Technical Question about Digital SPG - How does it measure? Does it measure absolute or gauge pressure?

[scottgrizzard]

So, I am wondering about the technical side of how the digital gauge works in a transmitter.

My understanding is that a traditional analog bourdon tube spg actually measures gauge pressure... the spg would read 44psi lower at 100 feet than at the surface (4ATA - 1ATA). Is this correct?

How does a transmitter's gauge work? Is it measuring absolute pressure or gauge pressure?

 

[gbirch]

Okay, so if I understand properly, a bourdon tube does work like a spring, but the tube on a scuba SPG is sealed at 1ATA, so it is always pushing back at the same rate, and will thus read consistently even as depth changes.

No. It is closed at one end and the other end is open to the High-Pressure source. The whole thing is encased in a sealed case, so it is always at 1 ATM regardless of depth... until its crush depth is reached .

Even if it were to be subject to ambient pressure then the ambient pressure would have no net effect since the ambient pressure would be applied equally to all surfaces of the tube - no net force to influence how the tube uncoils.

I think that the confusion is because the Wikipedia article refers to "gauge pressure", but the article is written from the perspective of a standard instrument used at (more or less) surface pressure (1 ATA). In SCUBA, gauge pressure has a different meaning, and refers to the hydrostatic pressure created by depth: i.e. Absolute pressure - 1.

 

[broncobowsher]

Most electronic pressure transducers I know of work in absolute. But the reading is zeroed at 1 ATA.

The other side of that, if you are to the point it matters if ambient pressure alters a gauges reading, you are cutting it WAY too close. Most gauges have way more error than what ambient pressure changes will show up as. So this is strictly a theoretical question and not one that will really matter at all. The only digital pressure gauges I ever care about the full display are when blending gasses. Even then you are dealing with pressure changes from heat which is way more error than the gauge has in it.

 

[mr_v]

Okay, so if I understand properly, a bourdon tube does work like a spring, but the tube on a scuba SPG is sealed at 1ATA, so it is always pushing back at the same rate, and will thus read consistently even as depth changes.

If you're interested in topics like pressure measurements inside diving systems, I recommend Life Support System Design by Nucklos, ISBN-13: 978-0536596161. From pages 18-19:

Pabsolute = Patmospheric + Pgauge

If a tank gauge shows 3000psig (psi, gauge), then the absolute pressure in the bottle is 3000psig + atmospheric pressure. If the atmospheric pressure is 15psia (psi, atmospheric), the absolute pressure in the bottle is 3015psia. The absolute pressure in the bottle will stay the same regardless of the local atmospheric pressure.

However...

If you take that bottle into an environment pressurized to 100psia, then the gauge pressure will be 3015psia-100psia=2915psig. That is how much gas you'd have available in the high pressurized environment, i.e., if you open that bottle the gas will come out until the pressure in the bottle is equal to the pressure in the environment. The remaining gas will be unavailable.

 

[lowwall]

Your reg isn't going to deliver gas once the tank gets below around 6 bar above ambient anyway.

 

[scottgrizzard]

Sounds like I need to put an analog spg and a transmitter on a regulator with a pony bottle, find a pressure chamber, and see what happens.

 

[tursiops]

Sounds like I need to put an analog spg and a transmitter on a regulator with a pony bottle, find a pressure chamber, and see what happens.

You only need to do that if you live on a flat earth. [/sarcasm]

 

[broncobowsher]

Sounds like I need to put an analog spg and a transmitter on a regulator with a pony bottle, find a pressure chamber, and see what happens.

The ocean makes a nice pressure chamber.

And what does it really matter anyway?
Measure with micrometer
Mark with chalk
Cut with ax
I have regs and gauges that can be over 100 PSI off between the two. Doesn't change anything.

 

[justinthedeeps]

Lots of sureties expressed here ...

My Shearwater transmitters have a little waterproof OPV nubbin in the bottom side of the housing, which gets neither ejected nor sucked in during dives to depths as deep as 100+ metres. So there is some level of equalization with the increasing external ambient pressure.

Not sure if or how this is experienced or included in any of the device's actual internal bits, readings or calculations.

In a now well documented flaw, some of these OPVs were blowing out, due presumably to internal high [tank] pressure escaping into the transmitter housing, unless there is another official explanation [not speculative, please]. If that is the cause, I would presume that yes in fact the external ambient pressure might factor into readings as well.

 

[scottgrizzard]

The ocean makes a nice pressure chamber.

And what does it really matter anyway?
Measure with micrometer
Mark with chalk
Cut with ax
I have regs and gauges that can be over 100 PSI off between the two. Doesn't change anything.

For the sake of knowing

 

[rjack321]

For the sake of knowing

Are you going to buy a couple dozen of each to calculate a 3-sigma difference between the means?
These instruments are not that precise that a one-to-one comparison is remotely meaningful.