Knowing your PO2 goes a long way towards making it safer to go deeper with an oxygen rebreather. If you want to go pure O2, it can be used to monitor how purged the loop is, and if you want to go a little deeper it can basically turn an O2 rig into a sort of mixed gas rebreather (or a full mixed gas rebreather with proper diluent addition).
In order to conveniently know my PO2, I have purchased O2 sensors. Having built in temperature compensation and having reliable manufacturing seems like a big plus vs fabricating, assembling, testing, QCing and calibrating my own.
For my first iteration I have started with just two cells. A third would be easy to add if this works out.
Layout and Logic
The electronics are going to be split into three parts- the cells/stuff in the counterlung, an electronics box, and a display. I decided that the only things in the counterlung should be the sensors themselves and a connector.
The “Electronics Box” will house the brains of the operation (an ESP32), and the battery. Batteries and other flammables will be kept outside of the oxygen rich environment of the rebreather, for obvious reasons. In the unlikely event of the battery shorting to the cells, hopefully the high impedance of the cells will limit resistive heating or fire. In the future, a USB port with a cap will be wired in for charging.
This box has been tested to ~80 FSW with just the cord grips+cord installed, and it passed without noticeable leaking. The cord grips are MSM-M SKINTOP connectors. They don’t seem like the should work, and yet they do. Mcmaster sells these as “submersible cord grips”. N.B. they make a face seal with the enclosure, and do not require a gland like an SAE o ring boss (ORB) fitting.
The main oring seal is a 1.5mm oring made from cord stock and superglued at the ends. You can see the join just above the middle heat set insert in this photo. Surprisingly this does not seem to create any significant leak paths, although there is always a slight possibility that I will have to eat my words on that someday.
The display will be upgraded to a HUD at some point, but for now it will be wrist mounted. It displays two PO2 cell readings, a compass heading, and (in the future) the depth. As you can see in the photo, the top row is “highlighted” to show a problem- the cells are disconnected and are reading a very high PO2.
Reading an off the shelf galvanic O2 cell is dead easy, since the temperature compensation and shunt resistor are built in. However, the output voltage is fairly low, and so it should not be fed directly into the ADC of a typical micro. It is possible to read such a voltage (~20mV), but it wastes a good portion of the resolution of the ADC.
For example, the maximum output expected is 2V (representing a PO2 of 2). With a 5v ADC, we are only ever using 2/3 of the range of the ADC, which effectively limits our resolution of PO2s to 2/3 the resolution of the ADC.
Since these signals are also not amplified or buffered in any way, it seems good to keep them away from the mcu. I have resolved to put them on an I2C DAC with an internal gain stage, which will let me both maximize resolution and keep the signal wires for the cells short. To this end, I used an ADS1015 breakout from adafruit.
Since it was on hand, I also threw in an LSM303 to use as an electronic compass. Since the compass has no “inertia” it has kind of jumpy readings, but doing some smoothing should help to get it to be a little less jittery. I could also try some compensation for nearby electronics, but they seem to have little effect. The LSM accelerometer/magnetometer lives in the wrist piece, although I did consider mounting it in the “head”, which would show you body heading, but not necessarily what you are looking at.
The display is the 128×64 OLED featherwing. Its easy to integrate, and it is fairly compact in terms of “extra space” for unused headers/buttons.
I headed to the mystical mystic lake to do some testing. The combination of near zero visibility to start with and a haze of sediment/algae/stuff I don’t want to think about made for a more-or-less night dive like conditions, even with a light, during the day. However, the little O2 cell reader and compass seemed to behave relatively well. Most importantly the firmware did not crash, and no water seemed to get in. Cant wait to test it somewhere actually fun!
In the last post I alluded to a larger pressure pot- the Science Sr. This was totally based off of the $50 Cell Checker from the wreckless diver, which is based off of a now defunct post on some other website. I’ll do my best to document what I have made here, since its an awesome tool. I’ll put a standard disclaimer on building one of these- don’t do it, it could blowup, etc, and it could really kill you, and it might hurt the entire time.
