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).
Here is a shopping list:
- 45 PSI OPV, mcmaster 48435K72
- Ball valve, mcmaster 4912K82
- HP brass pipe fitting, mcmaster 50785K233
- 2x NPT reducers, mcmaster 50785K61*
- 1x 1/8 npt to barb, mcmaster 5117K96*
- DGX bc hose to NPT
- Pentek 158117 (amazon or other)
*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:
- Temperature compensation
- 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!