Question: Why can't you calibrate SPG?

[Nemrod]

Absolute accuracy is not required of an SPG. It is the end points and current position and the ability to observe the needle fall that gives the diver the sense of being ok on air or not and his/her relative consumption f that resource. If you compare gauges to a known accurate standard and your gauge is off a hundred psi at 500 actual psi, is that really a big deal. Or that it reads 3100 at an actual 3000?

If the case floods the gauge will read off by the additional ambient pressure over surface sea level or the last time the case was vented and the needle set.

So, how do you know when comparing gauges, which one is the accurate one? I have has some read higher on one end and lower on the other end of the scale. You would have to have or have access to a recently certified reference gauge.

Analog, mechanical gauges are never absolutely accurate over their entire range, at least not simple bourdon tube gauges without a compound mechanism. Generally they are designed to be most accurate in the important range, and for an spg, that is below 1,000 psi or so.

N

 

[g1138]

dfx, corrosion can happen on just a certain portion of your metal wire spiral and not the whole thing throughout. So it only contracts evenly till a point and after that it's no longer "linear" (if we can use that word); that would explain any jumps along a gauge's dial.

 

[dfx]

I'll try to be more clear. This type of gauge is most inaccurate at either end of the scale. Lets assume it passed final test and shipped with a plus or minus 2% error. 0.02 x 4000 psi = 80 psi. So they put zero somewhere behind the pin to keep you from seeing the error. Back on post #4, second gague, you will see that the pin is at the 100 psi mark.

So let's say that the true zero wandered somewhere so that the indicator is just ready to lift off the pin. You can't see this by looking at the gauge. In addition, there is the gauge error to consider. So if my gauge is reading 200 psi, I can be pretty sure that I have at least 80 psi (10x the error, sorry about the two 80's *confusing*) in the cylinder if it was reading zero before the dive...

Ahh I see what you're saying now. I didn't notice that the "zero" was actually on the 100 psi mark until you pointed it out just now. Makes much more sense.

 

[lowviz]

Good, everybody is on the same page with respect to SPG's.

Now would be a good time for the reg whizzes to discuss what happens when your cylinder pressure drops below your reg's intermediate pressure. You may not have as many 'fumes' as you think...