Who DOESN'T analyze their gas?

[scubadude]

I have a couple of analyzers. I find that one (oxycheq expedition) that I hook to the bc hose with a flow restrictor and cap on the sensor interesting. If I remove the flow cap and calibrate ambient air in the house (climatized and not too much humidity) then calibrate using the cap and restrictor from a tank of air the readings are off more than I would expect from just a change in humidity since the temps of the ambient air and tank (which has been stored inside the home) are are essentially the same. I attribute it to a slight pressure increase in the flow cap, hence raising the PO2 for the tank/flowcap scenario. Tanks will always test about .6% higher when calibrated with the flow cap on an air tank as opposed to calibrating with the sensor exposed to air directly. I have an Oxycheq E'lCheapo that acts more like the expedition with the cap off.
Not a deal breaker, If I calibrate the expedition using ambient air I just set it to 20.5 ish to meet somewhere in the middle.

[tursiops]

I have a couple of analyzers. I find that one (oxycheq expedition) that I hook to the bc hose with a flow restrictor and cap on the sensor interesting. If I remove the flow cap and calibrate ambient air in the house (climatized and not too much humidity) then calibrate using the cap and restrictor from a tank of air the readings are off more than I would expect from just a change in humidity since the temps of the ambient air and tank (which has been stored inside the home) are are essentially the same. I attribute it to a slight pressure increase in the flow cap, hence raising the PO2 for the tank/flowcap scenario. Tanks will always test about .6% higher when calibrated with the flow cap on an air tank as opposed to calibrating with the sensor exposed to air directly. I have an Oxycheq E'lCheapo that acts more like the expedition with the cap off.
Not a deal breaker, If I calibrate the expedition using ambient air I just set it to 20.5 ish to meet somewhere in the middle.

Sounds like you need to turn down the tank knob and lower the flowrate when using the flowcap. Too much flow will indeed raise the apparent PP but there is no need for that much flow.

[jj1987]

when im in cave country I dont usually analyze the banked nitrox fills but always my trimix fills even if its banked.Up north here most shops have air fills only and if im getting 2-3 tanks filled i will usually only check one,, i guess now ill start using the analyzers i own a little more.

I'm glad you've come to that conclusion, but story time for those who haven't...

I've seen 3 cases over the last 2 years or so where someone I know or myself has gotten a fill where the o2 percentage was WAY off from what they expect (as in 40% or so when expecting 32%). The shops were all cave shops where this happened, and no it wasn't the same shop each time.

None of us would go cave diving without a guideline, even though we stand some chance to recover from that, following flow, scratch marks, reading the cave, etc. A hyperoxic event is almost always unrecoverable.

[scubadude]

Sounds like you need to turn down the tank knob and lower the flowrate when using the flowcap. Too much flow will indeed raise the apparent PP but there is no need for that much flow.

When using the flow cap there is always a flow restrictor between the cap and the first stage, so the flow is the same regardless of where the knob on the tank is, unless of course it is off
The restrictor was part of the analyzer package, so I just assume it give the needed flow, maybe not though.

[gschaut]

I have a couple of analyzers. I find that one (oxycheq expedition) that I hook to the bc hose with a flow restrictor and cap on the sensor interesting. If I remove the flow cap and calibrate ambient air in the house (climatized and not too much humidity) then calibrate using the cap and restrictor from a tank of air the readings are off more than I would expect from just a change in humidity since the temps of the ambient air and tank (which has been stored inside the home) are are essentially the same. I attribute it to a slight pressure increase in the flow cap, hence raising the PO2 for the tank/flowcap scenario. Tanks will always test about .6% higher when calibrated with the flow cap on an air tank as opposed to calibrating with the sensor exposed to air directly. I have an Oxycheq E'lCheapo that acts more like the expedition with the cap off.
Not a deal breaker, If I calibrate the expedition using ambient air I just set it to 20.5 ish to meet somewhere in the middle.

More likely that the .006 error is due to humidity. Normal room air has moisture content, a "properly filled" tank of air has a moisture content of near zero (dew point -60 or lower). So the room air shows a slightly lower O2 content due to "normal" humidity.

[tursiops]

When using the flow cap there is always a flow restrictor between the cap and the first stage, so the flow is the same regardless of where the knob on the tank is, unless of course it is off
The restrictor was part of the analyzer package, so I just assume it give the needed flow, maybe not though.

Check your assumption: run the analyser with the knob up high, then low, then almost off. Should be the same gas, but the analysis will drop. The "flow cap" mostly just protects the analyser's sensor so you don't blow it out. Best practice is turning the flow on first, feeling that it is only just barely on, then putting the flow cap against the valve.

[OFG-1]

Who DOESN"T analyze their gas, you ask? Why Chuck Norris does not. Oxygen gets Chuck Norris poisoning, not the other way around.

[scubadude]

I did check the assumption this way; Turn on the tank, the 1st stage is pressurized and the output to the BC hose is the 1st stage IP regardless of where the tank knob is set (unless off), then to the flow restrictor (which sees the IP only) then to the flow cap over the sensor. I can crack the tank valve open which pressures up all HP and LP hoses on the setup, then turn off the tank and the O2 reading stays the same as the pressure in all the hoses depletes to a point that affects the IP, then at that point is when a change in the O2 reading will occur. The bleed down takes tens of seconds to finish depending on the hoses on the reg setup. So the flow is stable no matter where the tank knob is until IP is affected.

The O2 reading change I was referring to above is .06 not .006, not sure if the humid diff would account for a whole .06, but I guess I could look that up somewhere.

Sorry if I hijacked the thread, just thought since we were on that topic, someone may have experienced the same thing.

On topic: I have personally witnessed a FullCave/Instructor that was diving with a group of request air instead of ean32 at one of the big cave shops in order to make some short dives with us, to save $ on fills I guess, since the dives were to be short. The air diver never tested the tanks since air was ordered. The rest of us ordered ean32. The result was we all got an ean32 fill. One of the group noted that the tanks seemed to all be filled together at the same time. Once the air tanks was tested it came in around 30%.

[curtschu]

It might not be completely the same but it's a funny story. We drove up to the Pike Peak visitor center, it is at about 14,000. my wife and I walked all around the area checking out things with no ill effect. Her Daughter was about 11 at the time took ten steps out of the car and just about passed out. The point being different people are effected differently and all of our data is Guidelines and must be verified independently for you.

[tursiops]

@scubadude
Sorry, man, ignore almost everything I said to you! I misunderstood your setup. I thought you were blowing air directly from the tank valve onto the cap on the analyzer; I missed that you were using a BC hose with a flow-restrictor on it.

The info you are looking for is in post #17 in this thread. It's pretty easy to get 0.6% error from the humidity in the room.

There are four main sources of error in the analysis: temp, humidity, barometric pressure, and flow rate onto the sensor. Calibrating to a tank of room temp air takes care of the first three; the flow restrictor takes care of the fourth.