Figuring out circuits for a Nitrox Analyzer

I'm quite interested in building my own nitrox analyzer. I can always purchase one- and in fact, I own one- and have a second one in pieces.

The reason I'm interested is that I'd like to do more than just have an analyzer- I'd like to understand it- and modify it to suit some of my needs. For example, if the battery powering the analyzer starts to go bad, then the analyzer will read incorrectly, so you might not know it. Or what about if you decide to use a different sensor? You'd have to get new analyzer circuitry, as the output of an oxygen analyzer (the little fuel cell) varies from manufacturer to manufacturer.

Fortunately, the overall concept is fairly easy- an oxygen sensor gives a tiny voltage when exposed to oxygen molecules. Just having a sensor and a voltmeter means you've got a rudimentary analyzer.




I can expose the sensor to air, take a reading, then expose it to a nitrox mix, take that reading, get the ratio between the readings, then multiply it by my start percentage of O2 (20.9). Sensors- fortunately- are linear, so this works. If my voltage reading is 10mV when I read air, and 20 when I read my nitrox mix, then my percentage of oxygen is (20/10)*20.9 = 41.8% O2.

But who wants to whip out the calculator every time- oh- the cost of those nitrox analyzers. Well, what if you could use the guts of a multimeter to do the calculation for you?

That's what most of these home-grown analyzers are. You can find plans for building your own in the book The Oxygen Hacker's Companion. I've also found some plans on the net- the ones I've been studying a lot because they go into detail about what to do, rather than just 'solder this wire'- is here. The heart of any multimeter is the ability to take an analog signal (usually milliVolts) and turn it into a display of decimal digits. Each panel is probably different, but it looks like all of them use a standard IC called an ICL7106. It can take a signal up to 200mV and display it on its display. It uses a voltage divider (fancy way to say resistors powered with a higher voltage) to create a reference voltage and has circuitry to convert that into the digits.

Here's the circuit of this reference voltage. It is important, because this is what we actually need to modify to reset it to our needs for our particular fuel cell. That being- we could just read the millivolts- but that doesn't help us- we want it to read in percentage of oxygen.


A voltage divider is a way to precisely take a stock voltage- 3V in this example, and output a lower voltage.

The way a voltage divider works is by simply following Kirchhoff's Law. The law states that all voltages in a closed loop must sum up to zero. So in our circuit above, we start off with a 3V on the 'right' that must have -3V of voltage drops on the 'left'. There are a couple of ways to figure out how this works- we know that V=IR. V3=IR2+ IR3 + IR4. Since the resistance values are known, I can solve for I and get each voltage drop. But, I don't need to know I at all. I know the voltage is going to drop proportionally according to the resistor associated with it. A big resistor means a big drop. Voltage at 36 is going to be proportional to the maximum of 3V. In other words, it's going to be a ratio of the drop at the big resistor compared to all the resistors.

V36=V* (R2 / (R2 + R3 + R4))

Putting in values (assuming the center for the variable resistor), we get V36 = 3000mV * (30k)/(30k + 909 + 100). V36=3000 * (30,000)/(31009). This ratio gives us 2902.38mV. That big resistor eats up a bunch- so what's the value going to be? Just subtract the start voltage, which turns out to be ~100mV. This is meant to 'center' the reading on the scale. If you notice, the variable resistor allows us to tune the reading slightly from 0-200 on the denominator so that if some of the resistor values aren't spot-on, we can factory calibrate the meter to read properly with a known 100mV source.

The circuit uses this as a 'reference' when deciding what the input voltage is. So, in the example circuit above, the voltage at (36) is 100mV. Which would be great for us if our oxygen sensors used this range and it matched up to percentage O2.

At this point, there are a couple of choices. We could modify the input to raise it to our decimal scale, or we can modify the output to change the display. And there's another consideration. We need to be able to adjust the sensor reading each day we want to test our mixes. Sensors as they age can creep up or down. Also, humidity and temperature can have an effect on the sensor as well, so we'll need to be able to calibrate, so we need a rheostat (which is a potentiometer used as a variable resistor).

How do we do it? We need for 100 to show up when there's 100% oxygen. All we do is find the range of our sensor for air, knowing that it needs to show 20.9 on the meter and make what the voltmeter thinks is 100mV be the max mV. We're going to move the range down (or up if there was a sensor that read >100mV on pure O2). Whatever sensor you use will have a specification for what it reads in air, or pure O2. We'll take that value to calculate what the reference value needs to be in the panel display.

