Red zone on SPGs

[wunat]

I have been doing some reading. In many discussions, it is stated that the calibration of the bourdon tubes (which the SPGs are made of) are made for accuracy in the mid range (the middle 1/3 of the operating pressure). Assume the gauges are designed to operate at 300 bar. The most accurate range is supposed to be between 100-200 bar.

Is this true?

 

[tbone1004]

Little bit of everything. These are analog gauges that are typically very accurate in the middle of their range, and not nearly as accurate at the extremes. This is true for just about every analog gauge out there, hell look at your speedo and how much it varies when you're below 10-15mph. There is a reason that analog gauges have a known "working" range, and the scale of the gauge is supposed to be based off of the working range being smack dab in the middle. For SPG's you want "turn pressures" to be in the middle. This is typically 5000psi or 350 bar in the US at least. Puts the sweet spot at 2500psi which thankfully is turn pressure for cave diving so your gauge is most accurate around those pressures. It is very accurate from say 1500-3500psi, and as you approach the extremes of the bourdon tube, the accuracy gets progressively worse. The red zone is in fact a zone in the pressure gauge where it is less accurate than the normal range, but the red zone is there for recreational divers to have the "oh sh!t" moment when they see it to come up because it also conveniently coincides with a rough amount of air based off of some rough cut numbers for a normal diver on an AL80 to come up from 100ft, do a safety stop, and get back on the boat without sucking the tank dry.

 

[Kharon]

My SPG has a red zone from 1000psi to 500psi. When I get to the red zone I know I'm getting close to 500psi and should be thinking about making it back. I've had a number of dives where a fellow diver saw my needle nearing the top of the red zone and called the dive because he thought I was running out of air when I still had plenty for the depth and distance from the boat. PITA.

 

[dmaziuk]

I have been doing some reading. In many discussions, it is stated that the calibration of the bourdon tubes (which the SPGs are made of) are made for accuracy in the mid range (the middle 1/3 of the operating pressure). Assume the gauges are designed to operate at 300 bar. The most accurate range is supposed to be between 100-200 bar.

Is this true?

What @tbone1004 said.

Strictly speaking, sensors tend to respond non-linearly and typically the "least unlinear" part is in the middle. In theory you could futz with the size, stiffness, etc. to make "the middle" cover a wider range but that usually makes the device less "slimline". In the case of the bourdon tube I imagine that to be much less slimline.

 

[Bob DBF]

Hi,

I have been told a while ago that the red zone on the SPGs (below 50 bar or 700/500 psi) are referring to the fact that the bar/psi measure by the gauge are not accurate anymore.
.

The red is just a visual reminder you are getting low on air. Older gauges did not have them and they were probably added to reduce OOA incidents.

Scuba Diving Dive US Divers High Pressure Gauge HP SPG compact travel light


Bob

 

[Dizzi Lizzi]

True, but at least that's better than the opposite where you have DMs sharing their tanks with people who have blown all of their air...sharing their tanks not to surface safely, but to continue the dive. <Shudder>

Are you kidding me? That is terrible, an incident waiting to happen.

 

[dmaziuk]

My SPG has a red zone from 1000psi to 500psi. When I get to the red zone I know I'm getting close to 500psi and should be thinking about making it back. I've had a number of dives where a fellow diver saw my needle nearing the top of the red zone and called the dive because he thought I was running out of air when I still had plenty for the depth and distance from the boat. PITA.

Ugh.

I find that someplace below 20m red goes dark and then I can't see the black needle quite as well against it. Not that I'd want to be at that depth with the needle in the red zone -- at least where the normal SPGs have the red zone -- but didn't anyone ever tell the people making these things about the colours down there?

 

[Neilwood]

Ugh.

I find that someplace below 20m red goes dark and then I can't see the black needle quite as well against it. Not that I'd want to be at that depth with the needle in the red zone -- at least where the normal SPGs have the red zone -- but didn't anyone ever tell the people making these things about the colours down there?

It would certainly be better if they had a bright needle against a darker background - wouldn't matter as much if the red zone had blended into the background then as you would still be able to tell the relative location of the needle

 

[Jim Lapenta]

I hate the few gauges I have with red zones on them. In low vis it makes the needle hard to see. Some of them have the zone starting at 700 psi. To me it's more of a marketing gimmick to make them stand out and APPEAR to be safer.

 

[Edward3c]

Hi,

I have been told a while ago that the red zone on the SPGs (below 50 bar or 700/500 psi) are referring to the fact that the bar/psi measure by the gauge are not accurate anymore.

Is this right? Any sources we can read about it further?

Thank you.

In the good old days. My first SPG had the red zone begin at 30bar. That was the pressure I was taught to terminate the dive and surface. A full cylinder was 160bar (if lucky).