SPG failure frequency?

[munitor]

Depends on how you define "failure". Calibration drift is the most common failure, but since diving doesn't need the accuracy of process control, being 2% off is not a big deal (100 psi per 5000). Problem is that inaccuracy is usually not linear, often being larger at minimum or maximum scale, or can be a fixed amount (needle offset) which makes for huge errors at low pressures.

SPGs should not only be serviced for the moving parts (o-rings, swivels) but also checked for calibration. Several OOA incidents have been attributed to out of calibration SPGs.

Significant calibration drift is present about 6 times per million hours of operation, assuming failure frequencies similar to what is seen in industry for bourdon tube gauges.

Electronic pressure transmitters that have self-diagnostics are usually 10 to 1000 times more reliable than analog instruments (usually because they will tell you when they are having problems), but I have no idea what is being used in dive computers. Even with electronic transmitters, calibration needs to be verified on an annual basis, so if you have an air integrated computer, don't forget to have it checked.

 

[DA Aquamaster]

Most SPG's are calibrated in the middle of their range which is nice as it assures the turn pressure is probably fairly close.

However serious calibration errors should not be an issue. If the SPG reads 200 psi when it is not connected to anything, the error is obvious and it is an ID10T error if the diver uses it and runs out of gas. If on the other hand it reads "0" when there is still 200 psi in the tank, it is not going to be evident in a pre-dive check. But it is also not going to be a problem as the diver should be out of the water by then anyway - and the error is in the divers favor as the reg will still be delivering air for awhile even after the needle hits the peg.

In short, serious calibration errors are not common and will only be a problem if the diver is clueless in noticing the SPG always reads something higher than 0 when disconnected or is destined to run out of air anyway regardless of what the SPG says. Blaming an OOA on a miscalibrated SPG is just a poor excuse for poor preparation and dive planning.

Also, to get a ball park idea on calibration you can compare the reading you get woith your SPG on a particular tank with the reading another SPG gets on the same tank. If they differ significantly, the opinion of a third SPG will usually tell you which one was off.

A method of seeing what "0" really means is to sit in your living room breathing normally off your reg and note how lang it tales for a 100 psi change. Then see how long the reg delivers air after the needs hits the peg. This will give you an indication if your SPG has a margin for error at the bottom end of the scale. It's not uncommon to find that "0" really means 100 to 200 psi remain.

Failure wise, electronic SPG's, either stand alone or in air integrated dive computers, are far more failure prone than a mechanical SPG. I have had 2 air integrated dive computer SPG failures and one electronic SPG failure and no mechanical SPG failures in over 20 years and 1000 plus dives.

The abscence of a low battery indicator on a computer or electronic SPG is not fool proof insurance against battery failure. I have had computers and elctronic SPG's that would give a normal battery inidcation at the surface and then fail at depth once the unit cold soaked as the lower temperatures reduce the voltage the battery is able to provide.

In any event, if a diver has been monitoring the SPG on a regular basis and menatally noting the readings, an SPG failure is not an issue as the diver will either recognize that the needle is stuck since the last observation or will notice the failure and be able to extrapolate what remains in the tank based on estimated usage since the last reading and be able to safely abort the dive. The only ways to run out of gas even with a failed SPG are to not observe the SPG frequently, not compare the reading with what should be expected based on depth, time and use rate, and to not abort immediately when you notice the failure. If you are extremely inexpereinced or careless enough look at the SPG without noting what it says or noticing it has read 1300 psi for the last 20 minutes, you will have problems, otherwise an SPG failure is almost a non event.

