SPG correction

[Thalassamania]

With that in mind, if one were to take a tank to 18,000 feet (please keep in mind, no one should ever dive at altitudes over 10,000 feet) our ambient pressure would be .5 ATM or 7.35 PSI.

Dr. Charles Brush III, who died recently, holds the world's record for high altitude diving at 19,455-foot in a volcanic crater on the border of Chile and Bolivia. But then, he used a Carlton Cobra pure oxygen rebreather.

 

[ClayJar]

Certainly the difference of 7psi on the outside of the cylinder could't make the internal pressure deviate 7psi on the SPG.

If the ambient pressure outside the Bourdon tube is that of a perfectly sealed, perfectly rigid container, then the ambient pressure surrounding the case would have no effect. If, on the other hand, the ambient pressure outside the Bourdon tube is that of the atmospheric pressure at that location, a difference of 7psi outside the cylinder would absolutely show up as a difference of 7psi in the SPG's measurement. That's the very definition of gauge pressure, i.e. the difference in pressure between one side of the gauge and the other.

So, if the SPG is sealed and rigid, it will be measuring the pressure difference from the cylinder to the inside of the gauge, and the measurement will not change (except very slightly, as there will be some all but imperceivable stretch in the materials of which the gauge housing is made). If the inside of the gauge (i.e. the outside of the Bourdon tube) is open to the atmosphere (i.e. a crack in the face, a bad seal, or whatever), the measurement will change by precisely the amount of the change in atmospheric pressure, but you're quite likely not going to be able to see such a small change.

Is that a better way to explain that I *was* already answering what you asked?

 

[Walter]

The correction depends upon the type of gauge. The above corrections are for the bourdon tube type depth gauge --- the ones where a 10' difference is about the same angular change, whether it is 0 to 10' or 90' to 100'.

Capillary gauges (where 0' to 10' is much bigger angular change than 90' to 100') have a whole different depth correction. IIRC, those corrections end up being identical to the equivalent sea level depth corrections and you can just dive your standard sea level tables using the depths displayed on the capillar gauge.

Digital gauges general don't need an altitude correction, as they automatically rezero themselves.

Excellent point.

 

[roakey]

(and for those who can't read, this is not the gauge that tells you how much air you have, but the one that tells you how deep you are)

And for those that are so quick to insult from a completely incorrect position, an SPG has never been, nor ever will be a depth gauge. The SPG is what tells you how much pressure is in your cylinder.

This definition is world wide -- you need to go back and read your open water textbook.

Roak

Ps. For the record, SPGs (the real definition, not a depth gauge) are so innaccurate, probaly more than 10%, that the small delta in pressure of diving at altitude will result in a difference well below the "noise level" of the gauge and can be complete ignored.

In other words, it's a moot question.

 

[JDog]

......((except very slightly, as there will be some all but imperceivable stretch in the materials of which the gauge housing is made). If the inside of the gauge (i.e. the outside of the Bourdon tube) is open to the atmosphere (i.e. a crack in the face, a bad seal, or whatever), the measurement will change by precisely the amount of the change in atmospheric pressure, but you're quite likely not going to be able to see such a small change.

Is that a better way to explain that I *was* already answering what you asked?

Yes I understand. My angle on this whole thing was...The imperceivable stretch(or compression) in the materials of which the housing (as well as the hose and cylinder) is made.

Spankey caused quite a ruckus with this thread. I believe we have answered the intent of the original question, even though he misspoke about the whole SPG depth gauge thing.

All in favor :nod:

 

[jetfixer]

AYE!! yay yay...