Building a Better Mousetrap (O2 Analyzer)
So you want to build your own O2 analyzer. There are a lot of good reasons to do so, not the least of which is to save a few bucks. At a minimum you will save about $80 over the cheapest ready-to-use unit and you can save as much as almost $300.00. And if the truth be known, there is virtually no difference between any of them other than the label and the outer case.
Legal Disclaimer
While this project is not difficult to complete, it does require a certain understanding of the information contained herein. If, when you read through the article, you dont have the faintest clue as to what I am talking about or describing, I would suggest that you forget it or seek out appropriate help.
Failure to mix/analyze your gas properly could lead to your injury or death. You should never engage in diving activities that are past your comfort and/or level of understanding.
If you like to sue people for spilling hot coffee in your lap while driving, getting fat from eating fast food, or getting cancer after ignoring the surgeon generals warnings for 30 + years, please dont try any of what follows.
What is an O2 analyzer and what does it do?
So what is an O2 analyzer? It has 3 components: a sampling device, a sensor and a display. The sampling device regulates the flow of gas to be sampled over the sensor. The sensor generates electricity in small amounts (millivolts) that varies based on the partial pressure of O2 in the gas being sampled. The display lets you read the amount of electricity generated.
In its simplest form you can use the tank valve to regulate the gas flow, hold the sensor in the stream of gas and hook the sensor to a voltmeter to read the voltage. You then do some math to convert the voltage to a representation of the percent of oxygen in the gas.
The downside to this approach the potential for error. Since the sensor actually responds to partial pressure of O2 rather than the percentage, if you direct the flow straight at the sensor, you could, at increased pressure, get a reading that is higher than the O2 in the gas you are testing. If you tried to use a voltmeter with an analog rather than a digital display, it wont have the resolution necessary to get an accurate reading. Lastly, if you screw up the math you are potentially in big trouble.
The Project
A much better idea is to modify a small, inexpensive digital panel meter to display O2 % directly. Since the sensor is the most expensive part of the project at a cost of $60-100, and the remaining parts only cost $30-40, this makes for a good D-I-Y project.
Basically what we are going to do is remove a resistor on the panel meter board and replace it with a potentiometer or pot (a variable resistor). This will allow you to adjust the resistance to display the % of O2 directly instead of the voltage.
When I first made my unit, I followed the instructions in The Oxygen Hackers Companion exactly and used an inexpensive ($2.00) Radio Shack pot in conjunction with a fixed value resistor ($0.30). It worked fine but was very touchy. Barely look at the adjustment knob and the reading would change. I could have changed the value of the fixed resistor to change the sensitivity but the more you decreased the sensitivity, the smaller your range of adjustment.
After using it for a year and reading some more articles, I decided to upgrade the unit. In place of using the RS 1 turn pot (360 degree adjustment) which cost $2.00, I decided to go first cabin and get a wire wound, 10 turn (3600 degree) 10 K ohm pot ($16.00). Instead of the fixed resistor ($0.30) I used a 15 turn, 100 K trimmer pot ($3.00).
What these modifications did for me was convert my unit from one that could be adjusted when reading air from a reading of 18.6% to 22.8% in one turn of the dial (360 degrees) to a unit that spread that same range of adjustment over 10 turns (3600 degrees). This allows for a ¼ turn to change the display 0.1 %. MUCH nicer to use. In place of the fixed resistor, the trimmer potentiometer allows you to adjust the range, if necessary, as your sensor output changes with age.
How To Do It
Illustrations/Pictures
Schematic Pic 1
Panel Meter unmodified - Pic 2
Pots Pic 3
Panel Meter End Pic 4
Case (outside) - pic 5
Case (inside) pic 6
1) Acquire the necessary parts
Panel Meter
Circuit Test (http://www.circuittest.com/)
Panel Meter - LCD Digital Panel Meter with Bezel, DMD-3500B, $12.95
I chose this because it uses my initials (DMD) in its name just kidding. Actually I chose it because it was recommended in the 3rd edition of Oxygen Hackers Companion which is where I learned how to do this stuff. The instructions that follow are for modifying the DMD-3500B.
