How to make a digital ppo2 display?

teomannaskali · 8th August 2007, 23:17

Does anyone know the design parameters for those o rings?

What what compresion ratio and what gland size? I have found different sources for O ring desings. But when I compare them to rebreathers then they do not hold.... so is there a different design guide for rebreathers ambient pressure o rigns?

Teoman

Drmike · 9th August 2007, 01:14

Quote: (Originally Posted by teomannaskali)

Does anyone know the design parameters for those o rings?

What what compresion ratio and what gland size? I have found different sources for O ring desings. But when I compare them to rebreathers then they do not hold.... so is there a different design guide for rebreathers ambient pressure o rigns?

Teoman

I use standard 25% compression

Rebreathing Red · 9th August 2007, 01:43

I wouldn't use a planel meter... I my option that's just asking for touble as the slope of the milli volts to po2 curve is different for every sensor. If you don't have a two point calibration you don't know what your breathing. Here is a overlay of some sensors.

steevke · 9th August 2007, 08:04

Quote: (Originally Posted by Drmike)

I want to make some ppo2 displays. Housing is a piece of cake but what I know sod all about is the electronics side.

Mike,
lascar sells a cheap panel meter that is very well suited for ppo2 display.
The datasheet is found http://www.lascarelectronics.com/data/pdf/SP%20200.pdf

It is very easy to recalibrate, just add a pot between RO and RI to GND to change the reference voltage fed into the ADC. No need for soldering.

It is fed by 5V, so either use a dedicated power supply or a standard 6V battery + diode. Add a reed switch to switch on/off and your set.

HTH

steven

paulraymaekers · 9th August 2007, 10:46

Quote: (Originally Posted by Rebreathing Red)

I wouldn't use a planel meter... I my option that's just asking for touble as the slope of the milli volts to po2 curve is different for every sensor. If you don't have a two point calibration you don't know what your breathing. Here is a overlay of some sensors.

hm...
strange, lucky most of us (the 'single point calibration' users) survive diving rebreaters

and even if I calibrate in dry air, and check with pure oxigen (just as a test) I 'm always in the range 0.98 - 1.01

wan't be so bad after all :-)

regards
paul

ps I use AI and Teledyne cells

DanFountain · 9th August 2007, 16:39

Quote: (Originally Posted by Rebreathing Red)

I wouldn't use a planel meter... I my option that's just asking for touble as the slope of the milli volts to po2 curve is different for every sensor. If you don't have a two point calibration you don't know what your breathing. Here is a overlay of some sensors.

Red,

Your graph doesn't look quite right. If you extend each line to the 0 millivolt level, they all converge, but at what would appear to be a PO2 of -0.105, rather than at a PO2 of 0. It looks like the X-axis is set up for -0.105 to 1.885 PO2 rather than 0 to 1.99 PO2. Sensors are fuel cells that consume oxygen to produce an output; they should (and do!) have zero output at zero PO2.

-Dan

AD_ward9 · 9th August 2007, 17:36

Quote: (Originally Posted by DanFountain)

Sensors are fuel cells that consume oxygen to produce an output; they should (and do!) have zero output at zero PO2.

-Dan

Logical, but it is not quite true. Have a look at http://www.deeplife.co.uk/or_files/DV_O2_cell_study_070329.pdf. It includes a test of an O2 cell with pure CO2 pulsed into it. Pure He does the same sort of thing. There is a small offset with 0% O2.

However, it is fairly small, so using a panel meter which you can scale (or put the input in via a pot as a potential divider), involves a smaller error than one gets through most calibration processes for the second point on the curve: i.e. assume 0 is zero, calibrate in air for the second point, then just do a check the sensors do not ceiling below a PPO2 of 1.6

Alex

koputai · 9th August 2007, 23:09

In reality O2 sensors (Teledyne at least) are pretty bloody linear.
When testing my displays, I calibrate in 100% O2, check for 0.20 - 0.21 in air, then test for 0.02 in Heliox02 (as supplied by the gas company). With a single point calibration they are always spot on. Well, until the sensor actually dies from old age.

Cheers,
Jason.

Rebreathing Red · 10th August 2007, 16:24

I have nothing to gain or loss in this. I'm just sharing what I know.
So let's ignore my prevoius graph and do a little math.

