing up) and gently lower into the water, such that no water gets in through the mouthpiece. You then measure the approximate depth the diaphragm is below the surface for a rough measure of cracking pressure. The D350/400 diaphragm is at an angle relative to the mouthpiece so the mouthpiece will not be exactly vertical when you do this correctly.

This is obviously a very rough measure - the *proper* way to do this is to do a bench test take off the mouthpiece and use a Magnehelic pressure gauge with a mouthpiece adapter. You really need the proper tools, training and experience to do this right.

Although you may notice a very small difference in breathing effort between 0.5 and 1.0" of cracking pressure, 0.5 is much too low in my opinion. I'll adjust a reg pretty much any way the customer requests (within reason) but the factory spec for the D350/400 is 1-1.4" as you stated. I've never had a customer request lower and I set them at 1.2". One of the most common complaints I do get is excessive free-flow on the surface (while gearing up, etc) and this is generally caused by too low a cracking pressure. If you tell me you want cracking pressure set to 0.6 inches, I can do it, but I'm going to ask you why. I would suggest you take the reg set to factory specs and try it out. If you have a good relationship with your tech there will be no problem bringing it back for further adjustment.

Note the the most significant contributer to Work of Breathing (WOB) is actually exhaust effort, and this is not adjustable. In the recreational range there is really no benefit at all to a lower-than-factory cracking pressure and most tech divers I service for actually want a *higher* cracking pressure (although most tech divers in our shop dive something like a G250 with an external adjustment).

Also note that cracking pressure is not the only adjustment - there are also lever height adjustments that affect the performance and "feel" of a regulator. Again, an experienced tech will have a good feel for the proper settings on these adjustments.

 

[DA Aquamaster]

In my experience, every company comes up with their own parts terminilogy. For example Scubapro calls the moving part in their balanced valve design a "poppet" while if you look at an Aqualung LX second stage, the exact came part (which is literally a combination of the older two piece scubapro poppet and the current 1 piece wing poppet) a suttle valve".

The fact is both of them have a rubber piece on the end that serves as the seat and it is often called a low pressure seat to differentiate it from the high pressure seat in the first stage. More specifically, both the D400 poppet and S-wing poppet used in most other Scubapro balanced second stage have a specific numbers part called a "seat" even though both are also available as an assembly. Getting critical about someone using a more generic and much mor readily commuicable and understandable term is counter productive. It is what it is - a low pressure seat.

I used to use the D400 exclusively and did so for about 15 years. They are superb regulators but are distinctly different and more complex to adjust properly. Other Scubapro balanced second stages are a single adjustment design where the only variable is the depth to which the orifice is adjusted. Scubapro's unbalanced second stages are a dual adjustment design where lever height is also adjustable. The D400 has more in common with an unbalanced second stage in that regard, but the order of adustment is much more important and there are a few nuances that are not immediately obvious or inuitive. Depending on when it was made there are also differences in levers, orifices and spring adjustment pads that all bear on the adjustment and tuning process.

Over the years the specified cracking effort on the D400 went up. This was do in part to an increase in freeflow resistance to meet truly useless CE standards and in part due to a decline in tech training, experience and competence with the D400. If you tune it below 1.0" of water, you really need to know what you are doing and a higher specification of 1.0 to 1.5" covers all manner of adjustment errors that would make a lower adjustment in the .5 to .8 range hard to achieve. And some of the later D400's with plastic orifices and the latest lever design just will not achieve it. The final lever used in the D400 had the unfortunate trait of increasing the effort required to sustain air flow after the initial inhalation and those regs breathe noticeably "harder" than earlier D400's. It was just one of the many steps SP took in engineering the performance out of the D400.

That said having used D400's for years, there is marked difference between one that functions at .7" of water and one that functions at 1.2" to 1.5" of water and a diver used to the former would return the latter to the shop for re-adjustment. Sadly, divers in general I think are not familiar with what is possible and are willing to accept mediocre performance in the 1.2 to 1.5" of water range in most high performance regulators as it has become the norm in part to meet really useless CE freeflow requirements. Techs who adjust D400's to anything over the current minimum spec of 1.0" of water do not help the situation any.

