Calibration frequency requirements...

Padowan · 21st June 2006, 15:01

Following on from an over-a-beer discussion with Mark regarding the calibration procedure of his Hammerhead, I was thinking about the following:

In the Inspo manual, you are advised to calibrate the unit before every dive, or at lease once a day. I'd like to get a better understanding why some other units don't need such a regular calibration frequency.

As far as I understand it, any Digital (not analogue) PPO2 monitor (whether incorporated into an eCCR or as a passive PPO2 readout) will essentially use the same fundamental technique to convert mV into PPO2. In the case of something like a KISS handset, the cell will put out a mV reading (in 100% O2 at ambient pressure) and you twiddle a potentiometer so that the electronics can "understand" this mV equals a PPO2 of 1.0. Then (assuming single point calibration) the mV to PPO2 conversions takes on a linear scale of 0mv = 0.0 PPO2, up to x.xmV = 1.0 PPO2 and then interpolates beyond x.xmV to give the PPO2s above 1.0.

Now for something like an Inspo, this calibration procedure is automated (for 3 cells simultaneously), and there is no potentiometer to tweak, so my assumption in that the software is taking the mV reading, applying a calibration frig-factor (well, 3 factors actually, one for each sensor) to essentially give the same PPO2 reading capability as above.

So, in the case of a KISS handset, if no-one moves the potentiometer, then any inaccuracy in the PPO2 can only be caused by a deterioration of the cell (excluding ambient pressure discrepencies), and the consequent mis-representation of the linear (single point) calibration assumption line. If you're confident in your cells lifespan and mV decay over time, you decide to only calibrate once a month, perhaps

Assuming that an electronic PPO2 meter (eg. Inspiration or HH for example) can remember the calibration frig-factor what is the difference, and therefore why as an Inspo diver am I being recommended to calibrate every dive, while my KISS mates calibrate once a month? My unit remembers the "total elapsed on time" with no problems, so any non-volatile memory in it surely works OK? The phrase that Mark used from KevinJ was that the HH "held calibration well" and therefore only needs calibrating once a month - surely an electronic PPO2 meter either remembers the frig-factor or it doesn't, I don't see how one can "hold better" than another?

Any thoughts? Would I (as an Inspo diver) be better off doing a really accurate, cells-in-a-bag type calibration, with well known O2 percentage and ambient pressure numbers, than religiously running through the auto-calibration process every dive (or day in my case)?

***jradomski*** · 21st June 2006, 15:08

Quote: (Originally Posted by Padowan)

Following on from an over-a-beer discussion with Mark regarding the calibration procedure of his Hammerhead, I was thinking about the following:

In the Inspo manual, you are advised to calibrate the unit before every dive, or at lease once a day. I'd like to get a better understanding why some other units don't need such a regular calibration frequency.

As far as I understand it, any Digital (not analogue) PPO2 monitor (whether incorporated into an eCCR or as a passive PPO2 readout) will essentially use the same fundamental technique to convert mV into PPO2. In the case of something like a KISS handset, the cell will put out a mV reading (in 100% O2 at ambient pressure) and you twiddle a potentiometer so that the electronics can "understand" this mV equals a PPO2 of 1.0. Then (assuming single point calibration) the mV to PPO2 conversions takes on a linear scale of 0mv = 0.0 PPO2, up to x.xmV = 1.0 PPO2 and then interpolates beyond x.xmV to give the PPO2s above 1.0.

Now for something like an Inspo, this calibration procedure is automated (for 3 cells simultaneously), and there is no potentiometer to tweak, so my assumption in that the software is taking the mV reading, applying a calibration frig-factor (well, 3 factors actually, one for each sensor) to essentially give the same PPO2 reading capability as above.

So, in the case of a KISS handset, if no-one moves the potentiometer, then any inaccuracy in the PPO2 can only be caused by a deterioration of the cell (excluding ambient pressure discrepencies), and the consequent mis-representation of the linear (single point) calibration assumption line. If you're confident in your cells lifespan and mV decay over time, you decide to only calibrate once a month, perhaps

Assuming that an electronic PPO2 meter (eg. Inspiration or HH for example) can remember the calibration frig-factor what is the difference, and therefore why as an Inspo diver am I being recommended to calibrate every dive, while my KISS mates calibrate once a month? My unit remembers the "total elapsed on time" with no problems, so any non-volatile memory in it surely works OK? The phrase that Mark used from KevinJ was that the HH "held calibration well" and therefore only needs calibrating once a month - surely an electronic PPO2 meter either remembers the frig-factor or it doesn't, I don't see how one can "hold better" than another?

