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rednose83
11-03-2009, 20:28
I was reading the other forum about the needle on the gauges moving when the air is not all the way on. My question is if you have a AI computer (with digital numbers) will these numbers fluctuate or is there another way to verify that the air is all the way on? (besides the obvious checking the valve) Just curious.

ScubaToys Larry
11-03-2009, 20:34
On analog or digital.. only real way to know is check the valve. It is true that if the valve is almost all the way closed... just a fraction turned on, a needle will bounce on an analog if you hit the purge hard... but if you have it turned on even half a twist - that will not happen.

rednose83
11-03-2009, 20:37
I should add that I always check to make sure that the valve is in fact all the way on- usually about 5 times.

ScubaToys Larry
11-03-2009, 20:44
I should add that I always check to make sure that the valve is in fact all the way on- usually about 5 times.

Good for you! That should do it!

CompuDude
11-04-2009, 11:12
And in answer to the other part of your question, no, digital will not react quickly enough to the pressure change to show a drop. It won't bounce like needles do.

rednose83
11-04-2009, 19:30
So, I tried it today (on land) just to see. My computer read 3125 after the 1st breath showed 2100 or so and after the second showed 500psi. On the 3rd breath, there was nothing- computer showed 0 and no air came out of reg.

CompuDude
11-04-2009, 23:46
So, I tried it today (on land) just to see. My computer read 3125 after the 1st breath showed 2100 or so and after the second showed 500psi. On the 3rd breath, there was nothing- computer showed 0 and no air came out of reg.

The test is designed to flag a "partially open" valve, not a valve that's actually OFF. If a valve is only partially on, you can suck the gas out of the hose faster than the valve refills it... the needle dips as you suck it out, and rebounds as the first stage refills the hose. This can cause more of a problem at depth than at the surface, is why we have the test to make sure this isn't the case.

With a digital gauge, they're usually just slow enough to respond that by the time it would start to register the slight dip in pressure, the pressure it back, so it doesn't really bounce. A slightly flicking digit is harder to monitor than a literally bouncing needle.

navyhmc
11-05-2009, 03:15
Where the partially open valve really comes into it's own it when you're at depth. When the air density increases, so does the resistance. What works nice and easy at 40' will be difficult as all hell at 90'.

I did this on one dive many years ago. Didn't get the valve turned on all the way and went in. Once down at 30', I didn't notice any problems. When I got to 70', it started breathing very hard. felt back and turned it on the rest of the way and presto! Easy breathing.