Note: There was a time when the 3500B was unavailable and it fell from favor and the PM28 was recommended in its place. The 3500B is again available and easier to modify than the PM 28 because the PM 28 now uses wave soldered surface mount technology which makes it more challenging to remove the resistor. The process for modifying the PM 28 is the same but the details differ. If you already have a unit based on the PM 28 (or any panel meter for that matter) you can read the range of adjustment you need and replace your potentiometer with a combination of a 10 turn wire wound pot and a 15 turn trimmer pot.
Sensor - $59.95-$79.95
SOURCES
Maxtec (http://www.maxtecinc.com/)
OxyCheq (http://oxycheq.com/main.html)
RC Dive Technology (http://www.oxygenanalyzer.com/sensors.htm)
Specific Parts - SOURCE - Digi-Key (www.digikey.com)
Potentiometer - 10 K OHM, Wirewound, 2 Watt, (part # 3540S-1-103-ND, $16.29)
Trimmer Pot 100 K OHM, ¾ Rectangular/Multiturn (15), Cermet/Industrial Sealed, 3009-104-ND, $2.72
Case Project box of your choice ($5.00-$10.00)
Other Parts SOURCE - Radio Shack
Rocker Switch - (get one that fits in a round hole like part # - 275-693 ($2.49)
9 volt battery connectors
Radar Detector Coiled Power Cord (part # - 270-032 ($6.95)
5.5O.D. X 2.1mm I.D. DC Power Jack (part # - 274-1563 ($1.99)
5.5O.D. X 2.1mm I.D. DC Power Plug (part # - 274-1569 ($1.99) (will vary based on sensor you use)
Control knobs of your choosing for ¼ shaft ($1.50-2.00 for 2)
Necessary Tools
15-40 watt soldering iron with sharp tip
electronics solder
small phillips screwdriver
18-22 gauge hookup wire
wire cutter
drill or drillpress with bits
coping saw, dremel or Roto-zip
file
Step 2 - Prepare the Case
Open the project box and mark on the inside the cutouts for the panel meter, the switch, the jack and the pot. If you are using a gray or tan case you can mark the case directly with a sharpie. If you are using a black case you can first put some masking tape inside and mark on that.
Once you have marked where you will install the components, use a hand drill or drill press to drill the holes for the switch, pot and jack. Then drill the corners and along the edges of the opening for the panel meter. You can then use a dremel with a cutoff wheel, a Roto-zip or a coping saw to cut the opening. It is best to cut the opening smaller and then file it to an exact fit. If you cut it too large there is little you can do except get a new case.
Step 3) Modifying the Panel Meter
Find the resistor at location R4 in the upper right of the panel board
(see picture)
Snip the resistor at the point indicated
(see picture)
Remove the 4 screws and lift board from the LCD display
Heat the underside of the 2 wire leads from the resistor one at a time and pull each out of the board.
Take 2 pieces, 6 long, hook up wire and solder them in place of the resistor. Leave the other end loose at this point. Later these ends will be soldered to the pot.
Take 2 pieces, 6 long, hook up wire and solder them to the input connections on the board. Leave the other end loose at this point. Later these ends will be soldered to jack.
Solder the two leads from the battery connector to the battery terminals on the board, red wire to the + and black wire to the -.
Step 4) Mounting the Components in the Case
Place the board back on the LCD display and attach with the 4 screws. Install the panel meter in the case.
Install the switch and the jack in the case.
Cut the red lead from the battery connector and solder the ends to the switch terminals.
Solder the two wires from the input connections to the two terminals on the jack.
The wire wound 10 turn pot has two contacts one on top of the other and a third that is off to the side. Solder one of the end leads of the trimmer pot to the bottom lead of the 10 turn put as shown. Use some double stick foam tape to secure the trimmer pot to the 10 turn pot. Solder the two wires coming from the R4 location on the board to the offset lead on the 10 turn pot and the other to the middle lead on the trimmer pot. You can cut off the remaining lead of the trimmer pot. Now mount the 10 turn pot in the case.
