Air pressure control for DIY reg service?

[HenrikBP]

I have a 0 to 600 psi test gauge that I compare my SPG's to and also check the activation pressure on my J reserve valves.

Do you get your test gauge tested (pun intended ) at regular intervals? Do dive shops/techs in general get their gauges calibrated?

I guess I'm wondering how exact certain pressures need to be. I would think the gauge for checking IP ought to be accurate to within 1-2 PSI since it seems the accepted IP range is about 20 PSI (125 - 145 PSI for instance).

The input pressure when testing the first stage may be less critical as long as it is high/close to the max pressure the first stage will see - as per Captain's balanced vs. unbalanced reasoning.

As for testing SPGs I would certainly like an accurate gauge for comparison, but it may be, that as long as I know a certain SPG is off and by approximately how much, I'd be ok. Of course I don't know of any way to calibrate/reset an SPG if I were to find one out of adjustment.

Thoughts?

Henrik

 

[captain]

Most common grade B gauges have +/- 3% overall accuracy, generally grade 3A test gauges have +/- .25% accuracy over the span of the gauge. Test gauges usually have a zero adjustment provision. Unless a gauge gets dropped or over pressured they will remain accurate.
If using a 0 to 200 psi gauge to set IP, than a gauge that has +/- 3% accuracy means the indicated pressure could be 6 psi higher or lower. It is always best to use a gauge that will read whatever pressure you are checking close to the middle of the scale. That is where it is most accurate.

McMaster-Carr.com is a good place to look for different types of gauges.

 

[beanojones]

Not at all on topic to the thread, but SB people pointed me to McMaster-Carr, and that place rocks. I just got a whole pile of O-Rings (in green, just to be different) and DIY LP seat material, and #5-40 nyloc nuts, and, and, and.

It is awesome, the internet is. (also being on the mainland so I don't pay more for shipping that I do for the product.)

 

[HenrikBP]

Thanks for the gauge info Captain. It was interesting reading up on the various types and accuracy levels of gauges. It looks like Scubatools.com chose an 1A type gauge for their IP/cracking pressure gauge console. Interesting to note that Scubatools.com finds the +-1% to be accurate enough for setting IP pressure.

Looks like McMaster has 2A gauges in that pressure range for less than $90. I'll need to find the various fittings to hook it up to the inflater hose.

Henrik

 

[halocline]

If you want to check IP at various tank pressures with one tank, it's easy. Start with a full tank, pressurize the reg, check the IP, close the tank valve, and start bumping the purge until the supply pressure is where you want it, then check the IP again. This is AWAP's trick.

Or, just buy another tank. AL80s are really cheap. It usually takes me a couple hundred PSI to service a reg because I like to cycle a new HP seat 50-60 times, and it's typical to pressurize, check 2nd stage cracking pressure, purge, remove 2nd stage to adjust, repeat, etc. I don't have an in-line adjusting tool.

Your little tank is going to drop in pressure more quickly, so just for convenience you're probably going to want a full size tank.

 

[pescador775]

Inline tools are nice but not necessary. I just received some goodies, including inline adjusters, from Scubatools.com which were used to check out the new MK25/G250HP from Leisurepro. As I said previously, I found that the IP on the new regulator was 175 psi. After adjusting to a more normal 140 psi, I found that the "balance chamber" spring in the second stage was adjusted almost all the way in (high tension). This suggests to me that the regulator was set up at the factory in such a way as to be in tune with the high IP. That is, I do not believe that the IP or 2nd stg had been tampered with or drifted but that the IP was set too high at the factory and the second stage tuned at the outset to conform. Adjustments and simple checks performed by the owner on older or even new regulators is a great convenience, perhaps a necessity.

This regulator breaths really nice. I do not have a magnehelic but the water bath tests puts inhalation at about 0.8 inches. Funny thing is, I have a 9 year old G250HP (the type with anti-set) and it can be adjusted to 0.5 inches.

 

[pescador775]

I use a military type aircraft tire gauge with 400 psi scale. I compared it against a common, utility gauge I bought for five bucks. It has a 160 psi scale. Both gauges read exactly 140 psi when checking IP. Generally, +- 3% of reading is good enough for me when checking IP or tank pressure. I think my precision lab gauge used for PP filling is closer to 1%. However, three percent of range is much less accurate and not good enough for using deco tables, etc. Interesting thing about SPG's, all the gauges that I have checked read 200 psi low at full tank press. My tank press gauge is very accurate at all but the lowest pressures. At a true tank press of 300 psi, the tank checker read about 360 psi. It has a course scale at pressures below 500 psi.

 

[Fishpie]

As for testing your SPG...you are testing the accuracy of the max and min every time you put your regulator on a full tank.
I always get my aluminium tanks at 3000 (+ or - 100). When I set up my kit I will check I have a full tank and then purge watching the SPS to make shure the needle falls smoothly and actually reads zero.
As for mid pressures, I have always heard that gauges are most accurate at the mid part of their range.

 

[HenrikBP]

As for testing your SPG...you are testing the accuracy of the max and min every time you put your regulator on a full tank.

If you know the people filling your tanks and trust that they fill to specifications - and that their gauges are accurate. Then yes, that would be a good test for the high pressure on the SPG.

Henrik