Originally posted by Dee
WAY deeper than you'll ever find me!
Boy do you have that right Dee!
POP QUIZ: How deep do you gotta be....
- Thread starter TexasMike
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Boy do you have that right Dee!
take away at least 10 atm of depth.
Roak is right, MOST (but not all) spg gauges are differential (not absolute) SOME are absolute, but calibrated to show approximate "gauge pressure" at sea level. A differential pressure gauge would show 0 psi at 6735 fsw. On the other hand, the OTHER pressure gauge we use (depth gauge) IS an absolute and calibrated not to show the very first atmosphere of pressure. It merely converts psi to feet on its scale.
Roak is right, MOST (but not all) spg gauges are differential (not absolute) SOME are absolute, but calibrated to show approximate "gauge pressure" at sea level. A differential pressure gauge would show 0 psi at 6735 fsw. On the other hand, the OTHER pressure gauge we use (depth gauge) IS an absolute and calibrated not to show the very first atmosphere of pressure. It merely converts psi to feet on its scale.
OP
TexasMike
Contributor
I was going on the answer that we came up with in class that the instructor said was correct:
- 3000 psi / (14.7psi/atm) = 204.082 atm
(204.082 atm * (33ft / atm) ) - 33 ft [1atm] = 6701 ft
JAW's answer was 6735 ft. And as he surmized, he didn't remove the 1 atm experienced at the surface.
Also, I didn't really intend to spark a huge scientific debate. The instructor was just using this to "liven" us up again during a marathon class session as he was reviewing the physics of why you can breath from a charged scuba tank. And I thought it would be a neat quiz for us to review.
But I guess I've learned, like many questions, there are lots of "right answers" if you don't provide every iota of exact information to start with
Mike
In a word, no.
It is the intermediate pressure rise that closes the high pressure valve/seat in the first stage. The high pressure valve is wide open with no pressure on the first stage at all.
Rick
Rick,
Mabey this belongs on the dumbest thing Ive ever done (or said)thread, but hey if you cant laugh at yourself
After reading your post I looked around and found a very good explination of how regulators work at Rodales. Not that I didnt trust you, just that I was tring to understand exactly what was going on, some day my life may depend on it. But the fact that I couldnt suck air through the regulater when it wasnt connected to an air tank seemed to contradict what it was saying. Upon further investigation, if you remove the dust cap, you can in fact suck air.
Well, live and learn.
Thanks,
Mike
Mabey this belongs on the dumbest thing Ive ever done (or said)thread, but hey if you cant laugh at yourself
After reading your post I looked around and found a very good explination of how regulators work at Rodales. Not that I didnt trust you, just that I was tring to understand exactly what was going on, some day my life may depend on it. But the fact that I couldnt suck air through the regulater when it wasnt connected to an air tank seemed to contradict what it was saying. Upon further investigation, if you remove the dust cap, you can in fact suck air.
Well, live and learn.
Thanks,
Mike
I was thinking that we were dealing with an instructor that was wrong but I wasn't going to say anyting until Mike came back with the "answer" from his instructor.
You don't remove 1 ATM because you're dealing with gauge pressure in ALL cases.
JAW's answer of 6735 was correct.
The reasoning goes like this: The cylinder has 3000 psig (the "g" being gauge) in it at the surface, since the gauge we use to mesaure cylinder pressure reads 0 psi when it's not hooked up to a cylinder, not 14.7 psi. The pressure at the surface is 0 psig since the depth gauge we use reads 0 feet at the surface, not 33 feet. At 0.44545 psi/foot, the point where water pressure reaches 3000 psig (notice the "g") is 3000/0.44545 = 6,735 feet.
Roak
No, it's not absolute, it's gauge too. It ignores the pressure of the atmosphere and reads zero at the surface. If you had an "absolute" depth gauge it'd read 33 feet at the surface.
The definition of "gauge pressure" is pressure relative to the ambient pressure.
True absolute gauges are pretty rare.
Roak
Some people clearly have too much time on their hands!!
The spg, is a differential gauge... it considers whats on the outside, and determines how much pressure is on the inside. It is off by another 14.7 psi every 33 ft or so. With the same internal pressure of gas, it would read lower and lower until at some point it would read 0 when the outside pressure and the inside pressure are equal. It's "standard" changes with depth.
The depth guage has it's own internal reference, and is determing the outside pressure from that sealed internal reference. While it is calibrated to "ignore" that first atmosphere of pressure it measurements are not "altered" by external events. It has no bulit in deviation like a differential gauge, and so gives a "true" or absolute reading. We could always print a new scale and change the "stop" so it we could measure the pressure in atmospheres, rather than in "feet". It's standard does NOT change with depth.
BTW, most digital SPGs are absolute too, with their calibration being off by 14.7 psi. Transducers are not usually affected by external influences. However, most digital depth gauges, while still absolute, calibrate themselves on the ambient surface pressure (altitude). They are better able to calculate Nitrogen tension at the end of a dive (tendency to bubble), because of that.
The depth guage has it's own internal reference, and is determing the outside pressure from that sealed internal reference. While it is calibrated to "ignore" that first atmosphere of pressure it measurements are not "altered" by external events. It has no bulit in deviation like a differential gauge, and so gives a "true" or absolute reading. We could always print a new scale and change the "stop" so it we could measure the pressure in atmospheres, rather than in "feet". It's standard does NOT change with depth.
BTW, most digital SPGs are absolute too, with their calibration being off by 14.7 psi. Transducers are not usually affected by external influences. However, most digital depth gauges, while still absolute, calibrate themselves on the ambient surface pressure (altitude). They are better able to calculate Nitrogen tension at the end of a dive (tendency to bubble), because of that.
Roak,
I agree with your reasoning about JAW's answer, but your (and JAW's) numbers are off a tad. JAW's answer is closer than Mike's instructor's, but still off. Where did you get "0.44545 psi/foot"?
I agree with your reasoning about JAW's answer, but your (and JAW's) numbers are off a tad. JAW's answer is closer than Mike's instructor's, but still off. Where did you get "0.44545 psi/foot"?
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