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@JonG1 and @stuartv
I do not run hot bottom mixes often. Most of my diving is cave oriented so it's long duration and I usually run 1.1. I usually run EAN32 and my dil ppO2's will often be 1.2 in certain caves. That does mean that a dil flush raises my ppO2 and I have to breathe it down. For reference that is usually 4ata diving with EAN32. In contrast, a more "traditional" configuration would use air for that dive and the dil ppO2 would be .84. With 0.84, you get to see the dil drop all the way down and then watch how the cells respond as you breathe it back up. For cave diving, the hotter dil mixes also give us the flexibility to use SCR mode in the event of either electronics or O2 supply failure without serious risk of hypoxia or a wicked premium to pay in deco time.
If diving EAN32, you would write down your expected mV's based on calibration data and linearity, and when you were at 4ata's, you would dil flush and validate both the ppO2 and mV's against what you have in wet notes.
Why is this important. IMO you should only dive up to a ppO2 that you are able to actually validate which is your dil ppO2. The 1.6 check at the beginning of the dive is critical, however what it can't do is show you how your cells are doing after 2-3 hours on the loop where they may start to behave differently than when they were nice and cool/dry at the start of the dive. They have now been fully saturated with humidity and are often times at a much higher temperature than they were at the beginning depending on surface conditions. The computer tells you the ppO2 that you are supposed to be at on the dil or if you're diving archaic readouts you can always just write it down somewhere, and then you have the mV's to check against just to make sure everything is behaving. With single point calibration units as said above, I think the mV validation is not nearly as critical as it used to be where you had two opportunities to screw up a calibration, but my Meg 2.7 does still require 2 calibration points, and you do not know that the calibration was correct until you do that 1.6 check. If the air cal point was wrong and the cells are acting super goofy, you at least have the mV's written down and can run by those.
On the subject of what @Bobby was getting at. If you are only running around 1.0 for the bottom, but you intend to run at 1.6 for deco, then you need the ability to plug in a 1.6 gas to the unit before you should feel comfortable running around at that high of a ppO2. I don't use onboard bottles because they're basically useless for cave diving and I firmly believe in the dilout concept. Onboard bottle does make a most excellent inflation bottle though for either suit, wing, or suit & wing depending on what you're doing. You have to have the 1.6 bottle for your planned OC deco ascent anyway and I have the ability to plug all of my bottles into something. Depending on your setup and preference that would either be a LPI or a QC6/CEJN on a regulator hose. The ascent schedule then has the same gas switches as you would for an OC ascent where you unplug your backgas and plug in the deco bottle following the same gas switch procedure as you would if you were doing an OC dive. I will sometimes make an exception to that for O2 stops where I will often stay on the onboard O2 if it's the last dive of the day. If it's first dive, I usually plug in since the O2 flushes on deco tend to kill the small bottles pretty quickly.
Why can't you just run a 1.3 and use O2 to bump it up? You have no earthly idea how much O2 to add. If the cells are limited, and heaven forbid they all are, you can keep adding O2 until the cell says 1.6 but the actual ppO2 may be well higher and deep into oxtox territory.
I do not run hot bottom mixes often. Most of my diving is cave oriented so it's long duration and I usually run 1.1. I usually run EAN32 and my dil ppO2's will often be 1.2 in certain caves. That does mean that a dil flush raises my ppO2 and I have to breathe it down. For reference that is usually 4ata diving with EAN32. In contrast, a more "traditional" configuration would use air for that dive and the dil ppO2 would be .84. With 0.84, you get to see the dil drop all the way down and then watch how the cells respond as you breathe it back up. For cave diving, the hotter dil mixes also give us the flexibility to use SCR mode in the event of either electronics or O2 supply failure without serious risk of hypoxia or a wicked premium to pay in deco time.
If diving EAN32, you would write down your expected mV's based on calibration data and linearity, and when you were at 4ata's, you would dil flush and validate both the ppO2 and mV's against what you have in wet notes.
Why is this important. IMO you should only dive up to a ppO2 that you are able to actually validate which is your dil ppO2. The 1.6 check at the beginning of the dive is critical, however what it can't do is show you how your cells are doing after 2-3 hours on the loop where they may start to behave differently than when they were nice and cool/dry at the start of the dive. They have now been fully saturated with humidity and are often times at a much higher temperature than they were at the beginning depending on surface conditions. The computer tells you the ppO2 that you are supposed to be at on the dil or if you're diving archaic readouts you can always just write it down somewhere, and then you have the mV's to check against just to make sure everything is behaving. With single point calibration units as said above, I think the mV validation is not nearly as critical as it used to be where you had two opportunities to screw up a calibration, but my Meg 2.7 does still require 2 calibration points, and you do not know that the calibration was correct until you do that 1.6 check. If the air cal point was wrong and the cells are acting super goofy, you at least have the mV's written down and can run by those.
On the subject of what @Bobby was getting at. If you are only running around 1.0 for the bottom, but you intend to run at 1.6 for deco, then you need the ability to plug in a 1.6 gas to the unit before you should feel comfortable running around at that high of a ppO2. I don't use onboard bottles because they're basically useless for cave diving and I firmly believe in the dilout concept. Onboard bottle does make a most excellent inflation bottle though for either suit, wing, or suit & wing depending on what you're doing. You have to have the 1.6 bottle for your planned OC deco ascent anyway and I have the ability to plug all of my bottles into something. Depending on your setup and preference that would either be a LPI or a QC6/CEJN on a regulator hose. The ascent schedule then has the same gas switches as you would for an OC ascent where you unplug your backgas and plug in the deco bottle following the same gas switch procedure as you would if you were doing an OC dive. I will sometimes make an exception to that for O2 stops where I will often stay on the onboard O2 if it's the last dive of the day. If it's first dive, I usually plug in since the O2 flushes on deco tend to kill the small bottles pretty quickly.
Why can't you just run a 1.3 and use O2 to bump it up? You have no earthly idea how much O2 to add. If the cells are limited, and heaven forbid they all are, you can keep adding O2 until the cell says 1.6 but the actual ppO2 may be well higher and deep into oxtox territory.
