Pressure droprate and flow warning?

jaap · 4th February 2007, 00:09

Hi

There are several computers that monitor the pressure of a cylinder. And there are computers for SCR linked to oxygen cells. Why not do away with the cells and still get some form of good/no good info for basic SCR-use?

Maybe there should be a niche for a computer that monitors tank pressure and give you a warning if the CMF-SCR is feeding a dangerously low flow?

If the diver enters tank size and flow prior to the dive the computer could warn the diver if the flow is dropping dangerously low?

I have never owed a computer that measures tank pressure but I guess this sort of thing should work if the sampling interval and precision of the pressure monitor is high. I imagine there are quite a lot of Rebreather-divers that have had troubles with O2-cells and that there is a fair amount of recreational SCR-users that only wish to dive and not need to bother too much about degrading and sometimes erratic cells giving them a headache. But maybe they still want some form of advance warning if the flow is dangerous?
I suppose a digital pressure sensor is generally way more robust and long lived compared to an oxygen cell.
Given the prices for add on pressure sensors for dive compters and the price of a commercial basic pO2-monitor a pressure sensor system for CMF-SCR monitoring should not be more expensive. And it should be alot less expensive if the user sticks to it for a few years "saving"-money and troubles not having to buy new O2-cells now and then. That is if there is a market for it.
Sure a pressure monitoring system cannot warn the diver if the wrong mix is used, or if the tank size & flow data is not entered correctly. Or in the unlikely case that the diver is working harder than the usually very conservatively set limits for a precribed mix/flow. And its alot less useful for a diver that wishes to experiment with non standard mixes/flows/procedures... Another thing is of course that an air calibrated oxygencell for basic SCR-use usually lasts quite alot longer than in the CCR-case.
So does anyone know if I have not thought about some major aspect that defeats this sort of thing?
Or is the market perhaps too small for such a thing?

**Freef** · 4th February 2007, 18:14

Quote: (Originally Posted by jaap)

So does anyone know if I have not thought about some major aspect that defeats this sort of thing?

The main problem you have already discussed-remembering to set the unit.

As user error is the most common form of Rebreather problem,, trusting a mis set computer would cause problems..."But the computer said i was OK".

The only dive instrument that is almost 100% reliable is a compass, everything else can or will fail at some point.

On my Dolph the O2 cell is still going strong 2 years later! I periodically check it's calibration with higher fO2 gas than I dive with and it is still giving readings to within 1%.

The pressure drop on my Dolph is very low as I dive on the slowest flow rate jet, so the gauge must be able to accurately detect a drop of under 1.2 bar/min when I dive on the 5L tank. When I start diving on the 10L it would need to detect a drop of less than 0.6 bar/min. Even with modern transducers these drops may be too low to accurately detect across the full spectrum of 50-200 bar.

Even if the gauge could detect the slow flow rates operation of the ADV would cause it problems.

The simplest way for SCR diving is to use a single cell and monitor the pO2 or fO2. One thing to turn on, and no remembering to set the computer each and every dive! Having loop monitoring is essential in SCR diving.

jaap · 5th February 2007, 09:30

Sure a working po2-meter is the best.

But the sensitivity of the pressure sensor is not a problem. Standard digital manometers are sensitive to less than 0.1 bar over the 0-300 bar range. Sure the computer could maybe get fooled if say the flow gets blocked but gas is still getting used by a demandvalve, bcd or suit...

So its probably a bad idea.

Quote: (Originally Posted by Freef)

The main problem you have already discussed-remembering to set the unit.

As user error is the most common form of Rebreather problem,, trusting a mis set computer would cause problems..."But the computer said i was OK".

The only dive instrument that is almost 100% reliable is a compass, everything else can or will fail at some point.

On my Dolph the O2 cell is still going strong 2 years later! I periodically check it's calibration with higher fO2 gas than I dive with and it is still giving readings to within 1%.

The pressure drop on my Dolph is very low as I dive on the slowest flow rate jet, so the gauge must be able to accurately detect a drop of under 1.2 bar/min when I dive on the 5L tank. When I start diving on the 10L it would need to detect a drop of less than 0.6 bar/min. Even with modern transducers these drops may be too low to accurately detect across the full spectrum of 50-200 bar.

Even if the gauge could detect the slow flow rates operation of the ADV would cause it problems.

The simplest way for SCR diving is to use a single cell and monitor the pO2 or fO2. One thing to turn on, and no remembering to set the computer each and every dive! Having loop monitoring is essential in SCR diving.

