Northwest Dive Club

I'm thinking about giving up on using NITROX because of repeated fill errors by my local shop and concern for the accuracy of the O2 monitor they provide for customer use to verify the FO2. At one point when the shop's only O2 monitor was out of service and I could not verify my tanks just filled, the owner said "get your own monitor." Yesterday my two tanks measured 4.0 higher than requested. After refills and many retests and re-calibrations by the fill operator and myself, it appears the shops O2 monitor was not in agreement with the fill process or a second monitor not owned by the shop. The shops O2 monitor read 4.0 higher. Fills included tank rolling to mix the gas.

I could make a longer drive to another shop or buy my own O2 monitor but how will I know when either of those monitors goes bad?
How often are shops required to provide service on O2 monitors? There are so many possible errors in this process - the fill, gas mixing, the O2 monitor accuracy, monitor calibration, monitor operation, recording the values, MOD calculating or table look up, computer value entry.

1) Buy your own
then
2) find another shop, anyone with that lackadaisical of an attitude towards nitrox (and analysis) should not be filling your tanks.

O2 sensors last about 2 to 2.5 years if not stored in a stupidly hot environment, batteries about the same. You can still get the semi home built el cheapo analyzer kits from oxy cheq. Get the optional "fine" tuning knob, its worth the extra $.

CaptnJack wrote:Get the optional "fine" tuning knob, its worth the extra $.

And to add... make sure you know how to use an O2 analyzer properly, too. Cells (sensors) do not register the percentage of oxygen in the gas. They register a value which corresponds to the partial pressure of oxygen only. So for an accurate percentage reading, you must make sure the flow across the sensor face is completely at ambient pressure. You won't believe how many people put way too high a flow rate of gas across the face and inadvertently screw up their readings by forcing more gas into the fittings than can be exhausted. That'll force a higher than normal reading for sure.

Also - be sure you know the variables in how oxygen sensors work. 1-2% error in accuracy in the output of a cell is well within manufacturer's tolerances for the device. Add that to the electronics in most of the cheap analyzers and you're increasing that a bit, too. That said, filling and measuring with the _same_ analyzer should be precise... but not necessarily accurate.

Rolling to mix gas is BS. Any chemical engineer will tell you that the flow into a scuba tank is completely turbulent from the get go... which means mixing occurs right away.

Finally, you should understand that all of this is theory and determined empirically with regards to your body's reaction. There's no "magic" point where at 130', 32% is going to be just fine, but 32.2% is going to kill you. Just doesn't work that way.

4% off? That's still probably sloppy mixing. 1-2% off? Live with it and realize they may be spot on and it's the measurement equipment that's off.

ohopdiver wrote:How often are shops required to provide service on O2 monitors?

They're not *required* at all. They're not even required to HAVE an O2 analyzer, or to make one available to customers. It's polite, it's prudent, and it's a good thing to do (TM) but there's no requirement. Not in the U.S. and not in any other countries I know of either.

-Adrian

Are these guys continuous blending? If so it is easy to miscalibrate the sensor by leaving it in the mixing stick. The O2 level in my stick stays a little high even several days after I'm done pumping. I've caught myself recalibrating to that level. Then it would take more O2 to get it to read the level they were mixing to.

Within 1% is as good as it gets. More than 2% and I've done something wrong.

You should have your own O2 analyzer. You should get so you understand how it works. You can tell when the sensors are getting old because the calibration starts jumping around.

camerone wrote: Rolling to mix gas is BS. Any chemical engineer will tell you that the flow into a scuba tank is completely turbulent from the get go... which means mixing occurs right away.

Not if you partial pressure fill a large or long and skinny cylinder on its end with a slow compressor (e.g. 3cfm). Definitely stratifies, although its mostly due to the temp difference. Laying it on its side during the fill pretty much avoids the issue as the surface area between the cool gas and the hot gas is much larger. Not typically applicable to shops as their compressors are larger and the faster fill creates more turbulence.

Is this the Bremerton shop?

Thanks for the info everyone.

Please no shop guessing, I'm not going to even hint as some or all of the issues may be my own doing or missunderstanding.

So what ever gas is added last is immediately and completely mixed, no shaking or rolling necessary? Not 4% error, it was off by 4.0 out of 32 or 12.5% error. I wondered why the reading went up when I increased the flow. It appeared to me the shop guys were holding the monitor port hard against the tank valve so that's what I did. It is a challenge to get my tank valves to give a slow steady flow.