Step 5) Making the Sensor Cord
Cut off the cigarette lighter connector and solder the appropriate plug in its place. I am currently using the Maxtec 250 sensor which has a female jack 5.5mm O.D X 2.1mm I.D. Other sensors will vary. The best thing to do if you are not sure is buy the sensor and take it to your local Radio Shack. They have all the different connectors on wires tagged with their sizes. You just search until you find the size that fits, check what the tag says and buy that size.
Step 6) Calibrating the Analyzer
Hook up a battery & connect the sensor cord to the sensor and the analyzer and turn the switch to on. You should have some number displayed on the panel meter. Turn the 10 turn pot to the middle position (5 turns in from either end). Now adjust the trimmer pot with a fine screwdriver until you get a reading of 20.9 (which represents the O2 content of air). Close up the case and you are done with the analyzer!
Step 7) Regulating Gas Flow Across the Sensor
While the analyzer is ready to go, you still have to figure out how to regulate the gas flow across the sensor. There are a number of options from cute little flow meters with a little floating ball that can be adjusted to give a precise flow rate which is then directed into the sampling tube, to a fixed step-down regulator that plugs into a BC hose and gives about 2 liters per minute (see pic). The former can cost up to about $50 plus BC connector to adapt them. The latter cost about $25 and are much simpler to use. That is what I used initially, purchased from the sensor manufacturer Maxtec and it is pretty much foolproof. I have seen plans to make similar units for a couple of bucks.
Currently I use a unit made out of a couple of inches of ½ cpvc pipe, a T, an end cap, and a piece of hose. The diverter is glued into the top end of the T, and a piece of pipe, with multiple holes drilled into it, is glued into either end. An end cap is glued to one end and a piece of hose is forced on the other. To use the sensor is screwed into the T and the hose end is held firmly against the tank valve, which has been opened just enough to let out a slight hiss. It is important that the tank not be opened up so far that there is a pressure buildup in the sampler, otherwise the reading will be inaccurate. After using this system for a year, I find I get identical results to using one of the more accurate systems. For the beginner, one of the other systems might be best but this works well for me and is much quicker when one has multiple tanks to test.
Closing Thoughts
Carrying Case - You will probably want something in which to carry your analyzer. Pelican or similar waterproof cases fill the bill quite well. I use a UK case I got for about $12. Instead of foam, I put some industrial grade 2 wide velcro strips on the back of the analyzer and in the bottom of the case. It doesnt move and has room for the cord, sensor and sampler. I added some foam to hold the sensor, sampler and cord.
Sensor Life and Care - For longest life the sensor should be kept in an airtight container. I use a pill bottle but have seen others use ziplock bags and such. Also heat can damage the sensor so keep it cool, in the fridge in the pill bottle has worked for me. However, let it warm up before you use it. My sensor is 3 years old and has shown no signs of degradation. Maxtec suggests that the sensor be replaced when its output drops by 30% which means for a 10 mv sensor to drop to 7 mv. This can and should be checked at least yearly. As the sensor drops in output you can recalibrate the analyzer with the trimmer pot.
Battery Life - The drain on the battery is so low that the battery life should be the shelf life of the battery, 4-5 years. At any rate check the battery at least each year and carry a spare. Being in a rush to get my first unit working and not having an extra 9v battery around, I stuck in a used one which was on its last legs. When the battery gave up the ghost, it still gave a proper readout as to percent of O2, but the display dimmed. So put in a new battery, check it yearly and replace it every 2 or 3 years.
Modifying Your Existing Analyzer If you already have an analyzer but would like to make it more sensitive, take the box apart and measure the range of resistance of the pot and replace the pot with an appropriate 10 turn pot and multi turn trimmer pot that will provide the same resistance.