Let's say our sensor reads 13mV on air, which is 20.9% oxygen. Since the panel reads 100 for 100mV, we want it to read 20.9 when it is at 13mV by changing what it thinks '100' should be. We just get this ratio, so 13/209 = 62mV.

Now using our formula above in the current divider, we need 20.9 on the meter to be associated with our input of 13mV, which means at 'reference', or 100, it needs to be 62mV. Let's assume R4 (the built-in adjustment rheostat) is centered at 100.

3000mV-Vref=V * (R2 / (R2 + R3 + R4))

• Vref is our reference voltage
• 3000mV is the input voltage
  
• R2 is our 'big' resistor which knocks the voltage down from 3V to where our readings live
• R3 is the value we're solving for
• R4 is the built-in adjustment on the panel (100)
  

3000 - Vref= 3000 * (30000/ ( 30000 + R3 + 100))
2938mV = 3000 * (30000/ ( 30100 + R3))
R3=(3000mV * 30000Ohms / 2938mV) - 30100Ohms
R3=533 Ohms

So, let's plug in our value at what our sensor will read in air and see what the output will be. The voltage that it reads is 13mV and it compares it to 62mv. 13/62 is the ratio with the '100' reference. So it is a percentage of 100....drumroll 20.9!

Now you may ask yourself- well, you just adjusted our sensor for air- what about if I only know what it reads at 100% O2? Not a problem- the ratio for our target mV is exactly the same. If we want it to read 100 at 100% O2, we need to know what the sensor puts out. Check your specifications, but most list it in air (and guarantee the linearity). So, we'll say that our sensor which reads 13 in air, will read 62mV (100/20.9 * 13). See a pattern? That's what we just fixed our sensor using 20.9 and 13mV.

Note that we can also change around our reference voltage by changing the R2 or 'big' resistor. We could have left the other resistors and calculated for R2 instead. In our case, our oxygen sensor puts out <100mv,> 100mV on some sensors, in that case, we'd shift it the other way and choose a larger R3/4/5 or smaller R2.

The above is the modification needed to allow the use of any oxygen sensor with a panel meter. Just by changing the value of the voltage divider, we reset the 'percentage of 100' that we want to use. When we calculate what 100 should be, our voltmeter now translates it into percentage of oxygen.

Now we know what to pick for R3 (or R2) to meet our needs. The next thing we need to decide is how to adjust the reading. R4 is a screw-adjusted rheostat which is used at the factory to set the voltage reference level (especially if the other resistors are a little out of spec). We could use this resistor, except that it's only accessible inside the panel's enclosure. The reason we need to set the reading is that the oxygen sensor tends to drift its base value over time. We need to reset it on air before we do other readings. We've got two choices- change the R3/4/5 resistor value by introducing another rheostat (R3/4/5..6), or change the voltage level of the measured voltage by putting a voltage divider on it.

Since a voltage divider can only lower a voltage, it is better that our adjustment rheostat be in the centering circuit, since a value can be chosen which allows it to be adjusted up or down, instead of just down in the case of the input voltage. Therefore (and as is shown in the oxy hacker book), the R3 resistor we've chosen should actually be two- a rheostat, and a fixed resistor. A 100 Ohm rheostat will give enough adjustment, and can be put in series with equivalent resistors, or a larger rheostat could be used, with a finer adjustment ability.

A Lethal Slayer of Fish

I've been fortunate to have a run of good weather, and accommodating friends which have allowed me a lot of diving lately. And either my luck is getting better- or my fish-finding skills are. I'm consistently bringing back big- and many- fish to the boat. To the point that I think I'm getting more invites because I've been generous sharing the fish.

I filled the boat's limit on grouper by myself recently- and managed to get the assorted other fish, sheepshead, triggers, and snapper to get the mixed-bag. Hogfish have eluded me, but I'm hoping to get another invite from Killer and Griller to fill that 'shelf' in the freezer.

I'm feeling a bit embarrassed with my run of luck lately, as I've been on the boat with a bunch of out-of-towners, who have been very complimentary of my fishing skills- to the point that one of them exclaimed I was a Lethal Slayer of Fish. A bit of hyperbole, but I'm very flattered.

The Compressor Saga begins

SCUBA diving has now surpassed racing as a hobby. And to feed the insanity and the engineer inside me, I've taken the plunge and gotten a Mako KA-7 compressor.

