POP QUIZ: How deep do you gotta be....

[TexasMike]

Walter and Roakey,

Please also understand that the instructor was not intending to provide us with "gospel" information. He was just trying to liven us up a bit with a "spur of the moment" thinking question during a long class session. And now that I review things in my head, I'm not certain he had the actual answer on his notes since he was doing the math along with us.

Again, this was more of a "fun" exercise than promoting a "gospel" fact.

 

[roakey]

Yes, really Pete...

You’re convoluting a very simple concept, Pete. A gauge that measures absolute pressure shows ALL pressure acting on it, a gauge that measures gauge pressure ignores ambient pressure is a GAUGE gauge (yuck, any better way of saying that? )

Key is, what does it read at the surface? If it reads 14.7 psi it’s an absolute gauge. If it reads zero, it’s a gauge gauge.

What does your SPG read at the surface when exposed to ambient surface pressure? Zero, it’s reading gauge PSI. What does your depth gauge read at the surface? Zero, it's reading gauge.

Now, given your depth gauge is “inside out” for lack of a better term makes no difference. Gauge versus absolute is about what a gauge reads, not how it’s built.

Therefore:

Originally posted by NetDoc
BTW, most digital SPGs are absolute too, with their calibration being off by 14.7 psi.

If it’s “off” by 14.7 than it’s reading gauge, who cares what the internal implementation is?

Again, “gauge” versus “absolute” is what the gauge displays; it has NOTHING to do with the internal implementation. A gauge, like the PSI Tronix I have on my O2 cascade is absolute-capable, because the transducer can measure absolute pressure (as you mentioned). But I ordered it in its gauge configuration. The difference between gauge and absolute gauges is simply what it shows me, not how it works inside.

Don’t be confused by the fact that the old mechanical depth gauges get screwed up at altitude. They’re *supposed* to be gauge devices, but because they’re capable of measuring absolute they become confused at altitude unless there’s a way to adjust them back so they’re reading gauge pressure accurately again.

Roak

 

[The Chairman]

I think we are both piling three different types of gauges into only two categories and are at odds as to how to separate them. I combined the sealed gauges in with the absolutes... Roakey combined the sealed gauges in with the "non-sealed". Here is what my research on the web AND my faulty memory has dredged up.

"Gauge" (non-sealed) noted by psig, is a differential value. It measures the internal pressure of a vessel against ambient pressure. While at sea level, a gauge may read 3000 psig when it in fact has 3015 psia in it. HOWEVER at depth (6735 fsw) this gauge will theoretically NOT read 3000 psig, but 0 psig when in fact it still contains 3015 [color= green]psia[/color]. The reading changes with ambient pressure.

"Sealed" noted by psis, is NOT a differential value but a "true value". It uses an internal reference to measure the internal pressure of a vessel. A sealed gauge may or may not be absolute. If it reads 3000 psis the pressure may very well be 3015 psia (depending on the reference). HOWEVER at depth (6735 fsw) it will STILL read 3000 psis. The reading does NOT change with ambient pressure.

"Absolute" noted by psia, is not a differential value but a true value calibrated from 0 psia. All absolute gauges are sealed, but not all sealed gauges are absolute. The same vessel would read 3015 psia at sea level, AND at 6735 fsw. The reading does NOT change with ambient pressure.

MOST SPGs give measurements in psig. The notable exceptions would be digital gauges that rely on a factory calibrated transducer.

Most Depth gauges give measurement in fsw, which could be converted to psis. This is why they misread the depth of fresh water, but still your tables are predicated on the PRESSURE at fsw and so having the gauge calibrated in fsw keeps us from having two sets of tables. Not that a foot or 4 would make a substantial difference in your on-gassing.

If that doesn’t relieve any of the pressure, then just pull my finger.

 

[rustyscubatool]

I certinly agree with whoever said that some people have way too much time on their hands. Why was this important anyways? What was the person (I think it was TexasMike that started this thing) trying to prove, or point out? As if I (or any of you other divers, who should have been working) is ever going to really go that deep and turn on a valve.
Which brings up a question.... what is the deepest anyone has ever dived on scuba?

 

[JustAddWater]

I think TexasMike was just posing a fun math conundrum related to the science of our sport, that would promote some varying thought and discussion.
Socrates would be proud of you, TexasMike.

 

[Stone]

I put a "Scuba Conversion" spreadsheet (for Excel users) on my website (Click on my URL below and go to the Dive Documents" page).

It converts Depth, Volume, and Pressure to Metric and English units commonly used in scuba circles (no tonnes per square hectare).

It looks like this:

 

[TexasMike]

Originally posted by rustyscubatool
What was the person (I think it was TexasMike that started this thing) trying to prove, or point out

Trying to prove? Nothing at all. I had absolutely no hatchets to bury, crosses to burn, or opinions to slam.

Like JAW said, I was just wanted to share an interesting "math" problem regarding the physics of diving. Intention was fun, not starting a debate ( after Pete )

It was some of the other contributors to the thread that have taken tangents that I had never dreamed of.

 

[The Chairman]

I learned QUITE a lot from this thread...

1) Rick was right in that the IP valving mechanism would be "wide open" at depth. That really never occcured to me, though it should have been obvious.

2) The difference between the different psi measurements; as in psig, psia, and psis. This maybe "minute" differences to some, but I just want to know the truth.

3) Roakey was right about the guage pressure being off by 14.7 psi and what the actual depth would be for it to equalise. Maybe he even sees now how he and I were both right and wrong on the gauge issues. But I want to go on record and say he was mostly right... I was using the wrong term, not him

4) Walter's numbers were good for me to know.

5) Like all Texans, Mike sure knows how to stir up a hornet's nest.

 

[SaltyWhale]

Hi Folks....

Assuming the factors of such;

- Sea Water
- Temperature unchanged
- Without attachment of Regulator (intermidiate pressure is ussually between 100-150psi)

The depth from my calculation is 6661.0774ft.

To my understanding 1 Atm = 32.8ft.

And we do have to knock off 1atm from the surface regardless of absolute / gauge pressure. An empty tank at the surface still contains 1atm not 0atm.

By the way....my conversion is translated from metric.

I do understand that this was an exercise to liven up your class, but I just wanted to share my opinion.

Also, I believe questions like this are fun....keep your mind working at all time.

Cheers

 

[TwoBitTxn]

Originally posted by rustyscubatool

Which brings up a question.... what is the deepest anyone has ever dived on scuba?

The open circuit scuba depth record was recently broken. Read Aprils edition of Skin Diver. Side note, he was using Apeks regs and an OMS wing. (Non bungied I would bet)

Tom