Any thoughts? Would I (as an Inspo diver) be better off doing a really accurate, cells-in-a-bag type calibration, with well known O2 percentage and ambient pressure numbers, than religiously running through the auto-calibration process every dive (or day in my case)?

I think once a month may be stretching it a bit depending on the use.. The calibration should be verified at regular intervals.. If I havent been diving for a few days.. I'll first expose the head to air and see what it shows.. then I'll do a quick flush.. if its close I'm set to go... I'll do a good cell linearity verification at the end of the dive at 6m.. If I want it dead on and I'm diving every day with long inwater times, with Teledyne cells I usually recall once every week or so..

***Beanie*** · 21st June 2006, 15:08

Good point well pointed out I was trying to think of a reason why the calibration would be out from the morning cal in the afternoon.
Someone did say something about pressure changes but then if the pressure changes the mV output changes relativly (?)

So what does it leave?
Cell deteriation
can't think of any others

Analysers => we 'calibrate' those each tiem we use them - single point in air.

I would be interesting to know what the calibration factors in the inspo electronics were each time to see the difference, same with the HH etc.

Padowan · 21st June 2006, 15:19

Quote: (Originally Posted by Beanie)

Good point well pointed out I was trying to think of a reason why the calibration would be out from the morning cal in the afternoon.
Someone did say something about pressure changes but then if the pressure changes the mV output changes relativly (?)

So what does it leave?
Cell deteriation
can't think of any others

Analysers => we 'calibrate' those each tiem we use them - single point in air.

I would be interesting to know what the calibration factors in the inspo electronics were each time to see the difference, same with the HH etc.

On your last point, you don't get to see the calibration factors on the YBOD, but you do get to see the "interpolated mV output for the cell in air" during the flushing process. This is (I assume) reversing the calculation:

mV x FrigFactor = PPO2

into:

20.9 / FrigFactor = x.xmV assumed in air

or to be more accurate:

20.9 / FrigFactor / (Pressure/1000) / (O2/100) = x.xmV in air
- where 'Pressure' and 'O2' are the pressure and O2 values entered in the calibration procedure.

I keep an eye on these to check for deterioration - but surely deterioration is going to occur first at the top end of the mV output scale, and O2 flushes at the end of a dive at ~6m are checking this...

***Beanie*** · 21st June 2006, 15:31

Quote: (Originally Posted by Padowan)

On your last point, you don't get to see the calibration factors on the YBOD, but you do get to see the "interpolated mV output for the cell in air" during the flushing process. This is (I assume) reversing the calculation:

mV x FrigFactor = PPO2

into:

20.9 / FrigFactor = x.xmV assumed in air

or to be more accurate:

20.9 / FrigFactor / (Pressure/1000) / (O2/100) = x.xmV in air
- where 'Pressure' and 'O2' are the pressure and O2 values entered in the calibration procedure.

I keep an eye on these to check for deterioration - but surely deterioration is going to occur first at the top end of the mV output scale, and O2 flushes at the end of a dive at ~6m are checking this...

hmmm sounds about right I guess doesn't help why some need more frequent calibration than others though

Of course it could be that they don't and its just diferent recommendations?
How many times has your inspo told you that you must calibrate?

***EBT*** · 21st June 2006, 15:35

Quote: (Originally Posted by Beanie)

Analysers => we 'calibrate' those each tiem we use them - single point in air.

This CAN be a bad thing. All too often you see a bunch of divers analysing their bunch of tanks after a fill, each re-doing the calibration within 30 seconds of taking the last reading (ie before the sensor has fully recovered from the last mix).

Personally my calibration frequency on the KISS varies by;

- Duration, I rarely re-calibrate between dives... if doing 3 days plus of diving, I re-cal.
- How committing is the dive (a 3 hour+ trimix cave dive will get a re-cal regardless)

Having said that, what I will do is a calibration CHECK every dive. Simple dives might only get a one point check, more complex will get two point.