List of what needs to be done:

• Wire up 220V
• Set up 3500 and 4500 banks
• Build a mixing panel
• Set up O2 cascade/bank

Here's an attempt at what the fill station should look like

A racing report from the past

Billy, if you're out there, give me a shout (you wanker)...

From: Bill Brownsberger <bbrownsb@comsys-opg.com>
To: Motorcycle Roadracing Listserver <race@micapeak.com>
Date: Wed, 11 Mar 1998 09:21:20 -0800
Subject: A backmarker's view of the 200



Hey everyone!

I resubscribed to The List to tell you all what a horrible
time I had at Daytona this year! Let me start by saying that
the AMA just doesn't have the Average Joe in mind when they
run this event, which really pisses me off! Come on, this
is supposed to be the biggest race in the country, and if
it isn't open to everyone, regardless of their financial
backing, then something just isn't right.

My week started off well enough, after almost a year's
hiatus, I really felt like I was getting back into this
roadracing thing with the right attitude. Since I've sold
off all of the racebikes, I walked out to my garage to
peruse my possible mounts for the weekend:

'86 VFR 750 - nahh, I'd hate to scratch the paint while dueling
with Russell and Duhamel
'94 ATK 406 - nahh, I just rebuilt it, it needs more break-in
miles on the trails before I can flog it
'84 RZ 350 - nahh, it only runs with the choke on
'94 KDX 200 - this will have to do it, it'll give a little
away up on the banking, only making about 25hp,
but I should be able to make that up in the infield

Race prepping consisted of shaving the knobbies down bald,
and of course the obligatory safety wire. We're there.

The AMA officials gave me some weird looks as I headed out for
the first practice session, most of them told me that the
supercross practice didn't start until later in the week.
Idiots! The KDX is an ENDURO bike, far from a supercross or
even a motocross bike. The sheer ignorance of some people amazes
me. I saw Laura Hardy directing some bikes out onto the course, and
made a beeline for her. She must not have recognized me, because as
soon as she saw me, she looked the other way. Oh well.
Once past the officials and onto the track, I started
to really blow those cobwebs out, and quickly got up to speed.
My wife was my pit crew, and she would yell my laptimes through
a bullhorn as I went by. You can't imagine the sense of
accomplishment I had as my laptimes lowered. I started off
doing :58s, then :56s, and before I knew it, I was down around
the 4:50 mark. I'd always heard Russell talk of getting into
the "forties" and figured I wasn't far off, I just might have
something for these guys come Sunday. I did think it strange
that most everyone had at least 100mph on me on the banking,
and they even went by quickly in the infield, but I must be
making time somewhere. I was waiting for SOMEONE to come by who
I could race with, but I guess Zack Jones's truck must have broke
on the way to the track. A few of the guys on superstock bikes
stopped by my pit to congratulate me on doing so well in
practice, it seems they were really glad to have me around.
Everyone loves a winner I guess.

Rather than make the 45 minute drive home that evening, I
decided to sleep in my van that night, and hang with the superstock
crowd, I figured this would let them know that I'm just as
needy and deserving of a 200 entry as they all were. We
sat around, drank cheap beer and lamented the fact that none
of us had the money to buy big-rig bike haulers or those
fancy-pants slick tires that some of the top riders had,
but we knew deep in our hearts that Daytona is the place
for us, and you can keep all of those "support" races, we
ain't nobody's underwear! And NASB? Forget it, the 200 is
the ONLY race we need mess with, Go Big or Stay Home! Sam
Fleming and I did get into some interesting discussions about
what lines to take on a bike making less than 140hp... I was
surprised that Fred Fartzignewton didn't show up, but then I
remembered that those high-horse AMA elitists only want 'experts'
and pros in the race. I told them that 5 years of racing is more
than enough to make you an expert, but they still want that piece
of paper! I even called Fred and told him that he needs to come
and argue his point, but he declined, saying that he'd rather
not race against guys who have been on a racetrack before, especially
if he can't protest everyone who beats him... Andy Birko came by
all of the non-factory-guys' pits, and kicked sand in our faces,
telling us to get the hell out and "Stay the fuck off the track!"...
Other than that, he seemed a likeable guy, although his
190lbs looked pretty funny on a 125.

Well, to make a long story short, it turns out that I didn't
make the cut for the 200, and boy am I pissed! How can they
deny me the right to race? I know that I may have been a little
off the pace, but come on, I can't afford the expensive tires
or 3 brakes like those other guys! Just because I'm a little
low on funds doesn't mean I shouldn't be allowed to get out
there and mix it up a little! I know I would have impressed
some people in the race, I really come alive once the pressure
is on. I don't mean to toot my own horn, but I did win
my fair share of regional races ya know, so I know I've got
what it takes to run with these spoiled punks.