 

[vel525]

On a recent trip my wife left her tank on during the 45min boat ride out to the dive site. Well, during boat ride, we heard a loud pop and we found that her pressure guage exploded. At first, my thought was it was because she left her tank on, but that doesn't really make sense. I thought about it some more and I'm guessing there was a small leak in the coil and pressure built up in the space in the guage and eventually popped the cover off. It's almost 7 years old so we're just going to go get a new gauge, but wondering if anyone else has had a similar experience. I'm also wondering if she had left her tank off, this probably would have occured 30min into the dive, so kinda glad she left her tank on cause I would've hated end my dive because her SPG blew up under water

Also, on the same dive my SPG started leaking badly and when I looked at the little pressure regulator thingy I noticed one of the o-rings was very worn. I replaced both o-rings and my SPG is fine. Kinda random that both our SPGs had issues on the same day though.

 

[drbill]

I had one years ago that developed a salt deposit or some other raised deposit at about the 800 psi mark. The needle wouldn't drop below that reading so I was never sure if I had 50 psi or 800 psi. Replaced it pretty quickly.

 

[Sam Miller III]

"QUOTE=Peter Guy;2870003]...
It seems to me that your depth guage and watch (BT) are MUCH more important than an SPG (and those are the two guages that were available when I first started diving). ...
Comments?"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
SPG first marketed by Mar Mac industries in 1954.

Used via a banjo fitting on doublehose regulators.

sdm

 

[DennisS]

I had one stuck on zero with full tank pressure. I gave it a whack on my hand and the needle fell off. Salt water intrusion would be my guess.

 

[fweber]

I've had one that would come up to pressure easily, but most concerningly, hangup at 500-800 psi on the way down. Threw it out without knowing if it was saltwater intrusion or mechanical interference. Then, a couple years ago I took the boots off my spg's. After denting the bezels to the point of leaking on 2 spg's and cracking the face of a third in less than 24 months, boots have gone back on for ocean diving. Haven't broken an spg since.

 

[ianr33]

I remember reading in Caverns Measureless to Man about an SPG failure that Sheck Exley had.Basically the SPG just stuck,so he thought he had more gas than he really did. Pretty scary failure mode. Something to do with the pressure at 700 feet or whatever compressing the gauge and causing it to stick. Not an issue for most divers fortunately.

 

[Charlie99]

I had an SPG that worked perfectly for about 6 years, and then gradually started reading higher and higher for a full tank. My guess is that the bourdon tube was getting close to failing. Maybe saltwater corrosion thinning of the tube walls.

I replaced it when it started reading 3400psi on a 3000psi fill.

 

[munitor]

Something to do with the pressure at 700 feet or whatever compressing the gauge and causing it to stick. Not an issue for most divers fortunately.

That brings up an interesting question about standards for gauges. I took a look at EN 250, the European standard and it said:

5.11.1 Pressure gauge
The pressure gauge shall be so designed and fitted
that the diver can take readings without difficulty.
If a flexible connection is required for this purpose it
shall be protected against damage by external
mechanical influences occurring during use. If the
connection has a cover which is not permeable to
gases, the space enclosed by this cover shall be
vented to the ambient atmosphere.
The connection point for the pressure gauge hose, or
if no hose fitted, the connection point for the
pressure gauge shall be so constructed that with an
upstream pressure of 200 bar it does not permit the
passage of more than 40 l/min of air measured at
standard temperature and pressure.
The display range of the pressure gauge shall
extend from zero to a value of 20 % in excess of the
rated working pressure of the air cylinder.
The scale divisions or increments shall not
exceed 10 bar. The range below 50 bar shall be
clearly differentiated so as to emphasize low air
supply. The accuracy of the pressure gauge shall
comply with the following tolerances measured at
decreasing pressure:
at 40 bar ± 5 bar
at 100 bar ± 10 bar
at 200 bar ± 10 bar
at 300 bar ± 10 bar
The pressure gauge shall be waterproof up to at
least 10 bar above atmospheric pressure for at
least 1.5 min. The transparent window shall be
splinterproof. The pressure gauge shall feature a
pressure-relief facility which, in the event of a leak,
protects the diver against injury.
Testing in accordance with 6.2, 6.3 and 6.10.1.

So bottom line is that probably no SPG is rated for more than 330 fsw, and accuracy is absolutely garbage by industrial standards (10%).