Scubapro D400/D350 cracking effort questions

[halocline]

As usual, Awap explained it. One thing I think you're getting a little confused about is your statement "as the seat wears, the less the cracking effort becomes". The cracking effort is the amount of force it takes to separate the orifice (sealing edge) from the seat. This is what lets the air flow. When the seat is in perfect shape, the orifice will seal against it with relatively little pressure. As the seat wears, it takes more pressure to keep the orifice sealed against it. You either adjust that with the adjuster, or someone does it by screwing in the orifice a little. So the cracking pressure increases, not decreases.

 

[ericpitar]

Awap said:"I assume you are referring to the venturi adjustment when you say it is already set to "least sensitive"."

I'm not sure what is the "venturie adjustment". What I was referring to were whatever adjustments the tech would make in the shop.

e

 

[redacted]

The tech is making 3 independent adjustments on the D-series. First the lever adjustment sets the lever to move the poppet as soon as the diaphragm starts moving, then the poppet spring sets the seating force on the poppet. Together these establish cracking pressure. Finally there is an internal venturi adjustment that regulates the tendency of the flowing gas to establish a vacuum behind the diaphragm to reduce WOB. As long as the reg is not freeflowing as soon as you pressurize it, this venturi adjustment should be the primary control for freeflow if cracking pressure is set OK.

 

[ericpitar]

mattboy said:When the seat is in perfect shape, the orifice will seal against it with relatively little pressure. As the seat wears, it takes more pressure to keep the orifice sealed against it. You either adjust that with the adjuster, or someone does it by screwing in the orifice a little.

I'm not sure if I'm confused or not. My premise at this point, is this: "The more the seat wears, the lesser the cracking pressure becomes--barring any adjustment."

Let me put it this way but use the G250 as an example since that's the schematic I have that's more readily understandable (for me)....On a brand new unit, think of the distance between the orifice and the far end of the spring where the spring pad is at. Let's keep the adjustment knob *fixed* on it *throughout* this example. Now, think of the force exerted by the spring against the seat and orifice at this point. Now, forward the time...When the seat wears and develops a groove like the "grand canyon", this allows more travel for the spring and "lengthens" it a tiny amount, doesn't it? If so, this would result to a lighter force pressing against the grooved seat. Wouldn't this result to a *decrease* in cracking pressure?

Now we can start introducing "adjustments" and other variables....To bring back the level of cracking pressure to the original, we in effect want to "increase" the now-lessened tension by "shortening" the spring by either using the adjustment knob or the orifice...More reason for this adjustment may be the irregularity (which I believe awap mentioned) on the seat that may require additional force.

Doesn't the same concept of seat wear and cracking effort apply to the D400? Do I have the concept correct?? Why are we spending so much time on this? Maybe I'm not diving enough and spending too much time on this board??

Thanks for bearing with me guys....'Just wanna learn!

E

 

[skalkot]

Cracking pressure is measured with a magnehelic. Water submersion only offers a gross estimate. Most (not all) dives will experience very little difference between in effort. A .6/.7 is (in my opinion) too sensitive for a D350/D400. it is not an inhalation admustable regulator. Therefore, the regulator may seep/flow when a diver swims into a current. The D400 is an excellent breathing reg at 1.0-1.4.

 

[skalkot]

The D400 does not adjust the same as other regulators. It has a stationary oriface.

 

[DA Aquamaster]

Cracking pressure is measured with a magnehelic. Water submersion only offers a gross estimate. Most (not all) dives will experience very little difference between in effort. A .6/.7 is (in my opinion) too sensitive for a D350/D400. it is not an inhalation admustable regulator. Therefore, the regulator may seep/flow when a diver swims into a current. The D400 is an excellent breathing reg at 1.0-1.4.

You've apparently never swam into an current with one. The purge cover on the D400 is small in area, well streamlined and fairly firm, so it does not tend to deflect under the pressure of a current and there is consequently no additional water pressure placed on the diaphragm that is sheltered completely behind it. A current that would cause it to freeflow would be about the same velocity as one that rips your mask off and based on my experience the mask will depart before the D400 will freeflow.

In addition, the pre-dive/dive button adds additional spring pressure on the lever that will further prevent any freeflow tendency, so while the D400 may not be adjustable in the sense of a knob that increases the pressure on the poppet, it is adjustable in terms of increasing both inhalation effort and freeflow resistance if desired by the diver in the water.

It does not have an inhaltion effort adjustment knob because it does not need one.
The reason most regs are not usable at cracking efforts less than about 1.0" of water, and the reasons conventional cased regs have adjustment knobs is due to the gemoetry of the case and the location of the exhaust valve and diaphragm. In short, in a face down position the upper edge of the exhaust valve is about 1" above the center of the diaphragm. So once you stop inhaling, the air in the case is initially at a slightly higher pressure than the water on the outside of the exhaust valve. Air consequently leaks out the exhaust valve until the pressure equalizes. That loss of air from the inside of the case causes the diaphragm to rise slightly, pressing on the lever. If the cracking effort is too low, the slight rise of the lever will open the valve and cause a slight freeflow between inhalations. The worst case oreintation for this effect is face down where the diaphragm is at its lowest point relative to the exhaust valve, creating about a 1" of H2O pressure difference and requiring a comensurate 1" of H20 minimum cracking effort.

In contrast, with the coaxial exhaust valve and diaphragm in the D400, the maximum vertical difference that can occur is 1/2 the diameter of the exhaust valve - about .5". And the angle of the diaphragm ensures that there is 0.0" of vertical difference in a normal swimming postion and the worst case position occurs at an angle divers seldom use (looking up at 45 degrees).

In short, anything over 1.0 on a D400 is a waste of reg performance.

 

[halocline]

Ericpitar: As the seat wears, yes, theoretically it moves a little toward the orifice and the spring is theoretically under a tiny amount less tension. But, remember, the cracking effort is the force needed to separate the orifice and the seat. If the seat wears a groove and the orifice fits into that groove, you still need to move the seat off the orifice to open the reg, now a tiny bit further distance.

In reality it's the integrity of the seat/orifice joint that determines how lightly you can set the cracking effort on land. In the water you have to think about case fault geometry as DA explained thoroughly. But, getting back to your question, as the seat wears, the orifice/seat joint gets knackered up a little, and you have to put more tension on the spring to keep it shut.

 

[ericpitar]

Thanks for the replies guys! Awesome awesome group here. 4 more questions on the orifice, seat, groove, and proper tuning :

1) am I correct in thinking that a slightly "aged" reg with a bit of groove on the seat would perform better since it would be better seated and sealed than one with a brand-spanking new seat and/or orifice that still needs some breaking in?

2) Can anyone post a photo of a groove on a seat that is nearing or is at the end of its useful life?

3) Generally, how many dives til a seat on a D350 needs replacement?

4) Once a tech has tuned a D350 at 0.7" cracking effort, can I be conficent that he is quite competent or can he be just a "fluke"?

e

 

[DA Aquamaster]

A new seat is better than an old one. The D300/350/400 all have very light spring pressure and seats do not wear out quickly at all. I have had D whatevers go years between seats.

The older D400's with a metal orifice tend to tune better as the sharper orifice will work well at lower inhalation efforts.

I like the older D400 poppets a lot better than the current ones. I have on occassion had a new poppet/seat assembly that would not seal at all and I have had a few plastic orifice and new seat combinations that would not meet even the 1.4" of water specification.