Next year...

Billy B.

PS: If that Scott Russell flicks me off 1 more time...

The Deathstick Tastes Blood!

Carpooled down to Harker's Island with Pete, aka Silent Killer on Friday but we got a really late start, not getting the boat out until noon. I wolfed down a couple of hotdogs from the gas station when we were gassing up- and ended up with chilidogs instead of my mustard/relish/slaw. The chili looked a bit 'experienced', but I was hungry.

The seas were a bit rough on the way out- and overcast. But the forecast called for no wind, and maybe a chance of showers. We were worried about vis because for the past week or so, there has been a layer of cold green water on the bottom. Silent Killer sneakily suggested I drop first when we were out at Big 10. The water looked beautiful on top. I started down and at about 40' started to get vertigo because I could see- but not the bottom. Sure enough- at around 60 it was like diving into a greenish cloud. I aborted because vis was all of 10 feet. I did a quick bounce on another spot and found structure and about 15' of vis, but nothing but juvenile fish.

We decided to head further out with the hope of better visibility. Good choice! SK drops on some great structure- but says the vis was just okay. He comes back with his biggest snapper of the year. He said he found a hole with three spiny lobster, so I grab my old gun and clip a lobster loop and bag to it. I back drop the spot and vis was amazing compared to the soup I was on earlier in the day. I manage a good gag, but the big fish were keeping well out of the way. Never did see the lobster.

Due to the generousity of Ken Jones of Deathstick Spearguns, and Spearfishing Planet, I was lucky enough to win a recent contest and had a beautiful custom-made Deathstick with me. I really wanted this first dive with it to be a successful one, but didn't have it rigged yet to be able to carry my plethora of crap with me when I was lobstering. Yes, I'm one of those divers who has everything. Trust me, it pays off.

SilentKiller drops on the submarine wreck out there. There are big grouper on it, guarded by bigger sandtigers but he came back up having only seen one grouper, and none of his buddies. It was more of a 'gotta dive the sub' event than a spearfishing one. He said the vis on the sub wasn't as good as the ledges we were on, so we head back to where we were. We marked a bunch of spots while I was waiting for my surface interval. My nitrox mix was light for the depths we were diving so I was getting close to deco on my drops. If anyone wants giant amberjack, come to NC. Some of these were as long as I was. I used to think they scared the fish away, but I'm of the mind now that it makes the big fish curious. About every dive, we found ourselves in the middle of a swarm of them.

Remember that chilidog from paragraph one? I do. As I puked it up in dead-calm seas, SilentKiller gives me some words of wisdom: never eat anything that's two-for-two-dollars while diving.

I dive again with my old gun (still hoping to get a lobster), and nail a snapper and struggle with getting him on the stringer, so I load up my 2nd shaft hoping some big fish will come take a look at the commotion. I pull back on the band and the gun slips off my hip. I figure it must have been pressing against my computer or something. I try again- and it happens again. Talk about timing- my old 3-piece gun is so worn that the trigger housing is flexing when I load up the 2nd band. Great. My new gun is on the boat. Of course, I spot some big grouper so I go up and tell SK where they are. He drops in and grabs a nice scamp, but the big boys stayed in the sand. We're not far from a spot where we spotted some huge grouper, and I still have half a tank from the early drop in bad vis.

I decide I need to give the deathstick a chance- it's just been on the boat looking pretty while I screw around with my old (and now broken) gun. I drop in on the spot, but don't see much. I'm hanging pretty far off the bottom, trying to save air and deco. A few big guys are out there, but I don't have the time to play hide and seek. I'm very bummed- my new gun is going to come back unblooded.

