SPG accuracy

[3ricj]

Hi there,

I've noticed that much scuba gear doesn't seem to have very accurate pressure gauges. I performed some basic testing and found some large errors on gear.

Reference Pressure gauge: Nist calibrated temperature compensated digital pressure gauge. Accurate to 0.1% of full scale (FS=5000psi, so 5 psi). I'm calling this the "real" PSI.

Tested gauges:

Halcyon bootless SPG
Oceanic V4.0 wireless dive computer

Results:

In general, the amount of error is non-linear. At lower pressures (where you could argue is more important) the error is lower. At higher pressures, the error is off by as much as 255 psi. I have not conducted an exhaustive data collection, so I only have two datapoints. Each charging and purge of the reg dropped the "real" psi in the tank by ~5 psi, so I've adjusted the "real" psi by that loss. Data was collected in 60degreeF amb and had a chance to become thermally stable. Yes, these pressures are higher than typical scuba tanks; these tests where conducted with parts rated for the pressures. "Don't try this at home".

At 3745 (+/- 5) real psi: Halcyon reported 4000 psi (an error of +255 psi), Oceanic reported 3590 psi (an error of -155 psi).

At 507 (+/- 5) real psi: Halcyon reported 650 psi (an error of +143 psi ), Oceanic reported 425 psi (an error of -82 psi).

I could not find any published information about the expected accuracy of an SPG; there are many who claim they are only accurate to 10%, but I still can't find any manufacturers who publish accuracy information on their pressure gauges. The two datapoints above show large error.

Modern digital pressure transducers are amazing. They are also expensive... a 1% accurate thermally compensated pressure transducer can be had for maybe ~$50. Do I think my $200 transmitter has this nice of a part in it? Not likely. So what's the deal, scuba industry: I understand there is measurement error in pressure transducers, but why not publish the expected accuracy?

For example, the Oceanic shows an "Resolution" of 5 psi, but no published information exists on the accuracy.

http://www.oceanicworldwide.com/us/media/wysiwyg/manuals/12-5213-r02_Eng.pdf

Can we expect the same amount of error on depth?

 

[Ken T]

While accuracy is always something to strive for, two points come to my mind.

1). A scuba divers gas planning should not involve being so low on air that the error in their gauge becomes a factor.

2). Is the cost of a highly accurate gauge worth it given the first point.

 

[Stoo]

I agree with Ken... I have always looked as the SPG as a guide, much like the gas gauge on my car. I don't think anyone (with a functioning brain) would drive around intentionally running right down to that little line with "E" beside it.

I have occasionally sucked my tanks "dry" when I am in very shallow water, and feel my gauges are accurate at the bottom end, but I certainly wouldn't swim around at 150' until the reading was 123 psi, 'cause that's what I calculated that I needed to surface "safely"...

Maybe the manufacturers should change the faces to simply say "More" and "Less" as to not mislead users...

 

[bl6394]

Consider a typical mechanical SPG built around a bourdon tube. These are subject to an incredible amount of impact stress and abuse (particularly rental gear.) Even if one was manufactured and calibrated to a high degree of precision - it's unlikely that it would stay that way. I doubt your $200 transducer has a high precision pressure sensor in it either.

There are times when a high degree of precision are needed. When I'm partial pressure blending trimix - I need to know the pressure to a few psi - in order to hit a 1% error in my final mix. Hence I use a $200 electronic pressure gauge - not submersible.

When we dive - we don't need that level of precision. +/- 300 psi accuracy in an SPG is sufficient for a prudent diver to effectively plan most - if not all - of their dives. Surface with at least 500 psi - and you'll never be OOA.

 

[3ricj]

While accuracy is always something to strive for, two points come to my mind.

1). A scuba divers gas planning should not involve being so low on air that the error in their gauge becomes a factor.

2). Is the cost of a highly accurate gauge worth it given the first point.

Ok. Let's unroll that a bit. I agree that proper gas planning is very important. My goal in testing the accuracy of SPGs was not to enable running down to E. My goal was to understand the amount of error such that this error could be included in the gas planning. Pressure gauges which tell you a tank is 30% more full at the start of a dive could easily result in an accident. Of course there is reserve air available with proper planning, however, if that reserve needs to be used and the information you have is wrong, that's where you get into trouble. While I appreciate ya'll challenging my assumptions, I'm really trying to plan better by using this error as part of planning; I am not using increased accuracy for increased risk.