***Beanie*** · 21st June 2006, 15:37

Quote: (Originally Posted by Beanie)

Analysers => we 'calibrate' those each time we use them - single point in air.

Quote: (Originally Posted by EBT)

This CAN be a bad thing. All too often you see a bunch of divers analysing their bunch of tanks after a fill, each re-doing the calibration within 30 seconds of taking the last reading (ie before the sensor has fully recovered from the last mix).

Opps badly owrded I meant everytime it comes out of the box not before each analysis.

You do have a valid point, that would really screw things up

sven · 21st June 2006, 16:01

My take on this: Different manufacturers have different levels of trust in the reliability of the sensors. Chemically and electrically there doesn't seem to be any reason for the different recommendations. I also think that "one unit holds the cal better than others" does not make sense at all, as the chemical/electrical processes have the same impact on the sensors regardless of the units. So as long as the same sensors are used, calibration should only be required in order to cope with deterioration of the cells.

In essence - the different recommendations are just a b_tt covering exercise and determined by the lawyers, more than anything else.

***jradomski*** · 21st June 2006, 16:53

Quote: (Originally Posted by sven)

My take on this: Different manufacturers have different levels of trust in the reliability of the sensors. Chemically and electrically there doesn't seem to be any reason for the different recommendations. I also think that "one unit holds the cal better than others" does not make sense at all, as the chemical/electrical processes have the same impact on the sensors regardless of the units. So as long as the same sensors are used, calibration should only be required in order to cope with deterioration of the cells.

In essence - the different recommendations are just a b_tt covering exercise and determined by the lawyers, more than anything else.

It may not make sense, but at least with the classic when I try and check the cal after a few days its always been off by more than a few percent.. With the HH its almost always still been on, and in these cases I check the cells readings and if they changed by more than a few MV in oxygen, I'll keep a close eye on them for replacement (which is usually necessary)....

***ScubaDadMiami*** · 22nd June 2006, 01:49

My cells seem to react to the moisture more than I expected. So, I will see some spread start to develop during the dive with at least one cell being as much as .15 off compared to the other two. Even if I flush the loop, I really don't see an improvement.

If it will be a while before the next dive (such as on a live aboard), I will open up my loop hoses to let things dry out a bit. Drying out does seem to stabilize the situation but does not necessarily lead to all of the cells coming back to reading the same thing. So, I will re-calibrate.

Sometimes, I will even re-calibrate during the surface intervals on dives where I won't open the loop (because it is too short of a time to justify doing it). It just makes it so that the readings on the cells are all the same at the start rather than a tad off. I also don't re-calibrate sometimes when I have these short surface intervals. I am still feeling this whole thing out.

21st June 2006, 15:01	#1 (permalink)
Padowan RBW Member Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Feb 2005 Location: Exeter, UK. Posts: 526	Calibration frequency requirements... Following on from an over-a-beer discussion with Mark regarding the calibration procedure of his Hammerhead, I was thinking about the following: In the Inspo manual, you are advised to calibrate the unit before every dive, or at lease once a day. I'd like to get a better understanding why some other units don't need such a regular calibration frequency. As far as I understand it, any Digital (not analogue) PPO2 monitor (whether incorporated into an eCCR or as a passive PPO2 readout) will essentially use the same fundamental technique to convert mV into PPO2. In the case of something like a KISS handset, the cell will put out a mV reading (in 100% O2 at ambient pressure) and you twiddle a potentiometer so that the electronics can "understand" this mV equals a PPO2 of 1.0. Then (assuming single point calibration) the mV to PPO2 conversions takes on a linear scale of 0mv = 0.0 PPO2, up to x.xmV = 1.0 PPO2 and then interpolates beyond x.xmV to give the PPO2s above 1.0. Now for something like an Inspo, this calibration procedure is automated (for 3 cells simultaneously), and there is no potentiometer to tweak, so my assumption in that the software is taking the mV reading, applying a calibration frig-factor (well, 3 factors actually, one for each sensor) to essentially give the same PPO2 reading capability as above. So, in the case of a KISS handset, if no-one moves the potentiometer, then any inaccuracy in the PPO2 can only be caused by a deterioration of the cell (excluding ambient pressure discrepencies), and the consequent mis-representation of the linear (single point) calibration assumption line. If you're confident in your cells lifespan and mV decay over time, you decide to only calibrate once a month, perhaps Assuming that an electronic PPO2 meter (eg. Inspiration or HH for example) can remember the calibration frig-factor what is the difference, and therefore why as an Inspo diver am I being recommended to calibrate every dive, while my KISS mates calibrate once a month? My unit remembers the "total elapsed on time" with no problems, so any non-volatile memory in it surely works OK? The phrase that Mark used from KevinJ was that the HH "held calibration well" and therefore only needs calibrating once a month - surely an electronic PPO2 meter either remembers the frig-factor or it doesn't, I don't see how one can "hold better" than another? Any thoughts? Would I (as an Inspo diver) be better off doing a really accurate, cells-in-a-bag type calibration, with well known O2 percentage and ambient pressure numbers, than religiously running through the auto-calibration process every dive (or day in my case)? __________________ Padowan The future's bright, the future's Yellow .....for now padowan.net - Latest Video: Scapa Flow Rebreather Diving
(Online)

