Measuring cracking pressure

[Bluey]

I was attempting to measure the cracking effort on my s600 last night and am wondering at what point to take the reading.
Should it be measured at the point where the smallest amount of air begins to release, or at the point when there is more of a breathable flow.

In the first case my reading is about .6" and in the second case about 1".

Thanks
Bluey

 

[Luis H]

Both are important. But, by definition the cracking pressure (actually suction) is just the required suction to initiate flow. At least, that is the way I have always seen it defined.

If your description is accurate it seams that you are not getting very much venturi assist when you have developed normal flow. That is unless you are really sucking to obtain that 1 inWC.

The 0.6 in WC is very respectable. You really can't get better than that without having some minor free flow when you are head down (when the exhaust is higher than the diaphragm).

 

[Bluey]

Thanks Luis
I did the test with the venturi assist set to "pre dive", so will try it again in "dive" position and see what the reading is.

 

[imasinker]

I was attempting to measure the cracking effort on my s600 last night and am wondering at what point to take the reading.
Should it be measured at the point where the smallest amount of air begins to release, or at the point when there is more of a breathable flow.

In the first case my reading is about .6" and in the second case about 1".

Thanks
Bluey

Would you mind giving a bit more info on how your actually measuring the crack pressure and what you are using to do it. Interesting to know how it`s done.

Thanks

 

[Nemrod]

I have used a homemade water manometer to measure cracking effort but a magnehelic is the best tool and they are relatively inexpensive I guess. My double hose Phoenix Royal Aqua Master measures about 0.4. However, being as my rig is homemade I don't expect it would be the bible as the manometer can be jiggly and hard to set up.

N

 

[Luis H]

Thanks Luis
I did the test with the venturi assist set to "pre dive", so will try it again in "dive" position and see what the reading is.

Yep, that explains your readings.

Are you using a Magnehelic or a U tube manometer? I would guess you are using a Magnehelic.

With a full venturi flow the vacuum should go down once you establish flow.

Several of my regulator will drop very close to zero. A few of my double hose regulators will actually go past zero into positive flow if I suck just right.

 

[redacted]

Yep, that explains your readings.

Are you using a Magnehelic or a U tube manometer? I would guess you are using a Magnehelic.

With a full venturi flow the vacuum should go down once you establish flow.

Several of my regulator will drop very close to zero. A few of my double hose regulators will actually go past zero into positive flow if I suck just right.

But to establish that flow will still take some additional force over the cracking pressure. So your cracking pressure should remain the same regardless of the flow vane position. But as you add more force, the flow with the venturi effect maximized should be progressively greater than with the venturi effect minimized.

 

[Nemrod]

Yep, that explains your readings.

Are you using a Magnehelic or a U tube manometer? I would guess you are using a Magnehelic.

With a full venturi flow the vacuum should go down once you establish flow.

Several of my regulator will drop very close to zero. A few of my double hose regulators will actually go past zero into positive flow if I suck just right.

I have noticed a similar effect with my PRAM. The "effort" reverses into positive or near positive flow. I only see this with my Aqua Masters and one single hose I have played with. The single hose was the Tekna T2100 with servo assist. I assume in the case of the double hose what we are seeing is the effective venturi assist effect on the large diaphram. In the case of the Tekna it is the pneumatic servo assist effect. I suspect this phenomena is somewhat artificial and would dissappear once in the water but then, you perhaps have more ideas on it.

I agree, the cracking effort would be the same with the venturi (pre dive) vane in or out--but--the effort to maintain flow afterward would certainly be effected. Something to think about.
N

 

[DA Aquamaster]

Would you mind giving a bit more info on how your actually measuring the crack pressure and what you are using to do it. Interesting to know how it`s done.

Normally with just a magnahelic you very gradually and carful increase the vaccuum as you inhale until the air just starts to flow. If you also have an IP gauge attached you will see the point where air begins to flow as a slight needle movement on the IP gauge.

On a full blown flow bench it is hard to actually hear when the valve opens with all the other air flowing, but the prinicple is the same and you see it on the IP gauge.

With enough flow many regs will have the inhalation effort drop to zero or even lower into the negative range, but that is often more flow rate than you'd normally be breathing.

One confounding variable is how you connect the magnahelic to the mouthpiece. For speed and conveninece some adapters slide inside the mouthpiece. They work great for setting cracking effort which occurs at very low flow rates. But the restriction they add will reduce flow out the mouthpiece and create increased pressure in the case at higher flow rates - so the magnahelic reading on a "normal" inhalation will be higher than it would be otherwise with an adapter that replaces the mouthpiece and fits on the outside of the case where the mouthpiece would be attached.

Conversely, using an adpater that replaces mouthpiece, takes any restriction created by the moutpiece out of the equation.

Double hose mouthpieces pose their own unigue problems as they have a different shape and no one makes a truly effective adapter - you have to come up with something that works on your own if you want measurments with the mouthpiece in place.

 

[Nemrod]

For single hose using my U tube manometer, I remove the mouthpiece and use a silicone tube to a PVC joint that is drilled and tapped for a fitting to the manometer line. I then attach a mouthpiece or vacuum source to the PVC joint. I am more interested in cracking effort to initial flow with my device than measuring vacuum during flow.

For a double hose I use an expended mouthpiece (bite tabs missing etc). I cut the mouthpiece section mostly off, inserted a slightly flattened on one end PVC pipe of about 4 inch length into the mouthpiece and then bonded it with sealant. The PVC adapter is drilled and tapped again for a fitting to the manometer. The manometer is a U tube with a ruler so that I can meausre vacuumin inches of water. The vacuum is either supplied by me or a large syringe. I note IP bump on attached IP guage and water column in manometer at the point of IP bump. That is the cracking effort I measure. On my Aqau Masters I can set it to nothing--lol---as in 0 --- but then of course that will not work because it will free flow continuously. The lowest practical settings on any regulator, single or double, are really around .4 to .6, otherwise they want to misbehave or free flow or such as that and are simply unstable day to day. The pilot valve, servo type regs like a Tekna T2100 or Oceanic Omega II are a different animal a different discussion.

N