SPG correction

[ClayJar]

I've heard 0.10 ata as the partial pressure of oxygen considered to be the lower limit for a normal person to remain conscious. That would put the total pressure at 0.48 ata, i.e. 7 psi.

Personally, I have never seen an SPG that was not rated for the entire "recreational" depth range, which means the SPG's readings are valid at least from the surface (1 ata) to about 130 fsw (5 ata).

As there is not a "depth correction" for the SPG, and the change in ambient pressure from surface to 130 fsw is 2.5 times the change in ambient pressure from the sea level to the altitude for 0.10 ata ppO2, it stands to reason that no SPG correction is required.

(It is also logical that a device which measures pressures 200 times ambient pressure at sea level should not be affected much by an ambient pressure change of one quarter of one percent of the measured pressure.)

 

[captain]

The gauge mechanisim is in the pressure proof housing and the pressure in the housing is what ever the atmospheric pressure was at the time the manufacture sealed it. External ambient pressure both positive or negative has no effect because it is not felt by the gauge mechanisim in the sealed housing.

 

[Charlie99]

The gauge mechanisim is in the pressure proof housing and the pressure in the housing is what ever the atmospheric pressure was at the time the manufacture sealed it. External ambient pressure both positive or negative has no effect because it is not felt by the gauge mechanisim in the sealed housing.

Obvious. Once you point it out, that is. I learn something new every day.

 

[ClayJar]

The gauge mechanism is in the pressure proof housing and the pressure in the housing is what ever the atmospheric pressure was at the time the manufacture sealed it.

The pressure in the housing is whatever the atmospheric pressure was at the time the manufacturer sealed it, assuming that the gauge has no leaks and is at the same temperature as it was when sealed. As the temperature can change by as much as 10% (from 95°F to 40°F) on a nice deep lake dive, the internal ambient pressure (i.e. that of the air space surrounding the Bourdon tube) can change by 10%.

Still, if you take a gauge with a freshly broken face and bring it down to 130 fsw, the change in ambient pressure surrounding the Bourdon tube is only about 60psi, which is less than the precision acheivable by reading the instrument.

 

[JDog]

I know I shouldn't even go here BUT, I am willing to guess if someone else is willing to do the sceintific research.
The multiple choice question is.

What would be the indicated pressure drop, on a SPG, for a Al80 filled to exactly 3000 psi, with the only difference being the air pressure surrounding the cylinder,which is 14.7 psi(sea level) versus 7.35 psi(18,000ft)? We will assume temperture remains the same.

A. over 10 psi
B. 1 psi to 10 psi
C. .1 psi to 1 psi
D. .001 psi to .1 psi
E. under .001

I guess it would be D

 

[scubak]

Correct me if I am wrong...but an air tank at altitude would be what it is on an SPG...what it is reading....much like the tires on your car that got you there.
Compressed air is air in whatever container...at what ever altitude....
K

 

[JDog]

You can see the effects of atmospheric pressure very easily taking a ballon filled with air from sea level for a drive up into the mountains, the ballon expands because the outside atmosphere has decreased in density. Physics is physics. The same would happen to an Aluminium cylinder only at a microscopic level just ask Walter.

 

[jetfixer]

I'm going to say "C"

 

[ClayJar]

What would be the indicated pressure drop, on a SPG, for a Al80 filled to exactly 3000 psi, with the only difference being the air pressure surrounding the cylinder,which is 14.7 psi(sea level) versus 7.35 psi(18,000ft)?

The correct answer is "the question is wrong" (or at least, that's what I'd argue if I ever saw that on a test -- I've argued against questions before and been upheld).

Now, if the question was asking about an AL80 filled to 3000.00 psi (pounds per square inch) at 7.35 psia ambient pressure (eg. 18,000 ft), and the question asked what the gauge pressure would be at 14.70 psia ambient pressure (eg. sea level) assuming the temperature was constant, and it did not specify an SPG (but was rather a question about gauge pressure in general, which is what the Bourdon tube in an SPG reads), then the answer would be 2992.65 psi (gauge pressure), i.e. a drop of 7.35 psi. The absolute pressure in the cylinder would still be 3007.35 psia, but instead of subtracting the ambient pressure of 7.35 psia, you'd subtract the ambient pressure of 14.70 psia.


scubak: The absolute pressure inside the cylinder remains constant (at constant temperature, and assuming you're not going to crush or stretch the cylinder or let gas out). Gauges do not read absolute pressure, however. They read gauge pressure (wonder how they came up with that name ), which is the difference between the absolute pressure on the inside and the absolute pressure on the outside. If you had a rather accurate high-precision pressure gauge, you could graph the gauge pressure difference in your tires vs. altitude. (Next time I go to Pike's Peak, I'm going to have to bring a lab-quality gauge to actually do that.)

(As an aside, those people who say the lunar rover's tires couldn't be made out of rubber because the pressure would make them explode are not physics majors. The pressure in your tires is about 2 ata, and they don't often explode. The real reason, of course, is that rubber doesn't take kindly to being alternately cooled to cryogenic temperatures and heated to high temperatures, which is what the tires would have to deal with in space, and so they made the lunar rovers' wheels out of piano wire. )

 

[JDog]

Set aside the fact that I am not a teacher (not that you need to be) and therefore cannot write a question properly. I tried to write it properly but OBVIOUSLY failed.

I think you know exactly what I was looking for.

Per Walters post I was trying to get someone to determine the actual SPG pressure difference from a Al80 that was filled to exactly 3000.0000 psi at sea level, then transporting that cylinder to 18,000ft, and that difference would be due entirly on the difference of the external atmospheric pressure (14.7psi vs 7.35psi) applied to the cylinder.

Is that better?

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

I could be wrong here, but if you filled an AL 80 at 18,000 ft and then brought it to sea level it would have more pressure in it not less.