21st June 2006, 15:08	#2 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Join Date: Mar 2005 Location: "Da" Bronx Posts: 3,113	Re: Calibration frequency requirements... Quote: (Originally Posted by Padowan) Following on from an over-a-beer discussion with Mark regarding the calibration procedure of his Hammerhead, I was thinking about the following: In the Inspo manual, you are advised to calibrate the unit before every dive, or at lease once a day. I'd like to get a better understanding why some other units don't need such a regular calibration frequency. As far as I understand it, any Digital (not analogue) PPO2 monitor (whether incorporated into an eCCR or as a passive PPO2 readout) will essentially use the same fundamental technique to convert mV into PPO2. In the case of something like a KISS handset, the cell will put out a mV reading (in 100% O2 at ambient pressure) and you twiddle a potentiometer so that the electronics can "understand" this mV equals a PPO2 of 1.0. Then (assuming single point calibration) the mV to PPO2 conversions takes on a linear scale of 0mv = 0.0 PPO2, up to x.xmV = 1.0 PPO2 and then interpolates beyond x.xmV to give the PPO2s above 1.0. Now for something like an Inspo, this calibration procedure is automated (for 3 cells simultaneously), and there is no potentiometer to tweak, so my assumption in that the software is taking the mV reading, applying a calibration frig-factor (well, 3 factors actually, one for each sensor) to essentially give the same PPO2 reading capability as above. So, in the case of a KISS handset, if no-one moves the potentiometer, then any inaccuracy in the PPO2 can only be caused by a deterioration of the cell (excluding ambient pressure discrepencies), and the consequent mis-representation of the linear (single point) calibration assumption line. If you're confident in your cells lifespan and mV decay over time, you decide to only calibrate once a month, perhaps Assuming that an electronic PPO2 meter (eg. Inspiration or HH for example) can remember the calibration frig-factor what is the difference, and therefore why as an Inspo diver am I being recommended to calibrate every dive, while my KISS mates calibrate once a month? My unit remembers the "total elapsed on time" with no problems, so any non-volatile memory in it surely works OK? The phrase that Mark used from KevinJ was that the HH "held calibration well" and therefore only needs calibrating once a month - surely an electronic PPO2 meter either remembers the frig-factor or it doesn't, I don't see how one can "hold better" than another? Any thoughts? Would I (as an Inspo diver) be better off doing a really accurate, cells-in-a-bag type calibration, with well known O2 percentage and ambient pressure numbers, than religiously running through the auto-calibration process every dive (or day in my case)? I think once a month may be stretching it a bit depending on the use.. The calibration should be verified at regular intervals.. If I havent been diving for a few days.. I'll first expose the head to air and see what it shows.. then I'll do a quick flush.. if its close I'm set to go... I'll do a good cell linearity verification at the end of the dive at 6m.. If I want it dead on and I'm diving every day with long inwater times, with Teledyne cells I usually recall once every week or so.. __________________ Joe Radomski CCR Trimix Instructor Trainer ANDI Instructor Trainer Director #10 All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.
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21st June 2006, 15:08	#3 (permalink)
*Beanie* Gallery Administrator Current Rebreather/s: Inspiration Classic Other Rebreather/s: Inspiration Vision Join Date: Jan 2005 Location: In da house! yeah baby! Posts: 3,311	Re: Calibration frequency requirements... Good point well pointed out I was trying to think of a reason why the calibration would be out from the morning cal in the afternoon. Someone did say something about pressure changes but then if the pressure changes the mV output changes relativly (?) So what does it leave? Cell deteriation can't think of any others Analysers => we 'calibrate' those each tiem we use them - single point in air. I would be interesting to know what the calibration factors in the inspo electronics were each time to see the difference, same with the HH etc. __________________ Beanie Gallery Admin & Library Assistant. www.outlawdivers.org.uk www.beandiving.co.uk www.beanengineering.co.uk
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21st June 2006, 15:19	#4 (permalink)
Padowan RBW Member Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Feb 2005 Location: Exeter, UK. Posts: 526	Re: Calibration frequency requirements... Quote: (Originally Posted by Beanie) Good point well pointed out I was trying to think of a reason why the calibration would be out from the morning cal in the afternoon. Someone did say something about pressure changes but then if the pressure changes the mV output changes relativly (?) So what does it leave? Cell deteriation can't think of any others Analysers => we 'calibrate' those each tiem we use them - single point in air. I would be interesting to know what the calibration factors in the inspo electronics were each time to see the difference, same with the HH etc. On your last point, you don't get to see the calibration factors on the YBOD, but you do get to see the "interpolated mV output for the cell in air" during the flushing process. This is (I assume) reversing the calculation: mV x FrigFactor = PPO2 into: 20.9 / FrigFactor = x.xmV assumed in air or to be more accurate: 20.9 / FrigFactor / (Pressure/1000) / (O2/100) = x.xmV in air - where 'Pressure' and 'O2' are the pressure and O2 values entered in the calibration procedure. I keep an eye on these to check for deterioration - but surely deterioration is going to occur first at the top end of the mV output scale, and O2 flushes at the end of a dive at ~6m are checking this... __________________ Padowan The future's bright, the future's Yellow .....for now padowan.net - Latest Video: Scapa Flow Rebreather Diving
(Online)