Why this could be a bad idea
The Science Sr. is based around an air filter canister. These are rated to 125 PSI…with water. they are 100% not for use with air as far as I can tell. Air, unlike water, is fairly compressible. That means if the amount of energy stored in pressurized air is huge compared to a hydraulic system at the same pressure. Filling this to 125 PSI might be fine, or it could explode due to some component being exposed to chemicals or because it has been cycled too many times or because it has been cycled too rapidly. Plastic will fatigue over time, and that will reduce the margin between exploding and not exploding.
Just like the wreckless diver, I have added an over pressure relief valve (OPV) to prevent going over ~45 PSI or so. This is not a guarantee that it will not go over 45 PSI- I have it hooked up to a scuba reg with an IP of ~120 PSI that can provide something like 100 SCFM (cubic feet/min of gas at some standard temp and pressure). That is fast enough to drain a typical AL80 scuba tank from 3000 PSI to 0 in under a minute. There is no guarantee that the OPV can keep up with this flow rate, so I am VERY careful operating the valve.
Also, in some kind of blue-moon case where my first stage reg goes haywire, its designed to fail open. This would be bad news because this will shoot tank pressure right out of all the low pressure hoses. In a dive situation this is great, because you have ~30s to breathe off a free flowing reg. In a pressure test situation this is bad, because up to 3000 psi (more for some tanks) will be shooting out of every reg and inflator hose, including the one stuck to the pressure tester, which will almost certainly explode it.
This was mitigated somewhat by testing with a mostly empty (500 PSI) tank, and by very careful control of the valve. For cell checking applications, it may be a good idea to fill this with water partially to reduce the amount of volume that is full of air, reducing the potential energy. Anyway, on to how to build it.
Science Sr. Construction
Here it is, in all its glory. On the top is basically a X manifold that houses all the important stuff. At 12 oclock is a NPT to BC hose adapter going into a ball valve. At 3 oclock is an 1/4-MPT to 1/8-FPT adapter and a 1/8 NPT pressure sensor. These are available on ebay and seem to work just fine. It seems like they are for some kind of automotive application based on the connectors, and they are available in a variety of ranges. At 6 oclock is a 1/4-MPT – 14-MPT adapter. This attaches the female pipe thread cross fitting to the female pipe thread of the canister head. At 9 oclock is a 1/4-MPT 45 PSI overpressure valve.
On the other end is a 1/4″ MPT- 1/8″ FPT adapter. The threads on the filter are one-time-use soft plastic NPTs, so I essentially “replaced” them with a metal 1/8″ npt. Screwed into this is a 1/8 MPT to hose barb adapter with a bunch of wires epoxied to it, creating an airtight pass through. I like the 60 second epoxies that have narrow mixing nozzles for this, because they cure FAST and the nozzle fits into the fitting well. However, they are expensive and usually only come with two nozzles, which is wasteful if you only use a few ml per fitting. Silicone seems to work ok for this, but its probably best to apply with some kind of syringe. While none of these fittings have shot out the epoxy/silicone plug yet, it is probably best to get as much plug in as possible, especially on the inside of the plug. This should create a step in the plug to make it much stronger.
It is important to install this on the side labeled IN, since there is a large pass through there, and the wires will be much easier to pull into the canister body (see the assembly picture above).
*I had these on hand but this mcmaster part should be equivalent
Testing and Results
After some initial testing, it seemed like I would need to return to the teflon tape as a membrane to get good sensor response. I suspect this will let electrolyte evaporate out over time (based on previous experience), but it does give a very satisfactory response time, and over short periods the sensor is serviceable enough. It was necessary to make the sensor up in the electrolyte to eliminate bubbles. This meant pouring the eletrolyte into a dish and then wearing gloves to assemble the sensor “underwater”. Excess KOH was washed off.
As a note- for this exact size of sensor, a ~510 ohm resistor seemed “right”. More discussion of that below.
Here is an example of the step response to pressure. With the ball valve, its a little nerve wracking to throw it fully open, and I wasnt keen on testing the opv, so the pressure step here actually has a rise time of ~250 ms. The rise time of the sensor is about the same- It does not seem like this step is fast enough to elicit any kind of delay in the sensor. The pressure here goes from 1 ATM to 1.5 ATM, and the sensor rises from ~220 mV to 325 mV, which is what we would expect.
