SPG accuracy

[raftingtigger]

1. yes
2. yes, though you should be orally inflating your wing for positive pressure checks and then tap the inflator a couple times to verify the OPV is working
3. no it doesn't, it just makes sure that it doesn't get hung up at any point in its cycle
4. yes, though you should be leaving your gear pressurized with the valve turned off during setup to make sure nothing gets into it
5. yes, which is the negative pressure check you're supposed to do with the dust cover on, but off is off on the tank valve

Off topic, but I'll bite:

2. Does it matter that I do most of the inflating orally or with my "more than ample tank volume" as long as I check both inflation methods?
3. Didn't say the top reading was accurate, just that I got a reasonable reading. I think we are agreeing in different words here.
4. Why do I want to leave it pressurized after set up? What are you worried will get into the system? Maybe you and I are diving different environments, but the only thing I can think of that might get into my regulator as things sit in my car is a bee. In that case keeping the regulator pressurized won't help. However, at least in my environment, DEPRESSURIZING the system has the big advantage of making it very obvious the tank valve is off.
5. Non-sequeter


Also the point I was trying to make in my original post is that by watching the SPG needle as the air is bled off should give you a pretty good idea of where zero tank pressure is.

 

[NAM001]

For me the only accuracy I need is the low end. This situation is also a major reason or mot pushing the dive to 200 psi. the 500 psi on the boat covers the most drastic inaccuracy's in a gage. A 10% full gage error on the low end says 200 Psi may read 500 psi ont he spg. I check all my gages with a reference at the start of each season at both full and near empty. easy to do when I have H valves . put the reg on one post and fill in the other. Gages match and im good to go, if not, can the gage and replace it at first opportunity. I carry spares but have never needed one.

 

[tbone1004]

Off topic, but I'll bite:

2. Does it matter that I do most of the inflating orally or with my "more than ample tank volume" as long as I check both inflation methods?
3. Didn't say the top reading was accurate, just that I got a reasonable reading. I think we are agreeing in different words here.
4. Why do I want to leave it pressurized after set up? What are you worried will get into the system? Maybe you and I are diving different environments, but the only thing I can think of that might get into my regulator as things sit in my car is a bee. In that case keeping the regulator pressurized won't help. However, at least in my environment, DEPRESSURIZING the system has the big advantage of making it very obvious the tank valve is off.
5. Non-sequeter


Also the point I was trying to make in my original post is that by watching the SPG needle as the air is bled off should give you a pretty good idea of where zero tank pressure is.

2. Yes and no, it ensures the oral inflator is functioning properly, and it doesn't waste gas. To each his own, but I prefer not to waste a few cf of air in the process.

3. top end is what it is, but it doesn't check it at the bottom end. It gives the same reading at 0 pressure whether it is on the tank and depressurized, or off the tank. That is what I was questioning, literally all it does is tell you what it thinks you should have *reason why I recommend everyone have a tank pressure checker that is large diameter and industrial grade preferably with a 6000/7000 psi rating, but 5000 is most common. This is an actually calibrated pressure gauge that comes with calibration specs. That will verify the top end of your gauge. These are like $40 with the adapter, and $15 if you have to replace the gauge

4. it tells you if the system is leaking. I still don't understand how knowing the valve is off is a big advantage, the valve is either on or off. You can turn it off, take a breath and it will sit at 2500psi or so and just leave it there, no reason to breathe it all the way down. If it's in the back of your vehicle that's one thing, but I try to leave everything pressurized in case someone tries to take it to the water and doesn't open the tank valve first.

 

[RJP]

And, that's why I added the 'maybe not'. However, perception (of greater accuracy) not reality (of greater accuracy), may be the key. A wise philosopher recently wrote, 'that's the very essence of marketing! The perception of my shoes is far more important that the reality of my shoes.'

When it comes to gas supply... "reality" trumps "perception" every time.

 

[Nemrod]

The spg is intended to be rugged and reliable. The simple Bourdon tube mechanism is not designed or expected to be nor needed to be accurate over it's entire range. N

 

[KeithG]

this all comes down to the proper solution for the proper problem. there is no need for "weights & measures" accuracy for simple diving. tank mostly full, tank not yet empty. that is good enough.

a simlar example is my bathroom scales. they are like my SPG. not real accurate (well unknown actually) but good enough. it is not good enough to weigh sliced pastrami at my local deli. it also costs way less than the scale in my local deli. and it does not need to be recalibrated and recertified on an annual basis to meet weights & measures laws.

do i need a highly accurate scale in my bathroom? no. is the technology readily available? yes. am i willing to pay for it? hell no!

 

[emoreira]

Just to add a little to the discussion.
In 2010 I bought a new TUSA Platima SPG. It has now something like 150 dives. The error at the high end is +/- 15 %
Two tanks filled in parallel to 3000 psi, my SPG measured 2600 and my buddy's SPG measured 2900. We changed tanks and the measure repeated.
Scuba SPGs are made with the Bourdon's tube principle, so inaccuracies are expected to happen. An almost linear mechanism to measure a non linear gas behavior.

