What's the reason for 500psi/50 bar?

[MauiScubaSteve]

The origional car training my dad gave me seems logical with regard to SPG's. Run it until it runs out, then you know where empty really is. My dive site has the last 150 yards of the dive at less than 15fsw, and all my gauges have been tested down to 100psi or less. With unknown rental gear I'd not go below 300psi. These statements have nothing to do with SAC and gas planning, learn those and be safe.

 

[diver 85]

I don't know why I never gave it any real thought, but it just occured to me that I don't know WHY we are so often told to end our dives at 50 bar/500 psi. Following a fruitless search, I thought I would ask the question here. I fear that I have learned the reason somewhere, but do to advanced age, I've forgotten...

Phil

To give the DM a few free drinks @ the end of the dive day when you've lost the bet.....

 

[Thalassamania]

As the metal age and fatigue, and without recalibration, my guess is that pressure gauges can be very inaccurate at ranges below 500 psi. I imagine, with a bathroom scale, at least you can re calibrate or re-tared. But with an spg, how would one do that??

A bordon tube relies on the difference in surface area between the outside and the inside of a helical tube, I've been told that metal fatigue is not relevent.

My curiosity would be how reliable the digital wireless transmitters are? With these sensors being so small and dependent on digital technologies, would you really trust them to be accurate at the lower pressure ranges? It sure seems nice to get rid of a hose and gauge, but I think I would still like a simple pressure operated spg.

The transducers used are quite acurate and precise, the Oceanic Atom 2 for example claims depth resoution to 1 ft down to 330 ft and tank pressure resolution to 5 psi from 0 to 5000 psi

 

[skynscuba]

The transducers used are quite acurate and precise, the Oceanic Atom 2 for example claims depth resoution to 1 ft down to 330 ft and tank pressure resolution to 5 psi from 0 to 5000 psi

Don't mistake "resolution" with "accuracy," unless the accuracy claim is also made.
Resolution is merely the smallest reading increment which can be displayed, or perhaps downloaded. OTOH, one part in 330 or one part in 1000 isn't a highly demanding requirement for the pressure transducers, and nada for the sensing electronics.

An example of resolution better than accuracy is the Oceanic computer temperature output, which reads in 1 degree increments, but only has a "room temperature" accuracy specification of +/- 3 degrees F, and has no specification at all over temperature.

 

[fisherdvm]

A bordon tube relies on the difference in surface area between the outside and the inside of a helical tube, I've been told that metal fatigue is not relevent.
The transducers used are quite acurate and precise, the Oceanic Atom 2 for example claims depth resoution to 1 ft down to 330 ft and tank pressure resolution to 5 psi from 0 to 5000 psi

I am not sure about the metal fatigue is not relevant. A spring has a constant, but over time, most springs either stretch or compresses. Eventually, all metals will change. The constant will change for that spring, but it still will be a constant for the day you use it.

My guess is if you do not use your spg ever, it will be accurate.

But an spg exposed to 4 tank changes a day, every day, for 365 days a year, will likely suffer changes.

I am not a metallurgist, but my guess is a metallurgist would say the tube will change. Certainly, it can be recalibrated so that zero matches with zero. And the rest of the ranges are fairly close. As the K constant will be slightly changed with the fatigue.

My worry is the lower ranges. At least with a bathroom scale, even if the spring is inaccurate, you can reset the needle to zero. With an spg, as with many depth gauges, you can't.

That explains why one of my depth gauge is always about 5 foot off.

 

[Thalassamania]

I am not sure about the metal fatigue is not relevant. A spring has a constant, but over time, most springs either stretch or compresses. Eventually, all metals will change. The constant will change for that spring, but it still will be a constant for the day you use it.

My understanding (which may be wrong) is that bordon tube devices are independent of any kind of spring, the rely solely on the difference in surface area between the inside and outside radius of the curve. Eventually I guess the metal will break and the device will not function, but that’s not what we’re talking about. We’re really talking about accuracy (how does it compare to reality) and precision (how repeatable it is).

My guess is if you do not use your spg ever, it will be accurate.

Sure’nuff.

But an spg exposed to 4 tank changes a day, every day, for 365 days a year, will likely suffer changes.

