Measuring cracking pressure with a Magnahelic

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JohnN

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I'm getting ready for the DRIS HOG service class on May 1. Except for the Magnahelic (required), I have the balance of the tools ready to go. I purchased a used Magnahelic from fleaBay, which has four ports, two marked low pressure and two marked high pressure. Am I correct I want to take a mouthpiece, punch a hole in it and connect a hose (barbed) from the mouthpiece to the Magnahelic?

Is the orientation of the Magnahelic critical (flat or at 90o)?

Any other handy tips?

Unfortunately, the DRIS instructor has not been of any assistance with this, other than to tell me to buy the Scuba-Tools kit.
 

Jim Lapenta

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The orientation of the mag should be more or less upright because what you will see is only going to be there for a fraction of a second and having it that way makes it easier to see if you have it mounted in some way. That said I have used one lying flat in a case, upright like the one I have mounted on my bench, and one laying at an angle. I saw no discernable difference in readings.
As to the hole in the mouthpiece method. This works very well. You can buy an official attachment with the wide rubber bands, but it does basically the same thing. I find that using a standard mouthpiece gives a little better feel and sensitivity.
One thing with the mag is making sure you choose the right barb to hook up to. That isn't rocket science. Put the hose on and try to take an easy breath. The needle will move in the right direction on the vacuum. If it doesn't, plug it into the other one!
 

Jim Lapenta

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John, I'll send you a pic of my setup. I can't figure out how to just put a simple image in a reply. It keeps asking for a url and I don't have one for a pic. Not worth the hassle of trying to figure it out.
 

abnfrog

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make sure you get a magnahelic that has a large
single type scale that does NOT have a pressure component to it ,as just vacuum one has a larger scale ...thus more arcuately read
 

Lorenzoid

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The orientation of the mag should be more or less upright because what you will see is only going to be there for a fraction of a second ....

I find this one of the more frustrating things about using the mag. As taught in my HOG course, you inhale, keep one eye on the mag and the other on the IP gauge, and just when the IP drop indicates cracking occurs, you note the pressure on the mag. I find it very difficult to watch both gauges. Is there nothing that can be done to "capture" the pressure reading? I suppose I could literally keep my camera pointing at it and press the shutter when I see the IP drops. Also, I find that I can only get my inhalation so steady; try as I might to keep drawing a steady vacuum, the mag needle jiggles around. So even with the goal of setting it to, say, 1.3 in. H20, I wouldn't be surprised if I couldn't do better than plus or minus 0.1 accuracy due to that jiggling. What do you pros do?
 

happy-diver

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Not worth the hassle of trying to figure it out.

Come on Jim, being the proud owner of
SCUBA: A Practical Guide for the New Diver By James A Lapenta
SEI Diving Instructor, CMAS 2 Star Instructor
#9 of 100

I make a formal request you learn!

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Lorenzoid

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