I will ask to see the monitor's instruction manual next time I'm in the shop and take another look at purchasing my own. Any recommendations?

As a 74 year old diver I'm well aware my body will not react like most test subjects and theories. So I try to increase my safety margins above what's recommended for the typical diver. That's why I'm concerned about the accuracy of the NITROX fills.

ohopdiver wrote:As a 74 year old diver I'm well aware my body will not react like most test subjects and theories. So I try to increase my safety margins above what's recommended for the typical diver. That's why I'm concerned about the accuracy of the NITROX fills.

I'm not there (74) yet, but I do share your concerns about safety margins. I figure that O2 gauge readings may be off by a percent or two, so I avoid diving near the MOD -- and never sweat the decimal differences between gauges. That, and using a 1.4 maximum PPO2 provides a good safety buffer.

If I'm tired, dehydrated, or otherwise not operating at 100% I'll even revert to diving on an air table while using nitrox. Peace of mind...

ohopdiver wrote:Thanks for the info everyone.

Please no shop guessing, I'm not going to even hint as some or all of the issues may be my own doing or missunderstanding.

So what ever gas is added last is immediately and completely mixed, no shaking or rolling necessary? Not 4% error, it was off by 4.0 out of 32 or 12.5% error. I wondered why the reading went up when I increased the flow. It appeared to me the shop guys were holding the monitor port hard against the tank valve so that's what I did. It is a challenge to get my tank valves to give a slow steady flow.

I will ask to see the monitor's instruction manual next time I'm in the shop and take another look at purchasing my own. Any recommendations?

As a 74 year old diver I'm well aware my body will not react like most test subjects and theories. So I try to increase my safety margins above what's recommended for the typical diver. That's why I'm concerned about the accuracy of the NITROX fills.

The meter is reading the partial pressure of O2, not the actual percentage. So at altitude it will read lower and it you crack the valve and a force of gas is blasting onto the membrane it will read higher. A flow restrictor addresses all of these concerns (except extremes of altitude). You can get one which goes in a BC inflator hose on your reg.

Its hard to say where your 4% discrepancy is coming from. Most commercial nitrox I have bought is actually too low not too high. O2 costs money and giving away a few percent more day after day is expensive and unwarranted.

Your 4% excess is obviously reducing your N2 loading but increasing your pulmonary O2 and CNS loads.

You could go to Tacoma Scuba if you are unhappy with their backtalk about getting your own meter. Although it looks like they don't know how to use their own.

ohopdiver wrote:It is a challenge to get my tank valves to give a slow steady flow.

https://www.divegearexpress.com/trumix- ... 2-lpm-flow or http://www.northeastscubasupply.com/bc-flow-restrictor This snaps onto a BC fitting and guarantees a steady flow at an appropriate rate. For a little more, you can get one that screws directly into a DIN port without a regulator, if you prefer, but I find the standard BC-nipple snap-on types more convenient.

For an O2 meter, they're not rocket science to build. The DIY one mentioned earlier will work, although isn't the world's most accurate design; many commercial units copy it, though, and its accuracy is good enough. If you want a prebuilt one: http://www.northeastscubasupply.com/oxy ... -analyzers

The only things to look for with any off-the-shelf O2 analyzer is the cell/sensor type. Some of them use pretty obscure O2 sensors, or in a couple of cases, were designed to be non-replaceable. Certain sensors generally aren't better or worse, but may be really, really hard to find or expensive when it comes time to pop a new sensor in. The Oxycheq version above uses a very common and relatively inexpensive sensor; most rebreather divers can tell you what is widely available and what to stay away from.

Best deal going if you can solder a little bit.
http://www.oxycheq.com/analyzers-sensor ... r-kit.html

Patrick has some of the worst customer service in the scuba industry imo. But I would recommend this DIY analyzer kit, works perfectly, the sensors are easy to find, and the cost of the complete kit is not much more than the sensor alone.

ohopdiver wrote:Thanks for the info everyone.

So what ever gas is added last is immediately and completely mixed, no shaking or rolling necessary? .

We can't answer that without more information. See Rjacks reply.

I ordered an analyzer and flow restrictor. I'll put the restrictor on a spare first stage with just a BCD inflator.

Thx all

On the tank that was 4% high what was the requested percentage?

CaptnJack wrote:On the tank that was 4% high what was the requested percentage?

32%

ohopdiver wrote: Not 4% error, it was off by 4.0 out of 32 or 12.5% error.

