Red zone on SPGs

[wunat]

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.

 

[CuzzA]

I haven't heard that; however, I have heard that you should plan your dive with a 100-200 psi buffer for possible inaccuracies and the potential for the first stage to not deliver air depending on the working pressure of the first stage as the pressure decreases in the tank.

I'm pretty sure the red is simply a visual aid to warn you. No different than the many red lines we see everyday, like on a tach or fuel gauge for example.

Edit: I'll add if that were true it certainly wouldn't give me a whole lot of confidence in an SPG if at 500-700 psi your SPG was no longer accurate. I wonder if this just isn't a myth created by dive op to scare (for lack of a better term) customers to be back on the boat with that amount of air.

 

[NYCNaiad]

I hadn't heard that about the red area. Interesting question. It'll be cool to see what others say.

 

[boulderjohn]

It may be a matter of the language used in describing things. The red zone is on the SPG, which varies to some extent by manufacturer and is not even present on all SPGs, is just a warning that you are getting near the end of your gas supply. Now, as it turns out, any inaccuracy in the gauge is also going to show up in that area--it could not possibly be detected until you are at the end of the gas supply. So someone might have said that the SPG is not accurate in that area (which is true), and that statement might have been misinterpreted to mean that the purpose of the red zone is to indicate that it is not accurate in that area (which is not true).

 

[Tbar]

For an SPG I expect that the accuracy would be based on "Percent of Full Scale" error rate rather than "Percent of Reading". What this means is that the same absolute error will apply at all points. If the gauge had an error of 1% Percent of Full Scale and a full scale reading of say 400bar then the error would be 4bar. In this example at a reading of 50Bar the pressure may be between 46 and 54bar. The scale is probably 10bar.

Accuracy can be impacted by quality of build impacting the Percent of Full Scale and Percent of Reading or size of face and graduations on the scale impacting Percent of Reading.

If you cannot trust the gauge at 50bar then it would have to have a very high Percent of Full Scale error, say 25%!

I have a couple of SPGs and find that with a full tank they read about 10bar different, maybe +5 and -5 on respective SPGs so between a 1 and 2% error. At 50bar they are closer.

For me the Red is to reinforce the 'safety' reserve gas. Not all SPGs have the red.

 

[boulderjohn]

For an SPG I expect that the accuracy would be based on "Percent of Full Scale" error rate rather than "Percent of Reading". What this means is that the same absolute error will apply at all points.

Sounds good, but...

How will you know if the SPG is inaccurate if it is telling you that you still have half a tank and everything is going fine?

The only way you will know is if you are suddenly out of gas and the gauge is still telling you that you have 150 PSI to go.

 

[DevonDiver]

Unilateral marks on the lowest round number of the gauge, relative to the scale of marking.

Nothing more, nothing less.

Makes me laugh when supposed 'pros' get anally retentive about customers/divers who 'violate' the red zone, especially for metric gauge divers - but have no basic understanding that imperial-metric is not even the same pressure (500psi = 35 bar, not 50 bar).

 

[Tbar]

How will you know if the SPG is inaccurate if it is telling you that you still have half a tank and everything is going fine?

The only way you will know is if you are suddenly out of gas and the gauge is still telling you that you have 150 PSI to go.

Well you check it! Do you have a clear idea of the expected tank pressure before you put your reg on? If your SPG is well off then check.
Did your gauge show zero before opening the tank, so it should return to zero and not be empty at 100bar.
When you finish the dive, note the SPG reading, and then occasionally check the tank with another gauge. If they vary to a point you are concerned then check the gauges.
If you repeated finish a dive with significantly more or less gas than your buddies, maybe check - my buddy always uses a lot less gas so that's what I expect

 

[Tbar]

Makes me laugh when supposed 'pros' get anally retentive about customers/divers who 'violate' the red zone, especially for metric gauge divers - but have no basic understanding that imperial-metric is not even the same pressure (500psi = 35 bar, not 50 bar).

Being a metric diver this is frustrating, and just a convenient number on the gauge.

 

[NYCNaiad]

Makes me laugh when supposed 'pros' get anally retentive about customers/divers who 'violate' the red zone, especially for metric gauge divers - but have no basic understanding that imperial-metric is not even the same pressure (500psi = 35 bar, not 50 bar).

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>