A straight line can be defined by the equation y=mx+b
So if po2 0.0 = 0 millivolts and sensor A has the following po2 0.21 = 12 mv (newer sensor)
Sensor B p02 0.21 = 9 mv (older sensor)
since po2 0=0 then b=0 (no linear offset)

therefore y=mx

A
13=0.21m
m= 61.905

B
9=0.21m
m=42.857
The slope is different on sensor A and B

so looking at a po2 of 1.6 we get the following
Sensor A =61.905*1.6 = 99.048 mv
sensor B = 42.857*1.6= 68.571 mv

you can make your own conclusions

AD_ward9 · 10th August 2007, 16:28

I think you may have missed something, or I did not understand the simplicity of your last post

One gets the O2 sensor on the panel meter, and adjusts the pot (either the potential divider in front of the meter or the calibration pot on the meter), to show 0.21.

It then shows PPO2 for the whole of the scale.

This is y= mx.

The "+b" term can be ignored: "b" does exist but is small. It gives an error at PPO2=0.21, and is multiplied 5 times by the time PPO2=1. The error is less than 5%.

Cheers,

Alex

Quote: (Originally Posted by Rebreathing Red)

I have nothing to gain or loss in this. I'm just sharing what I know.
So let's ignore my prevoius graph and do a little math.

A straight line can be defined by the equation y=mx+b
So if po2 0.0 = 0 millivolts and sensor A has the following po2 0.21 = 12 mv (newer sensor)
Sensor B p02 0.21 = 9 mv (older sensor)
since po2 0=0 then b=0 (no linear offset)

therefore y=mx

A
13=0.21m
m= 61.905

B
9=0.21m
m=42.857
The slope is different on sensor A and B

so looking at a po2 of 1.6 we get the following
Sensor A =61.905*1.6 = 99.048 mv
sensor B = 42.857*1.6= 68.571 mv