The next test was to wrap the sensor up in a bag with the inflator hose, and to shoot O2 straight into it. This should give a value for 100% O2 at 1 ATA.
I wasnt sure how much O2 to squeeze in there or where it was really saturating, so there is a little bump in the middle as I jostled the sensor. But after spewing out a good amount of gas, it looks like the maximum value is 500mV. This is an interesting result and shows that the sensor is actually no good up to 100% O2. If the sensor were linear, the resulting voltage should be 5x atmospheric what it is in normal “air” which is ~20% O2. That would be 1V, which it does not achieve. I believe this means the cell is current limited.
Here is the other end of that test- me tearing the bag to introduce normal air back in. From the initial jostle at 10s, it looks like it takes about ~1 min for the O2 to go back to normal levels. This is not representative of sensor performance because the gas actually has to get agitated for the O2 to go anywhere. In other words, it does not purely measure sensor response, but also incorporates the gas diffusing/getting blown away.
This is a plot of pressure (Y) vs sensor value (X). To produce this, the chamber was cycled several times from 1 ATA (.5V) to 2 ATA (1V). Cursors represent the starting point, and where the sensor “should go” at 1 ATA. Doubling pressure should double sensor value from 190 mV t 380 mV, and that is almost exactly what the line shows. Doubling the pressure represents going from a PO2 of .2 to .4. The voltage is below the 500 mV that the sensor saturates at, so it comes out beautifully linear.
Since the sensor was cycled several times, we can detect some other interesting sensor characteristics, namely a small amount of hysteresis and non-linearity. The cursor generally tracks on the right side on the way “up” in PO2/pressure, an tracks on the left as PO2 drops. This means a sensor will read slightly differently after it has been exposed to higher pressure- it has “memory”. The maximum value of this is almost 50mV!
There is also straight up non-linearity, as the shape of the curve is not straight. However, this seems to be fairly small.
Here is another hysteresis example, with the rise and fall done as slowly as possible. This should give an idea of the minimum hysteresis.
Here is an example of a slow and a fast pressurization and depressurization. I couldn’t color the two differently, but it is easy to see that the rightmost trace (which was a pressure drop) looks quite different- it drops to 0 pressure then moves tot the left to drop the voltage.
Current Limit/Resistor Size
I mentioned above that I used a 510 ohm resistor as a current shunt, and that the cell is current limiting. O2 cells, while read out in mV, are actually current-producing devices. Typically this is measured by putting a shunt resistor across the cell, and measuring the voltage. With low resistances, this produces a small voltage drop which can be difficult to measure. Higher resistances produce higher voltages- but you can only go so high! This is because the cell can only tolerate so much voltage across the cell*, and can only generate so much current. Therefore it is a balancing act to find an appropriate resistance to use- too small and readings get jumpy, too big and the cell wont change value at all, since it will be current limited.
*I am admittedly fuzzy on this but you probably do not want to get close to the open circuit voltage of the cell, which is 1.2v
Building a galvanic O2 sensor is possible and actually fairly easy. I suspect even the polarigraphic sensor would have been fine if capped in electrolyte, and if it did not leak. The sensors can obviously be tuned to give good responses, as this is exactly the same way that commercial O2 sensors work. However, building a really good sensor that could be used for diving, where the sensor is monitoring life support, requires a lot of sensor characterization which is way beyond what I want to do. There’s a lot more to an O2 cell than just getting it to spit out a voltage proportional to the O2 concentration- for example:
shelf life/storage condition determination
repeatable assembly/manufacturing processes
Any one of these could take a month (or longer) to do, and they would require a lot of units. I will stick to commercial O2 cells for now (and in the future), but now that I have a cell checker/pressure pot it should be interesting to compare a “real” cell to home made!