If you have any specific questions feel free to e-mail me at
dmdalton2000@yahoo.com
So you want to build your own O2 analyzer. There are a lot of good reasons to do so, not the least of which is to save a few bucks. At a minimum you will save about $80 over the cheapest ready-to-use unit and you can save as much as almost $300.00. And if the truth be known, there is virtually no difference between any of them other than the label and the outer case.
Legal Disclaimer
While this project is not difficult to complete, it does require a certain understanding of the information contained herein. If, when you read through the article, you dont have the faintest clue as to what I am talking about or describing, I would suggest that you forget it or seek out appropriate help.
Failure to mix/analyze your gas properly could lead to your injury or death. You should never engage in diving activities that are past your comfort and/or level of understanding.
If you like to sue people for spilling hot coffee in your lap while driving, getting fat from eating fast food, or getting cancer after ignoring the surgeon generals warnings for 30 + years, please dont try any of what follows.
What is an O2 analyzer and what does it do?
So what is an O2 analyzer? It has 3 components: a sampling device, a sensor and a display. The sampling device regulates the flow of gas to be sampled over the sensor. The sensor generates electricity in small amounts (millivolts) that varies based on the partial pressure of O2 in the gas being sampled. The display lets you read the amount of electricity generated.
In its simplest form you can use the tank valve to regulate the gas flow, hold the sensor in the stream of gas and hook the sensor to a voltmeter to read the voltage. You then do some math to convert the voltage to a representation of the percent of oxygen in the gas.
The downside to this approach the potential for error. Since the sensor actually responds to partial pressure of O2 rather than the percentage, if you direct the flow straight at the sensor, you could, at increased pressure, get a reading that is higher than the O2 in the gas you are testing. If you tried to use a voltmeter with an analog rather than a digital display, it wont have the resolution necessary to get an accurate reading. Lastly, if you screw up the math you are potentially in big trouble.
The Project
A much better idea is to modify a small, inexpensive digital panel meter to display O2 % directly. Since the sensor is the most expensive part of the project at a cost of $60-100, and the remaining parts only cost $30-40, this makes for a good D-I-Y project.
Basically what we are going to do is remove a resistor on the panel meter board and replace it with a potentiometer or pot (a variable resistor). This will allow you to adjust the resistance to display the % of O2 directly instead of the voltage.
When I first made my unit, I followed the instructions in The Oxygen Hackers Companion exactly and used an inexpensive ($2.00) Radio Shack pot in conjunction with a fixed value resistor ($0.30). It worked fine but was very touchy. Barely look at the adjustment knob and the reading would change. I could have changed the value of the fixed resistor to change the sensitivity but the more you decreased the sensitivity, the smaller your range of adjustment.
After using it for a year and reading some more articles, I decided to upgrade the unit. In place of using the RS 1 turn pot (360 degree adjustment) which cost $2.00, I decided to go first cabin and get a wire wound, 10 turn (3600 degree) 10 K ohm pot ($16.00). Instead of the fixed resistor ($0.30) I used a 15 turn, 100 K trimmer pot ($3.00).
What these modifications did for me was convert my unit from one that could be adjusted when reading air from a reading of 18.6% to 22.8% in one turn of the dial (360 degrees) to a unit that spread that same range of adjustment over 10 turns (3600 degrees). This allows for a ¼ turn to change the display 0.1 %. MUCH nicer to use. In place of the fixed resistor, the trimmer potentiometer allows you to adjust the range, if necessary, as your sensor output changes with age.
How To Do It
Illustrations/Pictures
Schematic Pic 1
Panel Meter unmodified - Pic 2
Pots Pic 3
Panel Meter End Pic 4
Case (outside) - pic 5
Case (inside) pic 6
1) Acquire the necessary parts
Panel Meter
Circuit Test (http://www.circuittest.com/)
Panel Meter - LCD Digital Panel Meter with Bezel, DMD-3500B, $12.95
I chose this because it uses my initials (DMD) in its name just kidding. Actually I chose it because it was recommended in the 3rd edition of Oxygen Hackers Companion which is where I learned how to do this stuff. The instructions that follow are for modifying the DMD-3500B.