My core question remains: Why is accuracy information not published? Why are SPGs not tested for accuracy on a schedule such as regs and valves and tanks?

I'm new to this board, so maybe a bit of background would help:

- ~300+ dives, mostly cold water pacnw.
- I am a scientist at a lab. I work on innovative things, and have access to one of every tool.
- I'm a friendly guy who frequently gets interpreted online as a jerk. I really do want to be nice, if I come across harsh, please talk to me, it's not my intention.
- I currently shoot a full frame sony a7r.

Cheers,
-3ric

 

[Akimbo]

I don't recall any manufacturer publishing tolerances. Most small industrial gauges are +/- 2.5-3%. I understand that the pressure sensor used on many computers AI can resolve much closer than that, but still no published data that I have seen. I have never noticed a discrepancy that large on any of the SPGs I have, usually less than 100 PSI compared to the +/-1/4% gauge on the compressor.

 

[underH2Oman]

Calibrated gauges are accurate for a little while but can become out of calibration by things like use and neglect- impact etc. Most divers I see have at some point had their gauges hit something.

Even more expensive sensors experience sensor drift. [h=2]Some manufacturers provide a value for the expected drift or long term stability, but these numbers are still based on use in very stable environments, making them somewhat irrelevant in normal use.[/h]Gauges used commercially often require calibration to verify accuracy. The local hydro facility requires it every 6 months.

Most scuba gauges are sealed units that do not allow for calibration. Of course if you could send it out for calibration at a cost near to the cost of a new one my guess is you would buy a new one.

So if you consider that your new gauge may have inaccuracies in its reading great. if you think yours is accurate think again.

There is a reason for promoting recreational divers to return with no less than 500psi. 10% of full scale should should allow some air to remain in the tank. Even if gauge is reading high by 7-8% of full scale the diver should notice the regulators performance decreasing as the pressure in the tank drops below the regulator intermediate pressure, but then they should be at the surface.

Here is what Scubapro lists for its pressure gauges.

50 bar: ± 5 bar
750 psi: ± 75 psi
100 bar, 200 bar: ±10 bar
1500 psi, 3000 psi: ±150 psi
300 bar: ±15 bar
4350 psi: ±225 psi

 

[Ken T]

Ok. Let's unroll that a bit. I agree that proper gas planning is very important. My goal in testing the accuracy of SPGs was not to enable running down to E. My goal was to understand the amount of error such that this error could be included in the gas planning. Pressure gauges which tell you a tank is 30% more full at the start of a dive could easily result in an accident. Of course there is reserve air available with proper planning, however, if that reserve needs to be used and the information you have is wrong, that's where you get into trouble. While I appreciate ya'll challenging my assumptions, I'm really trying to plan better by using this error as part of planning; I am not using increased accuracy for increased risk.

My core question remains: Why is accuracy information not published? Why are SPGs not tested for accuracy on a schedule such as regs and valves and tanks?

I'm new to this board, so maybe a bit of background would help:

- ~300+ dives, mostly cold water pacnw.
- I am a scientist at a lab. I work on innovative things, and have access to one of every tool.
- I'm a friendly guy who frequently gets interpreted online as a jerk. I really do want to be nice, if I come across harsh, please talk to me, it's not my intention.
- I currently shoot a full frame sony a7r.

Cheers,
-3ric

I agree for the purposes of emergency planning if/when things go sideways having a more accurate picture of the amount of breathing gas left is very desirable.

Especially if you are on some exploration technical dive into the great unknown.

For recreational diving, hopefully the diver is shallow enough to ascend if required, can depend on a buddy, or is carrying a redundant air supply.

I think that the inaccuracies in gas management is addressed through training, best practices and standard operating procedures.

This way we do not run out of air or have redundancy in air.

Your core questions remain:

Perhaps the manufacturers do not publish their accuracy numbers because they know they have a lot of inaccurate gauges out there.