Freeflow resistance on the D400 is improved with the dive/predive lever as well as with the venturi adjustment at the top of the aspirator assembly. If a diver desires more freeflow resitance they are better served by having the gate on the aspirator closed a notch or two than they are by increasing cracking effort. Taking an OW course and understanding that turning the mouth piece down on entry prevents freeflow as well as the fact that a flooded reg will not freeflow underwater also helps. I won't comment directly on the current state of diver education and the decline that has occurred over the years.

Work of breathing is indeed greatly impacted by exhalation effort, but the inhalation effort is much more noticeable. I can create a reg with an enormous exhaust vlave that inflates the diver on inhalation and get an extremely low WOB, but it will by no means breathe naturally nor will it be pleasing to the diver, especially with the wetness caused by the over sized exhaust valve. In my opionion based, on 25 years of diving and reg evolution, WOB numbers have taken the industry down the wrong path.

In terms of natural breathing the D400 was perhaps the peak and offers a rolls royce level of quality compared to a G250's less comfortable but more aggressive corvette level of performance. The R190 is more of a Ford Taurus offering reliability but not much of a ride or much of a thrill. In comparision to a higher end reg, the R190 breathes fairly rough even if tuned for a low cracking effort.

I do agree with you that .5" is too low, even though many D400's can achieve it, as the reg can be a bit touchy at that level. However .7 to .8 is a very useable number and is HALF of the maximum end of the current specification - it makes a huge difference in quality of breathing.

As a technical diver, I now used G250's (the originals, not the less than stellar G250HP) and they are all tuned to provide minimum inhalation effort with the knob all the way out. In the water, you have to adjust inward a turn or two in a face down position to eliminate any freeflow between breaths, but it does offer you minimum inhaltion effort for your particular orientation. I have swam into very high flow cave systems and have never had issues with freeflow. As a technical diver I like the adjustment for scootering, (an activity where the D400 was alwasy totally unaffected by water flow) but would never under any circumstances want a reg with a higher than neccesary inhalation effort as harder breathing leads to CO2 retention which potentiates narcosis and oxygen toxicity.

Some divers, tech or otherwise, operate under the misperception that a harder breathing reg will reduce their air consumption when in fact the opposite is true. It also ignore the issue that CO2 buildup, not falling 02 levels are the trigger for breathing and trying to stretch your gas consumption just elevates CO2 levels.

 

[halocline]

Having rebuilt a couple of D300s, I can vouch that for the DIYer they are a bit more complicated, but the beauty of the design makes them more fun to work on IMO. It's too bad some techs don't appreciate the coaxial exhaust and balancing system.

Then again, it's hard to get much simpler than G250s or other classic barrel poppet designs.

DA, a quick question, related to an earlier thread that was resurrected recently. When I got one of my D300s, the lever and dive/predive clip were oriented so that the vertical legs were "on the same side" of the reg, towards the hose side; I'm sure you know what I'm talking about. The lever looked to be backwards, so I turned it around when I rebuilt it. To be honest, I don't see how it could engage with the diaphragm installed backwards, but that's the way it was. I understand the orientation of these parts is different on the 3 D series regs. Can you explain that?

 

[redacted]

1 A person states his D400 lp seat needs replacing already and that the reg is set at its "least sensitive, otherwise it would freeflow". Despite the fact he stated it's set at its "least sensitive", does the fact that the lp seat "needs replacement" *and* that it is "near freeflow" imply it is likely still set at a relatively (relative to the normal cracking efforts on D350/D400's) low crackin g effort (.5" to 1") given the condition of his lp seat?

Any idea what performance parameters led that person to conclude that the LP seat (seat, molded, part # 11-015-106) needs to be replaced?

In general, I usually replace the LP seat (aka seat, molded seat, or duro poppet seat) when I can't get it adjusted to a low, stable cracking pressure. That often means the seating groove is starting to look like the grand canyon or I have managed to disturb the seat orientation and it is trying to cut a new groove.

 

[DA Aquamaster]

For reasons I have never been clear about, SP changed the direction the lever pointed when they switched from the the D350 to the D400.

The dive pre-dive arm should be behind the inlet barrel and the lever. I have always found removing the dive pre-dive switch and then putting it all back together to be very un-fun.

I also agree that the time to replace the seat/poppet in a D400 is when it will no longer seal with a normal amount of spring pressure.