Note: There was a time when the 3500B was unavailable and it fell from favor and the PM28 was recommended in its place. The 3500B is again available and easier to modify than the PM 28 because the PM 28 now uses wave soldered surface mount technology which makes it more challenging to remove the resistor. The process for modifying the PM 28 is the same but the details differ. If you already have a unit based on the PM 28 (or any panel meter for that matter) you can read the range of adjustment you need and replace your potentiometer with a combination of a 10 turn wire wound pot and a 15 turn trimmer pot.
Sensor - $59.95-$79.95
SOURCES
Maxtec (http://www.maxtecinc.com/)
OxyCheq (http://oxycheq.com/main.html)
RC Dive Technology (http://www.oxygenanalyzer.com/sensors.htm)
Specific Parts - SOURCE - Digi-Key (www.digikey.com)
Potentiometer - 10 K OHM, Wirewound, 2 Watt, (part # 3540S-1-103-ND, $16.29)
Trimmer Pot 100 K OHM, ¾ Rectangular/Multiturn (15), Cermet/Industrial Sealed, 3009-104-ND, $2.72
Case Project box of your choice ($5.00-$10.00)
Other Parts SOURCE - Radio Shack
Rocker Switch - (get one that fits in a round hole like part # - 275-693 ($2.49)
9 volt battery connectors
Radar Detector Coiled Power Cord (part # - 270-032 ($6.95)
5.5O.D. X 2.1mm I.D. DC Power Jack (part # - 274-1563 ($1.99)
5.5O.D. X 2.1mm I.D. DC Power Plug (part # - 274-1569 ($1.99) (will vary based on sensor you use)
Control knobs of your choosing for ¼ shaft ($1.50-2.00 for 2)
Necessary Tools
15-40 watt soldering iron with sharp tip
electronics solder
small phillips screwdriver
18-22 gauge hookup wire
wire cutter
drill or drillpress with bits
coping saw, dremel or Roto-zip
file
Step 2 - Prepare the Case
Open the project box and mark on the inside the cutouts for the panel meter, the switch, the jack and the pot. If you are using a gray or tan case you can mark the case directly with a sharpie. If you are using a black case you can first put some masking tape inside and mark on that.
Once you have marked where you will install the components, use a hand drill or drill press to drill the holes for the switch, pot and jack. Then drill the corners and along the edges of the opening for the panel meter. You can then use a dremel with a cutoff wheel, a Roto-zip or a coping saw to cut the opening. It is best to cut the opening smaller and then file it to an exact fit. If you cut it too large there is little you can do except get a new case.
Step 3) Modifying the Panel Meter
Find the resistor at location R4 in the upper right of the panel board
(see picture)
Snip the resistor at the point indicated
(see picture)
Remove the 4 screws and lift board from the LCD display
Heat the underside of the 2 wire leads from the resistor one at a time and pull each out of the board.
Take 2 pieces, 6 long, hook up wire and solder them in place of the resistor. Leave the other end loose at this point. Later these ends will be soldered to the pot.
Take 2 pieces, 6 long, hook up wire and solder them to the input connections on the board. Leave the other end loose at this point. Later these ends will be soldered to jack.
Solder the two leads from the battery connector to the battery terminals on the board, red wire to the + and black wire to the -.
Step 4) Mounting the Components in the Case
Place the board back on the LCD display and attach with the 4 screws. Install the panel meter in the case.
Install the switch and the jack in the case.
Cut the red lead from the battery connector and solder the ends to the switch terminals.
Solder the two wires from the input connections to the two terminals on the jack.
The wire wound 10 turn pot has two contacts one on top of the other and a third that is off to the side. Solder one of the end leads of the trimmer pot to the bottom lead of the 10 turn put as shown. Use some double stick foam tape to secure the trimmer pot to the 10 turn pot. Solder the two wires coming from the R4 location on the board to the offset lead on the 10 turn pot and the other to the middle lead on the trimmer pot. You can cut off the remaining lead of the trimmer pot. Now mount the 10 turn pot in the case.