Is a more accurate, durable gauge beyond the price point most divers are willing to pay?

As for annual testing- I do have some books/manuals that do recommend this.
Testing against a "known" pressure as you have done.

How wide spread of a practice is it though? No idea.

Perhaps for the layman without an accurate test tool, averaging out three readings from three different gauges?

 

[bl6394]

Ok. Let's unroll that a bit. I agree that proper gas planning is very important. My goal in testing the accuracy of SPGs was not to enable running down to E. My goal was to understand the amount of error such that this error could be included in the gas planning. Pressure gauges which tell you a tank is 30% more full at the start of a dive could easily result in an accident. Of course there is reserve air available with proper planning, however, if that reserve needs to be used and the information you have is wrong, that's where you get into trouble. While I appreciate ya'll challenging my assumptions, I'm really trying to plan better by using this error as part of planning; I am not using increased accuracy for increased risk.

My core question remains: Why is accuracy information not published? Why are SPGs not tested for accuracy on a schedule such as regs and valves and tanks?

I'm new to this board, so maybe a bit of background would help:

- ~300+ dives, mostly cold water pacnw.
- I am a scientist at a lab. I work on innovative things, and have access to one of every tool.
- I'm a friendly guy who frequently gets interpreted online as a jerk. I really do want to be nice, if I come across harsh, please talk to me, it's not my intention.
- I currently shoot a full frame sony a7r.

Cheers,
-3ric

I come from a technical background as well (Applied Physics - BS / Comp Sci - MS) I can appreciate your thought process and interest. At the same time - this feels a bit like a solution looking for a problem.

A high precision SPG is not widely available commercially. I believe this is largely because it is not necessary for the diving most people execute - recreational or technical. It would be expensive to acquire. It would also be difficult and expensive to maintain and use. I imagine periodic calibration against a known standard...

I'd disagree with your assumption that any amount of error in the gauge dramatically increases the risk to a recreational or technical diver. Any component in your regulator is subject to failure (hoses, o-rings, burst disks, valves, etc.) As a recreational diver - you mitigate that risk by diving with a buddy - or perhaps carrying a redundant air supply. Yes - it would be surprising if my SPG read 3900 - when I only had 3000 (30% error) and still read 900 psi when I was OOA - but no more surprising than a regulator free-flow - or o-ring failure.

As a technical diver - we assume that all equipment is subject to inaccuracy and failure. We mitigate this risk by using redundant equipment.

If there was a specific need for a high precision, durable, SPG that could be re calibrated by the end user - I believe someone would make it. The absence of such a product confirms my suspicion that this is not a needful thing. Sometimes even needful things - a carbon monoxide detector for example - are not terribly commercially viable. The CO detector I bought is no longer sold.

As to why the accuracy information is not made available. I suspect it is because (A) the manufacture is not required to provide the data, and (B) it would offer no competitive advantage in the market place. "Buy our new and improved ACME SPG! Manufactured by the cheapest labor we can find in Italy. It is (maybe) 5-10% accurate at some points on the scale - until you drop it - and then all bets are off!)"

 

[tbone1004]

couple things
almost all brass and glass SPG's are identical, come out of the same factory, just different faceplates. They are bourdon tubes, this means that per every other analog gauge, they are most accurate in the middle of their range. Most of these are 5000psi gauges which allow accurate pressures at "turn psi" for cave diving in the 2400psi range which is typical turn pressure. You should test that Halcyon gauge at the "important pressures", 3600psi, 3000 psi, 2400psi, 1800psi, 1200psi, 600psi. This is sixths of a typical "cave fill" for technical diving and is close enough to relevant pressures in recreational diving. Do the same with your Oceanic module, though since it is digital it should be a bit more accurate obviously. Once you do that, report back the numbers and then we can re-evaluate.

SPG gauge accuracy, at least by most of the divers that I know, is +_ one point of reference. I.e. a brass and glass SPG such as the halcyon is accurate to +_100psi, and the recreational ones are usually +_250psi, that is their "resolution" and accuracy can't really be seen any tighter than that, especially on the small ones. 250psi is enough to cause concern for me, but only if the other pressures are also off. Measure with a micrometer, mark with a sharpie, cut with an axe.