I go up really slowly because of the earlier close-to-deco. The vis is simply amazing at 20 feet of depth- I can see forever. I look around at the amberjack swimming below, enjoying the last dive of the day (and the first dive day in a month!). And I look again. Some of those amberjack look funny. THOSE AREN'T AMBERJACK! Screw it, I'm going back down. At about 50' I get close enough to the swarm of Cobia. There were at least 9 of them flying in formation. My gun was unloaded since I was coming up, and I struggle to get the band pulled back. I only get one band loaded, even though I'm going to be line shafting a long shot. One of them way down is a monster, and the one closest to me is legal, but dwarfed by the others. I split the difference and dive bomb on the next biggest (and closest one). I stretch way out, take careful aim as the cobia are starting to swim away. I purposefully didn't go for a head shot, but just behind it to hopefully get a solid hit. I aim, and the laser-accurate Deathstick delivers as promised. I see the shaft go through the fish enough for the flopper to stick, and I go straight up, not wanting a fight at this depth and with my computer bitching at me. I wrap the line around the gun a few times so I have a solid lever to pull on and look down and see what the cobia were after. A huge stingray was just sitting on the bottom and the cobes were just milling around above him waiting for the next buffet line to start up. I go up to the boat (still relatively slowly) and the cobia cooperates by swimming in circles below me.

If he'd swim up, there's a chance he would throw the shaft, so I'm happy with the situation. I make it to the boat and yell for SK that I have a fish. He's looking at me kind of weird because all he sees is my gun. I slowly reel the fish up to me, grab the spear and the Cobia goes apeshit. I'm holding the spear and the fish is kicking so hard that the 5/16th spring steel shaft is flexing back and forth. I stick the tip of the spear out of the water and yell for SilentKiller to help get the big boy in the boat. He grabs the gaff and hooks him and the fish calms down. He asks me to take the spear out before he pulls it in the boat. I was really hesitant because if he kicked off the gaff, I could still have a chance of grabbing him with the spear. I pull the spear out, Pete hauls the fish in, and what ensues, I can only describe as some kind of nautical cage fighting. I'm still in the water, and I just hear Pete cussing and the fish flailing around. It reminds me of that scene in A Christmas Story where the kid describes his dad as a furnace-fighter. After what ensued, the fish not having a spear with which to toss around was probably a very good idea. The scuffle finally stops by the time I get around to the ladder. As I climb up, Pete is sitting on the fish cooler, a good third of the fish sticking out of it. He says to me "sit on this thing so this fish will die!" There is blood all over the boat. I'm not sure of the battle that ensued after the fish went up, but I'm glad it is only the cobia's blood. Pete goes to grab a brush and start scrubbing down the boat and I'm getting my tank off when the fish goes nuts again, jumps out of the cooler and thrashes around the boat. Cobias are well known for causing damage when they come on the boat 'green'. Pete's yelling "stick something in it!"- so I grab my stringer, which has a sharp point on it and jab it in the side, and step on it to pin it down, then grab the knife off my BC and stab him in the head, hitting off-center the first thrust and then sinking it between the eyes. That did it- finally. Just in case, we leave the knife in a good while. The boat is filled with even more blood. I ask for a photo op to show off the new gun, but we only have Pete's camera-phone. No matter, as you can see how thrilled I was to take home my second-even, and biggest Cobia.

Spearfishermen- heck, fishermen in general consider Cobia to be "the best-eating fish in the ocean."

Pete was on a mission to get his own Cobia after I explained that there were a large group of them just milling around the ray. He grabs my tank and BC, which had about 720PSI in it so he can take a look. I turn on the pony bottle for him, so if he does get something, he's not pressed for air.

I'm hopeful he'll get a fish, and I idle around following his bubbles, but he comes up empty-handed. The ray- and cobia-escorts had moved on. I've seen giant rays before, but I really need to concentrate on looking 'up' to see if there are cobia waiting for a meal. Apparently, when rays swim along the bottom, they stir up all kinds of creatures in the sand that the cobia love to eat.

We spend a little while cleaning up, but we've got a long way to go to get home. Fortunately the seas were beautifully calm. But, it wasn't going to help- even though we were running flat out in ideal seas, the sun was going down faster than we were going in. We get to the difficult channel run to the Harker's Island boat ramp, and hit sand. Pete jumps out and it's knee deep. I have too much gear with me as usual- but my pair of dive lights come in quite handy to spot the channel markers as we slowly make our way back. It was a late night, getting home but getting the cobia made the trip!

Cornerspeeding

It's been a while, but I went back to the racetrack with Cornerspeed this year. I had an absolute blast with the students. I was a bit rusty myself, but by the end of the day I was back up to speed. However, the brake gremlins were still after me. Russell actually bought brand-new rotors for me to test out- and they helped with braking- but by the end of the day, the lever was coming back to the bar again. I don't know what else I need to do. My reputation for deep braking is ill-deserved, as it is because I can't get the bike slowed down!

Interstate Champion

In the highstakes game of interstate cash-only, probably illicit spearfishing tournament, it's anyone's guess who ends up on top.