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David, I understand your procedure when remixing. My point here was I use the same procedure whether remixing or starting from empty. My concern is even with a digital gauge, the sensing element is the weakest link. Both analog and digital gauges are prone to the same weakness. A digital gauge does not mean more accurate. The accuracy at the low end (especially during a mix from empty) is the area for concern. Regards Steve -----Original Message----- From: David Shimell (shimell) [mailto:shimell@se*.co*] Sent: Wednesday, April 21, 1999 7:10 AM To: Steve Hogan; kevin@nw*.co* Cc: techdiver@aquanaut.com Subject: RE: Trimix Question 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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