**Freef** · 5th February 2007, 16:18

Quote: (Originally Posted by jaap)

So its probably a bad idea.

It's not actually a bad idea having a seperate flow rate monitring system, but the Drager is designed to be pretty fail safe.

I thought of something else too, when diving in colder water there is a pressure drop as the tank cools as well as the drop through use, so the computer could be fooled into giving an OK when things arent.

I must admit I never thought of the system you suggest. It's probably a case of beign too limited for moost divers, and also longer to set up.

Look at it as an idea awating an application.

jaap · 5th February 2007, 16:56

Quote: (Originally Posted by Freef)

It's not actually a bad idea having a seperate flow rate monitring system, but the Drager is designed to be pretty fail safe.

I thought of something else too, when diving in colder water there is a pressure drop as the tank cools as well as the drop through use, so the computer could be fooled into giving an OK when things arent.

I must admit I never thought of the system you suggest. It's probably a case of beign too limited for moost divers, and also longer to set up.

Look at it as an idea awating an application.

Yes, well I think the cooling effect is over in a few minutes once in the water. But yes the pressure will drop by around 0.75 bar/Kelvin around 200 bar.

Infact Dräger already played with a somewhat related pressure warning (with a mask mounted bulb HUD!) in the SM 1, and that was almost 50 years ago...

4th February 2007, 00:09	#1 (permalink)
jaap Normal people worry me Current Rebreather/s: Classic Kiss Other CCR RB80 / Clone Ray Other SCR Home Build Other Rebreather/s: Other CCR RB80 / Clone Ray Other SCR Home Build Join Date: Jun 2005 Location: Stockholm Sweden Posts: 451	Pressure droprate and flow warning? Hi There are several computers that monitor the pressure of a cylinder. And there are computers for SCR linked to oxygen cells. Why not do away with the cells and still get some form of good/no good info for basic SCR-use? Maybe there should be a niche for a computer that monitors tank pressure and give you a warning if the CMF-SCR is feeding a dangerously low flow? If the diver enters tank size and flow prior to the dive the computer could warn the diver if the flow is dropping dangerously low? I have never owed a computer that measures tank pressure but I guess this sort of thing should work if the sampling interval and precision of the pressure monitor is high. I imagine there are quite a lot of Rebreather-divers that have had troubles with O2-cells and that there is a fair amount of recreational SCR-users that only wish to dive and not need to bother too much about degrading and sometimes erratic cells giving them a headache. But maybe they still want some form of advance warning if the flow is dangerous? I suppose a digital pressure sensor is generally way more robust and long lived compared to an oxygen cell. Given the prices for add on pressure sensors for dive compters and the price of a commercial basic pO2-monitor a pressure sensor system for CMF-SCR monitoring should not be more expensive. And it should be alot less expensive if the user sticks to it for a few years "saving"-money and troubles not having to buy new O2-cells now and then. That is if there is a market for it. Sure a pressure monitoring system cannot warn the diver if the wrong mix is used, or if the tank size & flow data is not entered correctly. Or in the unlikely case that the diver is working harder than the usually very conservatively set limits for a precribed mix/flow. And its alot less useful for a diver that wishes to experiment with non standard mixes/flows/procedures... Another thing is of course that an air calibrated oxygencell for basic SCR-use usually lasts quite alot longer than in the CCR-case. So does anyone know if I have not thought about some major aspect that defeats this sort of thing? Or is the market perhaps too small for such a thing? __________________ My initials: JAAP
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4th February 2007, 18:14	#2 (permalink)
Freef Custom Title Disallowed! Current Rebreather/s: Dolphin Other Rebreather/s: Dolphin Join Date: Jan 2006 Location: Land of the Freef, UK. Posts: 1,353	Re: Pressure droprate and flow warning? Quote: (Originally Posted by jaap) So does anyone know if I have not thought about some major aspect that defeats this sort of thing? The main problem you have already discussed-remembering to set the unit. As user error is the most common form of Rebreather problem,, trusting a mis set computer would cause problems..."But the computer said i was OK". The only dive instrument that is almost 100% reliable is a compass, everything else can or will fail at some point. On my Dolph the O2 cell is still going strong 2 years later! I periodically check it's calibration with higher fO2 gas than I dive with and it is still giving readings to within 1%. The pressure drop on my Dolph is very low as I dive on the slowest flow rate jet, so the gauge must be able to accurately detect a drop of under 1.2 bar/min when I dive on the 5L tank. When I start diving on the 10L it would need to detect a drop of less than 0.6 bar/min. Even with modern transducers these drops may be too low to accurately detect across the full spectrum of 50-200 bar. Even if the gauge could detect the slow flow rates operation of the ADV would cause it problems. The simplest way for SCR diving is to use a single cell and monitor the pO2 or fO2. One thing to turn on, and no remembering to set the computer each and every dive! Having loop monitoring is essential in SCR diving. __________________ David. Currently owner of two differently sized ankles. Last edited by Freef : 4th February 2007 at 18:28. Reason: Misread original post!
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5th February 2007, 09:30	#3 (permalink)
jaap Normal people worry me Current Rebreather/s: Classic Kiss Other CCR RB80 / Clone Ray Other SCR Home Build Other Rebreather/s: Other CCR RB80 / Clone Ray Other SCR Home Build Join Date: Jun 2005 Location: Stockholm Sweden Posts: 451	Re: Pressure droprate and flow warning? Sure a working po2-meter is the best. But the sensitivity of the pressure sensor is not a problem. Standard digital manometers are sensitive to less than 0.1 bar over the 0-300 bar range. Sure the computer could maybe get fooled if say the flow gets blocked but gas is still getting used by a demandvalve, bcd or suit... So its probably a bad idea. Quote: (Originally Posted by Freef) The main problem you have already discussed-remembering to set the unit. As user error is the most common form of Rebreather problem,, trusting a mis set computer would cause problems..."But the computer said i was OK". The only dive instrument that is almost 100% reliable is a compass, everything else can or will fail at some point. On my Dolph the O2 cell is still going strong 2 years later! I periodically check it's calibration with higher fO2 gas than I dive with and it is still giving readings to within 1%. The pressure drop on my Dolph is very low as I dive on the slowest flow rate jet, so the gauge must be able to accurately detect a drop of under 1.2 bar/min when I dive on the 5L tank. When I start diving on the 10L it would need to detect a drop of less than 0.6 bar/min. Even with modern transducers these drops may be too low to accurately detect across the full spectrum of 50-200 bar. Even if the gauge could detect the slow flow rates operation of the ADV would cause it problems. The simplest way for SCR diving is to use a single cell and monitor the pO2 or fO2. One thing to turn on, and no remembering to set the computer each and every dive! Having loop monitoring is essential in SCR diving. __________________ My initials: JAAP
(Offline)