you can make your own conclusions

8th August 2007, 23:17	#21 (permalink)
teomannaskali Custom Title Allowed! Current Rebreather/s: Home Build Other Rebreather/s: Not Bought Yet Home Build Join Date: Feb 2006 Location: Istanbul TR Posts: 350	Re: How to make a digital ppo2 display? Does anyone know the design parameters for those o rings? What what compresion ratio and what gland size? I have found different sources for O ring desings. But when I compare them to rebreathers then they do not hold.... so is there a different design guide for rebreathers ambient pressure o rigns? Teoman
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9th August 2007, 01:14	#22 (permalink)
Drmike Custom Title Allowed! Current Rebreather/s: MK 15.X Ouroboros Other CCR Home Build Other Rebreather/s: Inspiration Classic Other CCR Home Build Join Date: Feb 2005 Posts: 3,157	Re: How to make a digital ppo2 display? Quote: (Originally Posted by teomannaskali) Does anyone know the design parameters for those o rings? What what compresion ratio and what gland size? I have found different sources for O ring desings. But when I compare them to rebreathers then they do not hold.... so is there a different design guide for rebreathers ambient pressure o rigns? Teoman I use standard 25% compression __________________ Cave diving is a sport Wreck diving is a sport Diving in general is a sport 'Rebreather diving' is not a sport its the delusional obsession with a highly dangerous and often inappropriate piece of equipment
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9th August 2007, 08:04	#24 (permalink)
steevke Custom Title Allowed! Current Rebreather/s: Inspiration Vision Other Rebreather/s: Sport Kiss Join Date: Jul 2005 Location: Belgium Posts: 310	Re: How to make a digital ppo2 display? Quote: (Originally Posted by Drmike) I want to make some ppo2 displays. Housing is a piece of cake but what I know sod all about is the electronics side. Mike, lascar sells a cheap panel meter that is very well suited for ppo2 display. The datasheet is found http://www.lascarelectronics.com/data/pdf/SP%20200.pdf It is very easy to recalibrate, just add a pot between RO and RI to GND to change the reference voltage fed into the ADC. No need for soldering. It is fed by 5V, so either use a dedicated power supply or a standard 6V battery + diode. Add a reed switch to switch on/off and your set. HTH steven
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9th August 2007, 10:46	#25 (permalink)
paulraymaekers rEvo's daddy Current Rebreather/s: rEvo Other CCR Home Build Other Rebreather/s: rEvo Other CCR Home Build Join Date: May 2005 Location: belgium Posts: 1,437	Re: How to make a digital ppo2 display? Quote: (Originally Posted by Rebreathing Red) I wouldn't use a planel meter... I my option that's just asking for touble as the slope of the milli volts to po2 curve is different for every sensor. If you don't have a two point calibration you don't know what your breathing. Here is a overlay of some sensors. hm... strange, lucky most of us (the 'single point calibration' users) survive diving rebreaters and even if I calibrate in dry air, and check with pure oxigen (just as a test) I 'm always in the range 0.98 - 1.01 wan't be so bad after all :-) regards paul ps I use AI and Teledyne cells __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...
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9th August 2007, 16:39	#26 (permalink)
DanFountain Custom Title Allowed! Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Dec 2005 Location: Lake Superior Posts: 101	Re: How to make a digital ppo2 display? Quote: (Originally Posted by Rebreathing Red) I wouldn't use a planel meter... I my option that's just asking for touble as the slope of the milli volts to po2 curve is different for every sensor. If you don't have a two point calibration you don't know what your breathing. Here is a overlay of some sensors. Red, Your graph doesn't look quite right. If you extend each line to the 0 millivolt level, they all converge, but at what would appear to be a PO2 of -0.105, rather than at a PO2 of 0. It looks like the X-axis is set up for -0.105 to 1.885 PO2 rather than 0 to 1.99 PO2. Sensors are fuel cells that consume oxygen to produce an output; they should (and do!) have zero output at zero PO2. -Dan __________________ Try not. Do - or do not. There is no try. -Yoda
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9th August 2007, 17:36	#27 (permalink)
AD_ward9 So much more to learn Current Rebreather/s: Other CCR Other Rebreather/s: Other CCR Join Date: Jun 2005 Location: Scotland Posts: 1,732	Re: How to make a digital ppo2 display? Quote: (Originally Posted by DanFountain) Sensors are fuel cells that consume oxygen to produce an output; they should (and do!) have zero output at zero PO2. -Dan Logical, but it is not quite true. Have a look at http://www.deeplife.co.uk/or_files/DV_O2_cell_study_070329.pdf. It includes a test of an O2 cell with pure CO2 pulsed into it. Pure He does the same sort of thing. There is a small offset with 0% O2. However, it is fairly small, so using a panel meter which you can scale (or put the input in via a pot as a potential divider), involves a smaller error than one gets through most calibration processes for the second point on the curve: i.e. assume 0 is zero, calibrate in air for the second point, then just do a check the sensors do not ceiling below a PPO2 of 1.6 Alex Last edited by AD_ward9 : 10th August 2007 at 05:47.
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9th August 2007, 23:09	#28 (permalink)
koputai Subsea Systems Current Rebreather/s: MK 15.X Other CCR Other SCR Other Rebreather/s: Not Bought Yet Other CCR Other SCR Join Date: Jul 2005 Location: Sydney, Australia Posts: 326	Re: How to make a digital ppo2 display? In reality O2 sensors (Teledyne at least) are pretty bloody linear. When testing my displays, I calibrate in 100% O2, check for 0.20 - 0.21 in air, then test for 0.02 in Heliox02 (as supplied by the gas company). With a single point calibration they are always spot on. Well, until the sensor actually dies from old age. Cheers, Jason.
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10th August 2007, 16:28	#30 (permalink)
AD_ward9 So much more to learn Current Rebreather/s: Other CCR Other Rebreather/s: Other CCR Join Date: Jun 2005 Location: Scotland Posts: 1,732	Re: How to make a digital ppo2 display? I think you may have missed something, or I did not understand the simplicity of your last post One gets the O2 sensor on the panel meter, and adjusts the pot (either the potential divider in front of the meter or the calibration pot on the meter), to show 0.21. It then shows PPO2 for the whole of the scale. This is y= mx. The "+b" term can be ignored: "b" does exist but is small. It gives an error at PPO2=0.21, and is multiplied 5 times by the time PPO2=1. The error is less than 5%. Cheers, Alex Quote: (Originally Posted by Rebreathing Red) I have nothing to gain or loss in this. I'm just sharing what I know. So let's ignore my prevoius graph and do a little math. A straight line can be defined by the equation y=mx+b So if po2 0.0 = 0 millivolts and sensor A has the following po2 0.21 = 12 mv (newer sensor) Sensor B p02 0.21 = 9 mv (older sensor) since po2 0=0 then b=0 (no linear offset) therefore y=mx A 13=0.21m m= 61.905 B 9=0.21m m=42.857 The slope is different on sensor A and B so looking at a po2 of 1.6 we get the following Sensor A =61.9051.6 = 99.048 mv sensor B = 42.8571.6= 68.571 mv you can make your own conclusions Last edited by AD_ward9 : 10th August 2007 at 16:38.
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