21st June 2006, 15:31	#5 (permalink)
*Beanie* Gallery Administrator Current Rebreather/s: Inspiration Classic Other Rebreather/s: Inspiration Vision Join Date: Jan 2005 Location: In da house! yeah baby! Posts: 3,311	Re: Calibration frequency requirements... Quote: (Originally Posted by Padowan) On your last point, you don't get to see the calibration factors on the YBOD, but you do get to see the "interpolated mV output for the cell in air" during the flushing process. This is (I assume) reversing the calculation: mV x FrigFactor = PPO2 into: 20.9 / FrigFactor = x.xmV assumed in air or to be more accurate: 20.9 / FrigFactor / (Pressure/1000) / (O2/100) = x.xmV in air - where 'Pressure' and 'O2' are the pressure and O2 values entered in the calibration procedure. I keep an eye on these to check for deterioration - but surely deterioration is going to occur first at the top end of the mV output scale, and O2 flushes at the end of a dive at ~6m are checking this... hmmm sounds about right I guess doesn't help why some need more frequent calibration than others though Of course it could be that they don't and its just diferent recommendations? How many times has your inspo told you that you must calibrate? __________________ Beanie Gallery Admin & Library Assistant. www.outlawdivers.org.uk www.beandiving.co.uk www.beanengineering.co.uk
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21st June 2006, 15:35	#6 (permalink)
*EBT* Apprentice Luddite Current Rebreather/s: Classic Kiss Other Rebreather/s: Inspiration Classic Join Date: Feb 2005 Location: UK, Brighton Posts: 2,105	Re: Calibration frequency requirements... Quote: (Originally Posted by Beanie) Analysers => we 'calibrate' those each tiem we use them - single point in air. This CAN be a bad thing. All too often you see a bunch of divers analysing their bunch of tanks after a fill, each re-doing the calibration within 30 seconds of taking the last reading (ie before the sensor has fully recovered from the last mix). Personally my calibration frequency on the KISS varies by; - Duration, I rarely re-calibrate between dives... if doing 3 days plus of diving, I re-cal. - How committing is the dive (a 3 hour+ trimix cave dive will get a re-cal regardless) Having said that, what I will do is a calibration CHECK every dive. Simple dives might only get a one point check, more complex will get two point. __________________ Eagles May Soar, but weasels don't get sucked into jet engines! RBW Terms of service
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21st June 2006, 15:37	#7 (permalink)
*Beanie* Gallery Administrator Current Rebreather/s: Inspiration Classic Other Rebreather/s: Inspiration Vision Join Date: Jan 2005 Location: In da house! yeah baby! Posts: 3,311	Re: Calibration frequency requirements... Quote: (Originally Posted by Beanie) Analysers => we 'calibrate' those each time we use them - single point in air. Quote: (Originally Posted by EBT) This CAN be a bad thing. All too often you see a bunch of divers analysing their bunch of tanks after a fill, each re-doing the calibration within 30 seconds of taking the last reading (ie before the sensor has fully recovered from the last mix). Opps badly owrded I meant everytime it comes out of the box not before each analysis. You do have a valid point, that would really screw things up __________________ Beanie Gallery Admin & Library Assistant. www.outlawdivers.org.uk www.beandiving.co.uk www.beanengineering.co.uk
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21st June 2006, 16:01	#8 (permalink)