After a maddening time with polarigraphic sensor, I decided I would try to build the galvanic flavor of oxygen sensor. After reading this tech tip from Oakton Instruments, it seemed pretty obvious that galvanic cells have big advantages. The main draw for me was that the output was easy to measure, eliminating the need for the fancy DMM. This would also simplify the electronics needed for reading polargraphic cell.
Electronics Comparison- Galvanic vs Polarographic
Here is what I think would be needed to read a polargraphic cell- a precision buck or LDO to bias the cell, with a feedback pin at the top of the cell. This eliminates the burden voltage of the shunt resistor that is fed into some kind of stack of op amps that then produce a voltage on the other side. This might not be so bad, and given that we have a 0 drop shunt resistor, we no longer need to worry about having a tiny burden voltage.
For a galvanic sensor, its pretty much as simple as it can be- a single resistor and a high impedance amplifier to match the voltage output range to the desired ADC.
The first galvanic sensor I made just replaced the silver electrode with a zinc electrode. Platinum or gold (or likely any noble metal) makes a good anode for this system. Zinc, in contrast to silver or platinum, is a very, very agreeable metal to machine. I can easily take a millimeter or more off at a 25mm diameter. The rod I got from rotometals appeared to be cast, although without any apparent porosity after ~2mm into the diameter. The one offputting thing is that zinc fumes are toxic, and the melting point is alarmingly low~ 400C. So all the operations were done with a lot of coolant, and the soldering to the electrodes was done very gently to prevent or minimize any zinc vapors.
As you can tell from me stating that there was a first sensor, there is also a second sensor. The first sensor seemed to have the same drift problem as the polarigraphic sensor, which makes me suspect that the root cause of both sensors drifting is electrolyte loss through the membrane or leaking at the press fit of the metal to the delrin. I also wanted to increase the area of the zinc so that the electrode and the volume of the electrolyte. More zinc will alleviate any concerns about using up the electrode, and more electrolyte will reduce the impact of loosing small amounts of fluid, or bubbles. This is because each bubble or amount of lost fluid will be small compared to the sensor, since it is bigger.
Much like the polargraphic sensor, it kind of works. It certainly can detect a change in the level of oxygen, but it does it in kind of a non linear way. For example, I would expect that if 20% air is ~300mV, pure O2 should be 5x that, or 1500mV. It is possible that the cell just cant generate that much current, and that I should try a smaller resistor, but I certainly have not verified that yet.
With the improved sensor body, the sensor was also a lot more stable. It dropped a few mv over a fw hours, and its hard to know if that was related to temperature, drift, or the actual O2 concentration in the room. However, this stability was achieved over ~30 minutes as the sensor reached equilibrium. Likely the O2 in the bubbles in the electrolyte needed to be used up first, as they are in direct contact with the electrolyte. I suspect that after that happened, the sensor reached equilibrium with gas diffusing across the membrane and stopped sensing O2 trapped in the sensor.
On the other hand, it does seem very sensitive. Breathing on the sensor produces a small dip, and there is a noticeable difference in value (~30mV) from when I sit right in front of it and breathe on it vs when I leave the room- this is mostly anecdotal but interesting.
The linearity is not very good, as you can see. This is a plot of the pressure transducer vs the sensed voltage. Its all over the place, but is vaguely the right shape. Ideally the sensor should trace a straight line here, but there may be some hysteresis that causes non-linearity.
Unfortunately, just like with the step response, the change here should be much bigger. This test pressurized the sensor from 1 bar to roughly 6 bar- the reading should be about 6x as big, but it only went up a few mv! So this is not that impressive, as it shows either a non-linear sensor or some kind of enormous DC offset.
The last issue seems to be that the sensor leaks somehow. It may be that water vapor is permeating the membrane, because when left overnight the sensor dried out. In a humid environment like a rebreather this may not be a issue, but for storage it certainly is. This answers a question I have had for a while- why are rebreather sensors so slow? They are rated to a rise time of 6s to get to 90% of the final value. This is much slower than any of the sensors that I have seen, and does not seem to be an inherent characteristic of the sensor. My suspicion is that a much thicker membrane is used on rebreather sensors to reduce electrolyte water loss.
