Diver Daves' 02 Analysis

Electronic Oxygen Analysis (Surface Measurement)

Analyzing your mixes is an important part of rebreather diving. You can buy a good 02 analyzer for about $250, or you can cheap it out and make your own. I've made a few of these using Vince Harlow's book "The Oxygen Hackers Companion" which you can buy through through the mail. It has loads of other interesting 02 projects depicted as well, and you ought to probably buy a copy of it. Pat Duffy also sells the "El Cheapo 02 Analyzer" as a kit, and from what I'm told, it works just fine. If Pat would send me one, I'd put some nice pictures up here on the site (Yes Pat, that's a hint!). I'm just too cheap to buy one from him since I've already got three of the things here already.

Now, on to the good stuff:

This is the old tried-and-true standby, the Miniox. It's been a favorite of divers for years, as it is small, lightweight, and runs on internal battery power. The only problem with it is that it's expensive, running well over $200 without a sensor. I scrounged this one, and use it in the shop for gas analysis of incoming cylinders. I don't take it out on the boat, as I've built up the following system for that use.

Here are a couple of homebuilt 02 Analyzers. The larger one uses a nice calibration potentiometer and a separate on/off switch, and the little one uses a simple potentiometer with an internal on/off switch. Both cost under $25.00 to make, plus the cost of the sensor. The sensors are CAG-250 sensors, which provide a long life in air. When they finally die I'll probably replace them with Teledynes. These analyzers were build right out of the instructions in The Oxygen Hacker, and took no more than a half hour to build. Comparison to my Miniox shows them to be dead-on. In fact, I suspect that the LCD millivoltmeter, which is the core of any 02 analyzer, is identical to that used on the Miniox.

Controlling the flow from your gas supply across the sensor face can be done a number of ways. The easy way is to simply use a plastic bag to capture a sample of the gas. A better way is to use a flow-meter of some sort. You can buy these, or make them.

This is the "Elephants Trunk" which came with my Miniox. It allows a sensor to read directly from a SCUBA bottle. It works, but took a lot of space in my small Pelican box, and seemed crude. I figured I could do better, so here's what I did:

Here I have taken a small flow-meter that I purchased from McMaster-Carr and have fitted it with a BC hose fitting to supply it with gas from any regulator hooked to a cylinder, and at the bottom you can see that I have machined an adapter from Delrin to accept the sensor. Your system does not need to be as complex as this, although having the BC hose fitting on the flowmeter sure makes life easy.

Here's a better detail of the back of the flowmeter. The BC connector is at the top, and the delrin adapter that accepts the sensor is at the bottom. You can buy commercial adapters for the sensors, but I didn't have one handy and so I just turned one out on the lathe.

The flowmeter shown above worked great, but I wanted a simpler system for quick dive-boat testing of gas. I came up with this system, which uses a body (either brass or Delrin) with a small restricting orifice installed, a bore to accept a sensor, and a quick-connect to feed gas from a regulator. My first one is the brass one at the bottom, which accepts a standard BC inflator hose. The top one is made from Delrin, and is made to connect to the drysuit supply hose from my Viking suit. This Delrin one has proven to be strong and non-corrosive, and it works great. This allows me to analyze any gas that I can hook my utility regulator to, and the system has been very satisfactory. The orifice passes about 2 liters per minute when supplied with 140 PSI gas, which is perfect.

Here's the expedition box, which includes (2) 02 analyzers (both homebuilt), a ball-type flowmeter, the two smaller flow restricting orifice setups, (3) sensors, and a couple of cables. The whole box cost me about $125 plus the cost of the sensors, and that's a lot less than the cost of one Miniox. I'd never buy a commercial analyzer, and can recommend that anyone needing one simply build one. 02 analyzers are really just a millivoltmeter that reads the sensor voltage. The other circuitry just lets you calibrate the meter to show percent 02, and not just the raw millivolt reading of the cell. It's easy stuff, trust me. Just get a kit from Pat Duffy, or get the Oxygen Hackers Handbook.

One frequently overlooked use of an analyzer for the rebreather diver is to test your CCR sensors at higher PP02 than is possible on the surface. I've made up a jumper block that allows me to hook my Mark-15 sensors up to my analyzers, and then I toss the whole thing into the test-pot. Generally I blow down the pot with Nitrox to get a 1.6 PP02 reading at some reasonable depth. The analyzer, in that case will read "160%", which makes sense, since it's really just a PP02 meter with the decimal point in the wrong place. Here I've tossed a sensor into the pot and blown it down with air to 100 FSW. You can see the percent 02 is reading 98%, or really 0.98 PP02, which is about right. Sensors used in CCR's often fail at higher PP02 readings before they become bad on air, or even on pure 02 at the surface. I've found this to be an excellent way to keep track of the health of my rebreather sensors as they age. I make regular checks of the sensors before every dive trip, and keep a log of each sensor as it is tested. At the first sign of failure, I toss them. It's cheap insurance. My homebuilt analyzer has been in the pot here to 150 FSW literally hundreds of times, and shows no signs of distress. I'm not sure I'd have tried that with a $200+ Miniox!