 

[ericpitar]

Sphillips3 said: "For the field test of cracking pressure you need to put the reg in the water with the diaphragm parallel to the water (mouthpiece pointing up) and gently lower into the water, such that no water gets in through the mouthpiece.......The D350/400 diaphragm is at an angle relative to the mouthpiece so the mouthpiece will not be exactly vertical when you do this correctly."

Question: So do I lower it with the front face of the reg (with the ridges) flat on the surface of the water? I dont have the unit with me yet but based on photos, the mouthpiece would be pointing up but at a 45 degree angle, correct?

 

[halocline]

Question: So do I lower it with the front face of the reg (with the ridges) flat on the surface of the water? I dont have the unit with me yet but based on photos, the mouthpiece would be pointing up but at a 45 degree angle, correct?

Yes. It should start hissing long before you get to the mouthpiece. On mine, it starts around the depth at which the vents on the side of purge cover are submerged.

 

[ericpitar]

Hello all,

Awap said: "Any idea what performance parameters led that person to conclude that the LP seat (seat, molded, part # 11-015-106) needs to be replaced?"

My understanding is that it's still near freeflow even though the reg is already set at its "least sensitive". I think that was his basis though he didn't specifically say.

I have a question about the relationship between seat wear and cracking effort. Am I correct that the more the seat wears, the less the cracking effort becomes? In other words, if I have a brand new unit adjusted to 1" cracking effort, the more dives I do, the more seat wear occurs. The more seat wear occurs after x dives, the cracking effort decreases to *less than* the original 1" cracking effort I set it to? Thus, if I want to bring the cracking effort back up to 1", then I would need to ask a tech to adjust it to become "less sensitive". Am I correct in my understanding?

Thus, at the extreme end of heavy use/seat wear and after several adjustements to "lessen the sensitivity", eventually the cracking effort would unavoidably approach 0.4" or 0.5", thus nearing or at freeflow, correct? Is this where a reg would probably need a "new seat" because no amount of adjustment is possible due to the worn seat (i.e. I think this is what awap was referring to as resembling the "Grand Canyon")?


thanks all.

e

 

[redacted]

Hello all,

Awap said: "Any idea what performance parameters led that person to conclude that the LP seat (seat, molded, part # 11-015-106) needs to be replaced?"

My understanding is that it's still near freeflow even though the reg is already set at its "least sensitive". I think that was his basis though he didn't specifically say.

I have a question about the relationship between seat wear and cracking effort. Am I correct that the more the seat wears, the less the cracking effort becomes? In other words, if I have a brand new unit adjusted to 1" cracking effort, the more dives I do, the more seat wear occurs. The more seat wear occurs after x dives, the cracking effort decreases to *less than* the original 1" cracking effort I set it to? Thus, if I want to bring the cracking effort back up to 1", then I would need to ask a tech to adjust it to become "less sensitive". Am I correct in my understanding?

Thus, at the extreme end of heavy use/seat wear and after several adjustements to "lessen the sensitivity", eventually the cracking effort would unavoidably approach 0.4" or 0.5", thus nearing or at freeflow, correct? Is this where a reg would probably need a "new seat" because no amount of adjustment is possible due to the worn seat (i.e. I think this is what awap was referring to as resembling the "Grand Canyon")?


thanks all.

e

I assume you are referring to the venturi adjustment when you say it is already set to "least sensitive". This adjustment may have more effect on the severity of a freeflow rather than completely avoiding one in an otherwise well tuned regulator. Both my D-series regs take a little extra management to avoid freeflows when not in your mouth. The dive/pre-dive switch helps in some situations. But I find I just have to be more careful on the surface with the reg not in my mouth and more careful when handing the reg off UW. If I pull the reg from my mouth UW and don't get the mouthpiece pointed down, it will freeflow when well tuned.

LP seats will take a groove over time. Balanced seats not as badly as unbalanced due to the lighter spring pressures involved. I store most of my regs with the purge button depressed to prolong usable seat life. As long as the seat material remains in good shape and the groove is stable (not trying to cut multiple rings) you can probably readjust cracking pressure as needed. But as the groove gets deeper, the lever must travel a bit further to get the same amount of gas flowing also detracting from peak performance. Eventually, the seat compound will start to deteriorate (harden, soften, surface degradation, cracking, whatever) and it will be more difficult to set cracking pressure within specifications.