Does an spg measure vs atmospheric or ambient pressure?

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Rogerdd

Registered
Messages
49
Reaction score
13
Location
UK
If spg shows 200 bar at the surface (gauge pressure), that means the absolute pressure in the tank is 201 bar.

If we go down to 30m and don’t use any gas, the absolute pressure in the tank is still 201 bar (because the cylinder doesn’t deform).
But would the spg still show 200 bar, or 197 bar?

Basically 1) is an spg a gauge pressure vs atmospheric pressure (closed chamber in the spg), 2) or vs ambient pressure (atmospheric + hydrostatic at whichever depth the reading is made)?
 
If spg shows 200 bar at the surface (gauge pressure), that means the absolute pressure in the tank is 201 bar.

If we go down to 30m and don’t use any gas, the absolute pressure in the tank is still 201 bar (because the cylinder doesn’t deform).
But would the spg still show 200 bar, or 197 bar?

Basically 1) is an spg a gauge pressure vs atmospheric pressure (closed chamber in the spg), 2) or vs ambient pressure (atmospheric + hydrostatic at whichever depth the reading is made)?

SPGs are sealed. The indicated pressure will not vary with depth.

They also aren't accurate enough for it to matter. The standard allows a range of + or - 10 bar from 100 to 200 bar indicated, and +- 5 bar at 50 bar.
 
It's indicating gauge pressure, as opposed to absolute pressure.

Yes, in a sense it is always measuring the ambient pressure as well as the tank pressure to compare the two numbers

Absolute pressure is the pressure "in the tank" relative to complete vacuum. (PSIa in engineering terms)

Guage pressure is indicating the pressure "in the tank" relative to the surrounding ambient pressure (PSIg in engineering terms)

So in the sense of a SCUBA SPG, it's measuring against the atmospheric pressure when you're on the boat, and when your at depth it's measuring against the atmospheric pressure+ the water column pressure

A full tank at 3,000 psig would read a gauge pressure of zero PSIg if you submerge it deep enough so that the surrounding ambient pressure in the water column was 3,000 PSIa
 
It's indicating gauge pressure, as opposed to absolute pressure.

Yes, in a sense it is always measuring the ambient pressure as well as the tank pressure to compare the two numbers

Absolute pressure is the pressure "in the tank" relative to complete vacuum. (PSIa in engineering terms)

Guage pressure is indicating the pressure "in the tank" relative to the surrounding ambient pressure (PSIg in engineering terms)

So in the sense of a SCUBA SPG, it's measuring against the atmospheric pressure when you're on the boat, and when your at depth it's measuring against the atmospheric pressure+ the water column pressure

A full tank at 3,000 psig would read a gauge pressure of zero PSIg if you submerge it deep enough so that the surrounding ambient pressure in the water column was 3,000 PSIa
The person above you is saying the opposite. So I’m still not sure which one is it.

Tank pressure vs ambient pressure

Tank pressure vs 1 atm regardless of depth (not relative to ambient pressure)

[or even tank pressure vs vaccum = absolute]

In practice I agree it doesn’t matter much, it’s just for my knowledge
 
The person above you is saying the opposite. So I’m still not sure which one is it.

Tank pressure vs ambient pressure

Tank pressure vs 1 atm regardless of depth (not relative to ambient pressure)
It depends on the type of sensor used. Electric will tend to be absolute, others could be either.
 
Most gauges use atmospheric pressure as its zero point, also known as Gauge Pressure (PSIG). Pressure above atmospheric. Absolute pressure uses zero pressure(vacuum) as its zero point (PSIA). So, a standard analog SPG will be atmospheric pressure as it reacts to its surrounding(ambient) pressure, so a reading of 1000 psi actually includes an additional 14.7 psi of atmospheric pressure. However, a digital SPG can do calculations and can read in absolute pressure.
 
Pressure measurement - Wikipedia will tell you everything you want to know about pressure measurement and Pressure measurement - Wikipedia describes how Bourdon tube gauges work- every mechanical SPG is going to be a Bourdon gauge.

Now, every SPG has the Bourdon tube encased in a sealed housing along with the rest of the mechanical components. And that sealed unit should have an internal ambient pressure of 1 atm. So, the Bourbon will always be measuring tank pressure vs its sealed internal 1atm, unaffected by external pressure changes due to depth (that’s why it’s submersible.) And as the common standard for pressure gauges they set that 1atm internal pressure to be zero on the gauge
 

Back
Top Bottom