5th February 2007, 16:18	#4 (permalink)
Freef Custom Title Disallowed! Current Rebreather/s: Dolphin Other Rebreather/s: Dolphin Join Date: Jan 2006 Location: Land of the Freef, UK. Posts: 1,353	Re: Pressure droprate and flow warning? Quote: (Originally Posted by jaap) So its probably a bad idea. It's not actually a bad idea having a seperate flow rate monitring system, but the Drager is designed to be pretty fail safe. I thought of something else too, when diving in colder water there is a pressure drop as the tank cools as well as the drop through use, so the computer could be fooled into giving an OK when things arent. I must admit I never thought of the system you suggest. It's probably a case of beign too limited for moost divers, and also longer to set up. Look at it as an idea awating an application. __________________ David. Currently owner of two differently sized ankles.
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5th February 2007, 16:56	#5 (permalink)
jaap Normal people worry me Current Rebreather/s: Classic Kiss Other CCR RB80 / Clone Ray Other SCR Home Build Other Rebreather/s: Other CCR RB80 / Clone Ray Other SCR Home Build Join Date: Jun 2005 Location: Stockholm Sweden Posts: 451	Re: Pressure droprate and flow warning? Quote: (Originally Posted by Freef) It's not actually a bad idea having a seperate flow rate monitring system, but the Drager is designed to be pretty fail safe. I thought of something else too, when diving in colder water there is a pressure drop as the tank cools as well as the drop through use, so the computer could be fooled into giving an OK when things arent. I must admit I never thought of the system you suggest. It's probably a case of beign too limited for moost divers, and also longer to set up. Look at it as an idea awating an application. Yes, well I think the cooling effect is over in a few minutes once in the water. But yes the pressure will drop by around 0.75 bar/Kelvin around 200 bar. Infact Dräger already played with a somewhat related pressure warning (with a mask mounted bulb HUD!) in the SM 1, and that was almost 50 years ago... __________________ My initials: JAAP
(Offline)

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