Yeah I am not accustomed to addressing percent error of percentages. Especially because you could easily and correctly say the N2 was off and it was 64% instead of 68% - a 6.25% error. Bottom line is that the percent error is irrelevant to me, the absolute discrepancy in fO2 and which direction its off is what counts.

36% has a MOD of 95ft (ppO2 1.4) compared to 110ft for 32% which for recreational diving is really the only concern. OHOP were you limited on what you could do with these tanks based on the shallower MOD?

This is a cool analyzer


I am a gas mixer for a dive shop and there is no excuse for having an analyzer that is 4% off.
We use a couple separate analyzers on the tanks and if they are all not within 1% we look into whats up.

We don't roll or shake the tanks to mix the gas

CaptnJack wrote:
36% has a MOD of 95ft (ppO2 1.4) compared to 110ft for 32% which for recreational diving is really the only concern. OHOP were you limited on what you could do with these tanks based on the shallower MOD?

Yes, my plan was for max of 105 on first dive at China Wall, Blakely Rock so they were re-mixed to lower FO2 ending at 33.0 MOD 107.
You have convinced me that it was likely excessive flow giving a false high reading. I will try to confirm.
I am also concerned about the N2 affect of entering a false high FO2 in my computer.

ohopdiver wrote:

CaptnJack wrote:
36% has a MOD of 95ft (ppO2 1.4) compared to 110ft for 32% which for recreational diving is really the only concern. OHOP were you limited on what you could do with these tanks based on the shallower MOD?

Yes, my plan was for max of 105 on first dive at China Wall, Blakely Rock so they were re-mixed to lower FO2 ending at 33.0 MOD 107.
You have convinced me that it was likely excessive flow giving a false high reading. I will try to confirm.
I am also concerned about the N2 affect of entering a false high FO2 in my computer.

Personally I would have just dove it at China wall. I have never hit 110ft there in probably 25+ dives, more like 98ft as my max max. Typical maximum is more like 95ft and that's only on the deepest part of the western end. Perhaps the tide was higher for you, but I've never planned for 110ft there.

I noticed cave adventurers has a sale on an analyzer right now. No construction required. I have never used this brand before (but they are all basically volt meters on a fuel cell) and not affiliated with Edd or his shop. But $109 was a damn good price.

http://www.caveadventurers.com/nrc-pick ... lyser.html

Be careful with the Oxycheq El Cheapo. It's not just a bit of soldering, and installing dials and switches, which is what I thought when I bought it. You get the hard plastic box with no holes cut in it at all, and it's not easy to cut. You have to remove some components from the circuit board and replace them, and the instructions are not particularly clear. We eventually gave up and asked a much DIY handier friend to do it for us (thanks, airsix!). In retrospect, I would have paid for the assembled one (which we had previously had, and had sold to a friend in MX, where they are extremely hard to come by and very expensive).

I'm happy with my Oxycheq Expedition with plug-in sensor. The problem was the shop did not, and still doesn't, have an adequate flow restrictor.
So for Nitrox fills I bring my own analyzer with a BC flow restrictor that I put on a first stage, plus my own pressure gage and an Ipad with a MOD algorithm. I occasionally check against the shops analyzer but using my flow restrictor. After all that falderal my Data Max Pro Plus 2 FO2 entry is in integers only so I round down for highest N2 - most conservative no-deco.
My understanding is the Oxycheq sensor should be replaced annually even though I only do about 50 Nitrox tanks per year.

There are so many factors involved that you should not be trying to split hairs. This is all guesstimating to start with. Each diver is different and the tables are only numbers of a chart that is based on "normal" What is normal and who is normal? All of the chart numbers are very conservative. I have friends that have dove with a 1.5 set on there computers for years without the slightest problem. Max depths? that is based on the assumption that you will do most of your dive at that depth and how often will that happen? if your MOD is 95ft and you drop to 110 for ten minutes are you going to die? Not likely at 1.4 ppo Always ere to the safe side and you'll be fine. Don't push your limits. There are way to many variables and assumptions to set hard numbers and live by them. If your tired, if you are doing repetitive dives over several days, your general condition on that given day, stress levels and yes your age? If you ride the line sooner or later your going to get into trouble. Always allow a buffer.

ohopdiver wrote: My understanding is the Oxycheq sensor should be replaced annually even though I only do about 50 Nitrox tanks per year.

Just replace it when it dies. Its not actually an oxycheq sensor its a Teledyne. They are fine and then drop off a cliff performance wise and you will be turning the dial way up. Its pretty obvious when they fail.

Mine last about 2-2.5 years.