Step 5) Making the Sensor Cord
Cut off the cigarette lighter connector and solder the appropriate plug in its place. I am currently using the Maxtec 250 sensor which has a female jack 5.5mm O.D X 2.1mm I.D. Other sensors will vary. The best thing to do if you are not sure is buy the sensor and take it to your local Radio Shack. They have all the different connectors on wires tagged with their sizes. You just search until you find the size that fits, check what the tag says and buy that size.
Step 6) Calibrating the Analyzer
Hook up a battery & connect the sensor cord to the sensor and the analyzer and turn the switch to on. You should have some number displayed on the panel meter. Turn the 10 turn pot to the middle position (5 turns in from either end). Now adjust the trimmer pot with a fine screwdriver until you get a reading of 20.9 (which represents the O2 content of air). Close up the case and you are done with the analyzer!
Step 7) Regulating Gas Flow Across the Sensor
While the analyzer is ready to go, you still have to figure out how to regulate the gas flow across the sensor. There are a number of options from cute little flow meters with a little floating ball that can be adjusted to give a precise flow rate which is then directed into the sampling tube, to a fixed step-down regulator that plugs into a BC hose and gives about 2 liters per minute (see pic). The former can cost up to about $50 plus BC connector to adapt them. The latter cost about $25 and are much simpler to use. That is what I used initially, purchased from the sensor manufacturer Maxtec and it is pretty much foolproof. I have seen plans to make similar units for a couple of bucks.
Currently I use a unit made out of a couple of inches of ½ cpvc pipe, a T, an end cap, and a piece of hose. The diverter is glued into the top end of the T, and a piece of pipe, with multiple holes drilled into it, is glued into either end. An end cap is glued to one end and a piece of hose is forced on the other. To use the sensor is screwed into the T and the hose end is held firmly against the tank valve, which has been opened just enough to let out a slight hiss. It is important that the tank not be opened up so far that there is a pressure buildup in the sampler, otherwise the reading will be inaccurate. After using this system for a year, I find I get identical results to using one of the more accurate systems. For the beginner, one of the other systems might be best but this works well for me and is much quicker when one has multiple tanks to test.
Closing Thoughts
Carrying Case - You will probably want something in which to carry your analyzer. Pelican or similar waterproof cases fill the bill quite well. I use a UK case I got for about $12. Instead of foam, I put some industrial grade 2 wide velcro strips on the back of the analyzer and in the bottom of the case. It doesnt move and has room for the cord, sensor and sampler. I added some foam to hold the sensor, sampler and cord.
Sensor Life and Care - For longest life the sensor should be kept in an airtight container. I use a pill bottle but have seen others use ziplock bags and such. Also heat can damage the sensor so keep it cool, in the fridge in the pill bottle has worked for me. However, let it warm up before you use it. My sensor is 3 years old and has shown no signs of degradation. Maxtec suggests that the sensor be replaced when its output drops by 30% which means for a 10 mv sensor to drop to 7 mv. This can and should be checked at least yearly. As the sensor drops in output you can recalibrate the analyzer with the trimmer pot.
Battery Life - The drain on the battery is so low that the battery life should be the shelf life of the battery, 4-5 years. At any rate check the battery at least each year and carry a spare. Being in a rush to get my first unit working and not having an extra 9v battery around, I stuck in a used one which was on its last legs. When the battery gave up the ghost, it still gave a proper readout as to percent of O2, but the display dimmed. So put in a new battery, check it yearly and replace it every 2 or 3 years.
Modifying Your Existing Analyzer If you already have an analyzer but would like to make it more sensitive, take the box apart and measure the range of resistance of the pot and replace the pot with an appropriate 10 turn pot and multi turn trimmer pot that will provide the same resistance.
If you have any specific questions feel free to e-mail me at
dmdalton2000@yahoo.com