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	This is the old tried-and-true standby, the Miniox. It's been a favorite of divers for years, as it is small, lightweight, and runs on internal battery power. The only problem with it is that it's expensive, running well over $200 without a sensor. I scrounged this one, and use it in the shop for gas analysis of incoming cylinders. I don't take it out on the boat, as I've built up the following system for that use.
	Here are a couple of homebuilt 02 Analyzers. The larger one uses a nice calibration potentiometer and a separate on/off switch, and the little one uses a simple potentiometer with an internal on/off switch. Both cost under $25.00 to make, plus the cost of the sensor. The sensors are CAG-250 sensors, which provide a long life in air. When they finally die I'll probably replace them with Teledynes. These analyzers were build right out of the instructions in The Oxygen Hacker, and took no more than a half hour to build. Comparison to my Miniox shows them to be dead-on. In fact, I suspect that the LCD millivoltmeter, which is the core of any 02 analyzer, is identical to that used on the Miniox.
	Controlling the flow from your gas supply across the sensor face can be done a number of ways. The easy way is to simply use a plastic bag to capture a sample of the gas. A better way is to use a flow-meter of some sort. You can buy these, or make them. This is the "Elephants Trunk" which came with my Miniox. It allows a sensor to read directly from a SCUBA bottle. It works, but took a lot of space in my small Pelican box, and seemed crude. I figured I could do better, so here's what I did:
	Here I have taken a small flow-meter that I purchased from McMaster-Carr and have fitted it with a BC hose fitting to supply it with gas from any regulator hooked to a cylinder, and at the bottom you can see that I have machined an adapter from Delrin to accept the sensor. Your system does not need to be as complex as this, although having the BC hose fitting on the flowmeter sure makes life easy.
	Here's a better detail of the back of the flowmeter. The BC connector is at the top, and the delrin adapter that accepts the sensor is at the bottom. You can buy commercial adapters for the sensors, but I didn't have one handy and so I just turned one out on the lathe.
	The flowmeter shown above worked great, but I wanted a simpler system for quick dive-boat testing of gas. I came up with this system, which uses a body (either brass or Delrin) with a small restricting orifice installed, a bore to accept a sensor, and a quick-connect to feed gas from a regulator. My first one is the brass one at the bottom, which accepts a standard BC inflator hose. The top one is made from Delrin, and is made to connect to the drysuit supply hose from my Viking suit. This Delrin one has proven to be strong and non-corrosive, and it works great. This allows me to analyze any gas that I can hook my utility regulator to, and the system has been very satisfactory. The orifice passes about 2 liters per minute when supplied with 140 PSI gas, which is perfect.
	Here's the expedition box, which includes (2) 02 analyzers (both homebuilt), a ball-type flowmeter, the two smaller flow restricting orifice setups, (3) sensors, and a couple of cables. The whole box cost me about $125 plus the cost of the sensors, and that's a lot less than the cost of one Miniox. I'd never buy a commercial analyzer, and can recommend that anyone needing one simply build one. 02 analyzers are really just a millivoltmeter that reads the sensor voltage. The other circuitry just lets you calibrate the meter to show percent 02, and not just the raw millivolt reading of the cell. It's easy stuff, trust me. Just get a kit from Pat Duffy, or get the Oxygen Hackers Handbook.
	One frequently overlooked use of an analyzer for the rebreather diver is to test your CCR sensors at higher PP02 than is possible on the surface. I've made up a jumper block that allows me to hook my Mark-15 sensors up to my analyzers, and then I toss the whole thing into the test-pot. Generally I blow down the pot with Nitrox to get a 1.6 PP02 reading at some reasonable depth. The analyzer, in that case will read "160%", which makes sense, since it's really just a PP02 meter with the decimal point in the wrong place. Here I've tossed a sensor into the pot and blown it down with air to 100 FSW. You can see the percent 02 is reading 98%, or really 0.98 PP02, which is about right. Sensors used in CCR's often fail at higher PP02 readings before they become bad on air, or even on pure 02 at the surface. I've found this to be an excellent way to keep track of the health of my rebreather sensors as they age. I make regular checks of the sensors before every dive trip, and keep a log of each sensor as it is tested. At the first sign of failure, I toss them. It's cheap insurance. My homebuilt analyzer has been in the pot here to 150 FSW literally hundreds of times, and shows no signs of distress. I'm not sure I'd have tried that with a $200+ Miniox!