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?

 

[tursiops]

Absolute pressure at 100 ft is 3ATM (water) plus 1 ATM (atmosphere).

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?

The SPG reads gauge pressure. At the surface it reads tank pressure (e.g., 3000 psi) plus 0 ATM.

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

The transmitter also reads gauge pressure, identical to the SPG.

The SPG/transmitter tells you the pressure in the tank; what depth you are at is irrelevant.

 

[gbirch]

Neither.

Internally, most digital pressure gauges use a diaphragm so, unless the gauge is assembled in a vacuum, it is atmospheric pressure on one side of the diaphragm, so the gauge reads relative to atmospheric pressure. For example, a full cylinder, when tested on the surface is 3000psi above atmospheric, and an empty cylinder reads 0psi (above atmospheric).

Gauge pressure varies with depth, your digital SPG/transmitter reading does not vary with depth.

 

[scottgrizzard]

Absolute pressure at 100 ft is 3ATM (water) plus 1 ATM (atmosphere).

The SPG reads gauge pressure. At the surface it reads tank pressure (e.g., 3000 psi) plus 0 ATM.

The transmitter also reads gauge pressure, identical to the SPG.

The SPG/transmitter tells you the pressure in the tank; what depth you are at is irrelevant.

Umm... I think I may have miscommunicated the question...

A bourdon tube works like a very sophisticated spring... so the pressure from the ambient also pushes back... just like a stick tire gauge works.

So, when I take it down, the ambient pressure will push "harder," since the pressure increases. At 100ft, the ambient should be pushing against the gauge by another 44psi (the three additional atmospheres of the water). Thus, if I understand it correctly, an spg on a tank should read 44psi lower at 100ft than on the surface (if the temperature of the tank was held constant).

Does the electronic pressure gauge in the transmitter work the same way, or is there some other way it is measuring pressure?

 

[scottgrizzard]

Neither.

Internally, most digital pressure gauges use a diaphragm so, unless the gauge is assembled in a vacuum, it is atmospheric pressure on one side of the diaphragm, so the gauge reads relative to atmospheric pressure. For example, a full cylinder, when tested on the surface is 3000psi above atmospheric, and an empty cylinder reads 0psi (above atmospheric).

Gauge pressure varies with depth, your digital SPG/transmitter reading does not vary with depth.

Ah! Perfect! Thank you for explaining about the diaphragm!

 

[tursiops]

Umm... I think I may have miscommunicated the question...

A bourdon tube works like a very sophisticated spring... so the pressure from the ambient also pushes back... just like a stick tire gauge works.

View attachment 880308

So, when I take it down, the ambient pressure will push "harder," since the pressure increases. At 100ft, the ambient should be pushing against the gauge by another 44psi (the three additional atmospheres of the water). Thus, if I understand it correctly, an spg on a tank should read 44psi lower at 100ft than on the surface (if the temperature of the tank was held constant).

Does the electronic pressure gauge in the transmitter work the same way, or is there some other way it is measuring pressure?

Neither the SPG nor the transmitter (which work the same way, they just have different sensors) is exposed to the ambient pressure. They only see the pressure in the tank.

 

[gbirch]

"A bourdon tube works like a very sophisticated spring... so the pressure from the ambient also pushes back... just like a stick tire gauge works"

A bourdon tube doesn't work like that. It is a hollow, curved, closed-end tube filled with gas at one atmosphere. As the pressure increases, more gas enters the tube, forcing the curved tube to want to straighten. The "uncoiling" of the tube is transmitted with a pointer onto a calibrated gauge card. So, the bourdon tube is not affected by ambient pressure.

 

[scottgrizzard]

"A bourdon tube works like a very sophisticated spring... so the pressure from the ambient also pushes back... just like a stick tire gauge works"

A bourdon tube doesn't work like that. It is a hollow, curved, closed-end tube filled with gas at one atmosphere. As the pressure increases, more gas enters the tube, forcing the curved tube to want to straighten. The "uncoiling" of the tube is transmitted with a pointer onto a calibrated gauge card. So, the bourdon tube is not affected by ambient pressure.

Oh wow! Okay!

Thank you for explaining that... I totally misunderstood then! The wikipedia article on Pressure management must be incorrect...or scuba gauges must be different. I will need to read more...

AWESOME SAUCE! Thanks again!

 

[tursiops]

OP, the Wikipedia article you link seems pretty goo. It even says, "Bourdon tubes measures gauge pressure, relative to ambient atmospheric pressure, as opposed to absolute pressure."
What do you think is incorrect in the article?

 

[scottgrizzard]

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.