 

[blatter]

Absolute accuracy at the upper end of the SPG scale is not that important. Other variables such as depth and excercise are going to be more signifent when consisdering ramaining air supply on a dive. However, I think it is important that the SPG does reliably read zero, so you have a good idea of when you are going to run out of gas. That is something that can be easily checked and in my view should be checked by the diver on a regular basis.

You can check an SPG reads zero easily and at the same time do a crude sort of calibration whenever it is being used, at least to the level of accuracy required for normal diving.

When you hook up the reg/SPG while setting up gear to go for a dive, check the SPG before turning on the valve. The SPG should read about zero. Whatever it is reading it on the scale is actually zero (above ambient pressure). If the needle does not match zero on the face then the SPG may be damaged and its reliability should be considered.

Turn the gas on. I was taught never to look into the face of the gauge in case it blows out - but if you do happen to see the SPG indicating increasing pressure, perhaps with the aid of saftely glasses, the needle should move smoothly to full pressure.

When the pressure equalizes the SPG will be reading whatever the pressure in the cylinder was. If the cylinder has just been filled you should know what the expected pressure is, and can compare the SPG reading with that to get a sense of how accurate it is. It is not actually that important that the absolute pressure above ambient is accurate. It is good if it is repeatable and reliable and you know what it should be after a fill. Checking against other guages can help, and one would expect that a just filled cylinder will be at a known pressure at the fill station which should have the use of a more accurate and reliable guage. If it is drastically wrong the SPG may have been damaged and its reliability should be considered.

The most important thing is to check guage function. To do this, turn the valve off and bleed off the pressure in the reg slowly while looking at the SPG needle. Slowly purging the second stage or very gently breathing through the reg will do this. What you are looking for is the smooth movement of the needle as presure above ambient reduces to zero. The needle should not stick and then jump, or be stuck at a position above zero. Sometimes SPGs will stick for a short time then reduce to zero. This indicates the guage is worn of damaged and caution should be used. On a dive, with an even more gentle/gradual reduction in pressure there is a risk that the needle may become stuck before reaching zero. indicating that there is more gas remaining than is actually the case. Sticking needles as pressure is reduced usually indicates the SPG is due for replacement.

Having said that, I once had an SPG that was started to stick at around 40 bar/300 psi. I replaced it but then started using it for a less critical application - on a reg driving a booster pump - and discovered that the problem was in the HP hose and the SPG was fine.

Anyway, I think it is important to check that SPGs are working as they should on a regular basis. This should not be hard to do and as part of my dive routine I check that the SPG gives an expected reading at the start of every dive, and that the needle does not stick at the end of every dive when I purge the gas to remove the reg.

 

[iain/hsm]

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.

Can we expect the same amount of error on depth?

Submersible pressure gauges used by all SCUBA, cave and technical divers are all pretty much the same. Poor quality components and design, described as sports equipment and classed for use within a recreational diving pastime.

They are in effect sports equipment.

To differentiate a divers submersible pressure gauge.

With analog submersible pressure gauges suitable for diving we have always used the accuracy based on the standard class of 2.5 to EN837-1 (Previous to that we used British Standard ( BS1780: 1985) That is a 2.5% accurate (FSD) Full Scale Deflection calibrated movement.

In order to keep and ensure repeatability of the gauge accuracy it is the design of the "movement" that is critical.

Further dependant on bourdon tube material choice the temperature affect on the bourdon tube needle is also within a variance of around 0.4% FSD +/- 10C from the calibrated room temperature point (around 21C)

Materials typically used are Beryllium Copper, Monel, Inconel, 316 stainless and Phosphor Bronze. depending on pressure and application.

The gauge type movement used is known as a jewelled movement using synthetic sapphire bearings for high accuracy and shock resistant and repeatability for the pinion movement to which the display needle is attached.

Other design details such as the "glass:" also can be either sapphire glass or 10mm toughened soda lime glass or Borosilicate glass again depending on the application depth etc. Case material also including non magnetic applications and requirements.

A typical divers submersible pressure gauge of this type rated for 400MSW (1320 FSW) costs around £130 GBP ($200 USD)

By comparison the typical recreational sports diver however "technical" he may claim, pays substantially less. Iain Middlebrook

 

[3ricj]

With analog submersible pressure gauges suitable for diving we have always used the accuracy based on the standard class of 2.5 to EN837-1 (Previous to that we used British Standard ( BS1780: 1985) That is a 2.5% accurate (FSD) Full Scale Deflection calibrated movement.

Very interesting post! Is there an equivalent US standard to EN837-1? I can't seem to find any SPGs sold here which meet this standard, so I wonder if another one is used.