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Kevin >David, nothing personal, but everything I've seen, heard and read is not as >you say. No offence taken. To keep this short, your point is that readings at the low end of the scale are more inaccurate than those towards the end. I accept this to be true if there is only one point of calibration i.e. the end point. Thus the closer one gets to the point of calibration the more accurate the reading. Hopefully we agree on this. My point is that I have a gauge which is reasonably linear. Before I use it I calibrate the zero setting by measuring ambient atmospheric pressure. I am reading the O2 close to this point of calibration i.e. 10 bar. This principle is the same as an O2 analyser or any other instrument. <50% calibrate in air >50% calibrate in pure O2. One of us is wrong, and if it is me, I would be grateful if you would explain it in horsey-duckey terms, so that I can be even more amazed at the accuracy of my hot mixing strategy! Of course, the real "inaccuracies" are due to temperature changes are not considered, e.g. if I mix a 232 bar mix, I pump to 254 bar and let it cool to around 232. David Shimell Project Manager, Sequent Computer Systems Ltd., Sandton, South Africa. Email: shimell@se*.co* <mailto:shimell@se*.co*> -----Original Message----- From: Kevin Connell [SMTP:kevin@nw*.co*] Sent: Wednesday, April 21, 1999 4:36 PM To: David Shimell (shimell) Cc: techdiver@aquanaut.com Subject: RE: Trimix Question David, nothing personal, but everything I've seen, heard and read is not as you say. Guages are almost always rated at an accuracy of FULL SCALE. IE/ if you have a 300bar gauge, 1% of that is 3bar, not 1% of the current reading. I know people with the fancy digital gauges, and even super duper 1/4% temp compensated and zero calibrated test gauges. They all agree that filling the O2 second is important. For example, when I mix 34% nitrox on my gauge (.5% accuracy, 4000PSI), if I fill it to 545psi of O2 first then 3300 air, I get a mix of about 36%. If I fill it to 200 psi of Air first, then up to 745 of O2 tben 3300 of air, I get 34% every time. (This is a zero calibration problem) When I mix trimix, I put the helim in first, and really fast. If I'm in a hurry, I'll overfill by 10%, but usually, I just let it sit for an hour or so, and then top to the desired pressure. Then I add O2, let sit a while, analyse the heliox mix, and correct if necessary. Then fill as fast as possible with air. I believe this is the preferred method. If you add O2 at the bottom of the process you are dealing with three potential problems: Gauge linearity, gauge zero calibration, and in deeper mixes, the amount of O2 you add is so small, it's hard to tell wether or not you actually added any O2 at all with an empty tank (See WKPP II) For helium accuracy: who cares. 5% won't make a lick of difference, so just go with an END of 90-100fsw to be safe with the narcosis. Always analyse the heliox if you're paranoid, like me. My .03 At 03:10 PM 4/21/1999 +0100, "David Shimell (shimell)" <shimell@se*.co*> wrote: >Steve > >I wrote: > >>I hot fill, O2 first, Helium then air. > >Your replied: > >>When I read this I was surprised. Why don't you mix in the helium first >>and the O2 second? Adding the O2 second allows the gauge to be in its' >>most linear range (hence most accurate) for the O2 addition. The helium >>when added first does not need to be as precise as the O2. > >I add O2 first to get more accurate O2 fills for the following reasons: > >1. No other gas has been added to the cylinder to cause it to heat up >and affect the pressure readings. >2. My gauge will read more accurately in the low end readings. > >You will note that my second point disagrees with your point. I use a >digital pressure gauge which has been factory calibrated to ensure its >accuracy at the 350 bar reading. The linearity 0-350 bar is also >documented. However, as we all know, this calibration and documentation was >only accurate at the time of testing and may have changed over time. What >will not change is the zero reading i.e. atmospheric pressure, since this is >checked and/or tared out to zero after switching on the gauge for use. >Since I am adding only 10 bar or so of O2 to my Trimixes, we are measuring >extremely close to the point of calibration (zero), hence the readings will >be very accurate. > >Another point you have to consider is that a 1% error in the gauge reading >of 10 bar is only 0.1 bar which is the accuracy of my gauge's display. > >I agree with you point about Helium not needing to be accurate. > >>Do you use the same procedure when starting with drained tanks? > >Yes, but I rarely dump the remaining gas - I usually remix it. > >David Shimell >Project Manager, Sequent Computer Systems Ltd., Sandton, South Africa. >Email: shimell@se*.co* <mailto:shimell@se*.co*> > >-----Original Message----- >From: Steve Hogan [SMTP:Steve.Hogan@tr*.co*] >Sent: Tuesday, April 20, 1999 9:08 PM >To: shimell@se*.co*; kevin@nw*.co* >Cc: techdiver@aquanaut.com >Subject: RE: Trimix Question > >David, > > >>I hot fill, O2 first, Helium then air. By hot fill I mean add the O2 >>slowly, then the Helium at a moderate rate. Two quick blasts of air to >>"swirl" the gas and moderate fill rate for the rest of the air. I over >pump >>each of the gasses by 10% to allow for cooling and compressibility. > >>Often I remix and use a booster. > >When I read this I was surprised. Why don't you mix in the helium first >and the O2 second? Adding the O2 second allows the gauge to be in its' >most linear range (hence most accurate) for the O2 addition. The helium >when added first does not need to be as precise as the O2. > >I think we all agree to add air last...... > >Do you use the same procedure when starting with drained tanks? > >Steve > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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