This sensor seems a lot easier to use, but it seems like a lot of the issues I have noticed may be due to my membrane selection and leaking. I have parts on order for a larger pressure pot (the under $50 cell checker) to see if I can get the larger sensor to behave in a linear way with a polyethylene or FEP (or even teflon tape) membrane. I think this cell checker will be very useful for a number of other things like depth gauges/computers/ingress testing so I am excited to have it on hand. I will have to make an effort to keep my pressure pot electrolyte free this time!
Testing the oxygen sensor will need to be done over a wide range of temperatures and PPO2s, and the cheapest, easiest, safest way of doing this seems to be to not use pure O2 gas. Not only is O2 a somewhat “spicy” gas, but I don’t have a huge tank of it sitting around in my house.
Instead, I intend to increase the PPO2 by increasing the pressure of the air- this will also prove out that the sensor works “at depth”. While that might seem magical, its pretty easy to imagine- as gas density increases at higher pressures, there are just more oxygens per volume bouncing around. The odds that one of these O2s bounces off the sensor go up as the pressure increases, and it is linear with depth.
The Science Junior
With this in mind, I set about what I am calling the Science Junior, since it looks like a generic science widget from KSP. Basically its just a small pressure chamber with a window and some NPT ports, which can be used for various purposes:
Pressure port via schrader valve from bike pump
Sensor wire pass thru
Gas infeed (?)
For those interested in the construction, the O ring is a just superglued out of cord stock and the sensor wires are run through a 1/8-27 NPT hose barb and epoxied in place.
Designed for a maximum pressure of 150 PSI, the 1/2″ thick polycarb cover and 8x M3 bolts should be more than enough to keep things together. There is about 1lbf per PSI so at 150 lbf I didn’t bother with the math.
One thing I would do differently is to use something removable for the sensor wire infeed, probably something that would get dropped in through the front and get captured by a lip on the inside, as shown in the sketch.
Testing and Learning
For testing, I set the bias voltage and logged the pressure (using a pressure to voltage transducer) and current of the sensor simultaneously while varying the pressure, which controls the PPO2. When the pressure is plotted against the current, it should be roughly linear. The chart above shows the sensor working reasonably well- however there is an odd drop of in current after being pressurized which manifests as a non-linearity in the chart above.
If we turn the scatter plot into a line plot to represent it as a time series, it looks a lot like a typical plot of hysteresis, but that seems like a red herring.
Looking at the normalized time series of the test, we can see that the sensor seems initially very linear, but then the current drops off after being exposed to pressure.
Here is some data from another test- the same strange trend occurs.
Looking at several sets of data we can see some that are complete garbage (blue data, yellow data, orange data) while some seem highly linear (green, red). Not exactly a good look for a mission-critical sensor that is helping you make life support decisions. Imagine trying to drive the speed limit if your speedo didn’t work!
I strongly suspect that pressure is playing a role here. First, sometimes gas is trapped under the sensor membrane, which causes the sensor to always read high, and to actually drop in current as pressure is applied. This seems to happen as the gas contracts and the membrane basically vaccum seals to the cathode. While oxygen is now coming into contact with the cathode, there is no opportunity for it to interact with the electrolyte, and this causes the current to drop.
Another factor is that in order to get the membrane closer to the cathode, I have had to burp out some electrolyte manually. This probably causes a slight negative pressure on the sensor as the membrane tries to regain its shape- I suspect that this can pull in gas and cause the gas blocking problem.
To solve this, I tried reducing the gap between the top lip of the sensor and the cathode, and making the sensors up “underwater” in electrolyte. This did seem to help with bubble elimination, but I still had a maddening and slow loss of current over time- uA over minutes or hours. these sensors show that good linearity can be achieved, but this drift is unacceptable for rebreather applications.
Additionally, I suspect that the sensor may not be fully watertight. That’s no good, since there needs to be electrolyte in there or it wont work! Some of the slow DC drift that I see could be due to this, or to evaporation through the teflon tape. The volume is <<1ml so even a small amount of evaporation could have an effect. I may try to remedy this or I may try to make a galvanic sensor…it turns out all I need is a little zinc or lead.