I don’t think so.

I am not a metallurgist, but my guess is a metallurgist would say the tube will change.

Neither of us have any idea what a metallurgist would say.

Certainly, it can be recalibrated so that zero matches with zero. And the rest of the ranges are fairly close. As the K constant will be slightly changed with the fatigue.

It can not be “recalibrated” except to have the gear drive fixed and the needle repositioned. As to constant K, there is none since it is falls out in the division.

My worry is the lower ranges. At least with a bathroom scale, even if the spring is inaccurate, you can reset the needle to zero. With an spg, as with many depth gauges, you can't.

The lower ranges are the only real concern, who cares if it’s 3100 or 3050? You can actually re-zero the needle, bit that’s it.

That explains why one of my depth gauge is always about 5 foot off.

It likely jumped one tooth.

 

[ArcticDiver]

I was reading the specification of a new analog SPG, and at least while new, they guaranteed accuracy to plus or minus 72 psi at ranges less than 580 psi, and plus or minus 145 psi at greater than 1450 psi.

As the metal age and fatigue, and without recalibration, my guess is that pressure gauges can be very inaccurate at ranges below 500 psi. I imagine, with a bathroom scale, at least you can re calibrate or re-tared. But with an spg, how would one do that??

My curiosity would be how reliable the digital wireless transmitters are? With these sensors being so small and dependent on digital technologies, would you really trust them to be accurate at the lower pressure ranges? It sure seems nice to get rid of a hose and gauge, but I think I would still like a simple pressure operated spg.

My Oceanic wireless is dead on at all pressures tested, 100 - 3000 psi. I'm told this is normal.

Both my SPGs(different regulator setups) agree with my wireless at 300 psi and at other pressures when I've thought to compare. But, I often do an entire dive and never look at my analog SPG. With the wireless on my wrist and a pretty good idea where I should be as a check there really isn't a lot of reason to check the analog.

 

[1_T_Submariner]

Where you reading this in a manual or on-line. I'm just curious makes an SPG sound like the Idiot Temp gauge/fuel gauge my car comes with. I work with transducers that are good to .25% of full scale witch a 5000PSIG gauge/transducer could be off by 13 PSI. IF there is a link please post or PM me. Thanks.

I was reading the specification of a new analog SPG, and at least while new, they guaranteed accuracy to plus or minus 72 psi at ranges less than 580 psi, and plus or minus 145 psi at greater than 1450 psi.

As the metal age and fatigue, and without recalibration, my guess is that pressure gauges can be very inaccurate at ranges below 500 psi. I imagine, with a bathroom scale, at least you can re calibrate or re-tared. But with an spg, how would one do that??

My curiosity would be how reliable the digital wireless transmitters are? With these sensors being so small and dependent on digital technologies, would you really trust them to be accurate at the lower pressure ranges? It sure seems nice to get rid of a hose and gauge, but I think I would still like a simple pressure operated spg.

 

[Thalassamania]

On a yearly basis I used to have to test all SPGs that all of our divers used and produce a calibration card for each. Any that were more than 5% high between zero and 500 PSI had to be discarded. I don't remember throwing and away[FONT=&quot].
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[fisherdvm]

Sure’nuff.I don’t think so.Neither of us have any idea what a metallurgist would say.

Actually, I have two sisters who are metallurgical engineers, one with a PhD. She had a cool collection of EM photos of metals that she did for her masters. But I am not going to involve her with gauges.

That is good to know that you can recalibrate the gauges. If you can recalibrate your bathroom scales, I imagine it is no more difficult once you get the cover off.

Here is a nice diagram of the gauge: http://www.engineersedge.com/instrumentation/bourdon_pressure_detector.htm

Lets assume that my theory that metal fatigue will happen to the tube, and the arc will become more like a line.

Lets assume that a standard SPG will obey this diagram.

You would note that the gear will push the needle against the zero mark. Thus you will need a higher initial pressure (greater than 0) to move the needle away from the zero pin.

If my theory concerning metal fatigue is correct, the older the SPG, the more likely it will read a LOWER tank pressure than it really is.

Therefore, SPG's were designed with safety in mind, that as they age, if the metal do fatigue, it is going to err on the safety side, am I correct??