sven Blogs Admin / Forum Mod Current Rebreather/s: Inspiration Classic Other Rebreather/s: Evolution Join Date: Nov 2005 Location: Sydney Posts: 551	Re: Calibration frequency requirements... My take on this: Different manufacturers have different levels of trust in the reliability of the sensors. Chemically and electrically there doesn't seem to be any reason for the different recommendations. I also think that "one unit holds the cal better than others" does not make sense at all, as the chemical/electrical processes have the same impact on the sensors regardless of the units. So as long as the same sensors are used, calibration should only be required in order to cope with deterioration of the cells. In essence - the different recommendations are just a b_tt covering exercise and determined by the lawyers, more than anything else. __________________ Regards, Sven The Sydney Project website: http://www.sydneyproject.com My Blog: http://sven.rebreatherworld.com "Whenever you find yourself on the side of the majority, it's time to pause and reflect." Mark Twain
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21st June 2006, 16:53	#9 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Join Date: Mar 2005 Location: "Da" Bronx Posts: 3,113	Re: Calibration frequency requirements... Quote: (Originally Posted by sven) My take on this: Different manufacturers have different levels of trust in the reliability of the sensors. Chemically and electrically there doesn't seem to be any reason for the different recommendations. I also think that "one unit holds the cal better than others" does not make sense at all, as the chemical/electrical processes have the same impact on the sensors regardless of the units. So as long as the same sensors are used, calibration should only be required in order to cope with deterioration of the cells. In essence - the different recommendations are just a b_tt covering exercise and determined by the lawyers, more than anything else. It may not make sense, but at least with the classic when I try and check the cal after a few days its always been off by more than a few percent.. With the HH its almost always still been on, and in these cases I check the cells readings and if they changed by more than a few MV in oxygen, I'll keep a close eye on them for replacement (which is usually necessary).... __________________ Joe Radomski CCR Trimix Instructor Trainer ANDI Instructor Trainer Director #10 All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.
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22nd June 2006, 01:49	#10 (permalink)
*ScubaDadMiami* Moderator Current Rebreather/s: Optima Other Rebreather/s: Join Date: Sep 2005 Location: Florida, USA Posts: 2,215	Re: Calibration frequency requirements... My cells seem to react to the moisture more than I expected. So, I will see some spread start to develop during the dive with at least one cell being as much as .15 off compared to the other two. Even if I flush the loop, I really don't see an improvement. If it will be a while before the next dive (such as on a live aboard), I will open up my loop hoses to let things dry out a bit. Drying out does seem to stabilize the situation but does not necessarily lead to all of the cells coming back to reading the same thing. So, I will re-calibrate. Sometimes, I will even re-calibrate during the surface intervals on dives where I won't open the loop (because it is too short of a time to justify doing it). It just makes it so that the readings on the cells are all the same at the start rather than a tad off. I also don't re-calibrate sometimes when I have these short surface intervals. I am still feeling this whole thing out. __________________ Howard Packer IANTD CCR Instructor Miami Beach, Florida CCRDiveTraining.com
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