Nitrox stick: Measuring after compressor

[estresao]

HI everyone,

I have already made my nitrox stick and I've been using it for a while and once I got used to everything my mixes are pretty accurate..... until nowadays.... In my country (Spain) a famous saying goes "if it works... don't touch it"...... but I did... I touched it.

Well, this was in the aim to get more accurate measurements to reduce mental adjustments. I've had constructed an O2 analyser using an ESP32 processor collecting values from two cells: in the future I want to mix trimix, but so far I'm measuring twice the nitrox to compare and fine tune.

I'm using a static mixer which diameter is 40mm, and I have to reduce de diameter to 20mm which is the inlet of my Bauer PE-100, which can do 100 litres per minute. To measure I'm using an used rebreather cell and a brand new Vandagraph O2 Sensor R-17VAN (which is the same my Vandagraph Tek-Ox uses so I'm expecting having very similar results)

I have not found yet a place where my cells (mainly my Vandagraph O2 Sensor R-17VAN) reads accurate values... sometimes I've got similar, but not good enough and I end up making mental calculus. My guessing is that gas flow is too much for the cell, which seems to "work with a flow rate between 100 millitres per minute to 2
litres per minute" (SIC from Vandagrash site)

I've tried in different ways:
1) mounting a 40mm PVC T after the mixer and locating the cell in the side branch.... including these variants:
1.a) Located in a PVC lid using a Flow Divertor (9711000)
1.b) same than before but with no flow divertor
1.c) attached one more PVC T and located the cell and flow divertor in the 2nd T side brand in vertical
2) Mounted 20mm PVC T after the reduction from 40mm mixer and right before the hose to the compressor.

My next tries are following options:
1) Setting a bypass from the 40mm section to the 20mm section to get some Venturi effect and locate in that bypass the O2 cell and a flow reducer
2) Same than 1 but connecting to the bypass to a Vandagraph Quick-Ox Sampler (Reference 9730210) which is the same my analyser uses


Those who made similar sticks, are you guys getting accurate values?

 

[estresao]

After running some test I still cannot get proper measurements from the cells located in my stick. I managed to add a bypass with venturi effect to control the flow crossing the O2 cell.... but even controlling that never got proper values.

I managed to use both, my home made analysed based on an ESP32 microcontroller and directly using my pocket analyser i both case using exactly the same O2 cell... while my pocket analyser provided lower values compared to when I measure directly in the tanks, the ESP32 based was even worse, so I'll get a commercial analyser to add to my stick.

I've got two questions for those who are using commercial analysers
- Are you getting proper values? (I know there would be around 0,5% because the humidity difference)
- Where and how have you installed it?
- Are you using a flow divertor? Like this one: Flow Divertor (9711000)

 

[clownfishsydney]

I could never get accurate results from the stick, so I just did some sampling of the final result in tanks and worked out a graph showing % Nitrox from various oxygen regulator psi. I can get to half a percent every time.

 

[Wookie]

What pressure are you feeding into your compressor? When I was in the nitrox making business, I sampled lp air, hp compressor suction and HP compressor discharge. The lp air was to be able to calibrate the analyzer.

We did not measure hp compressor suction when the hp compressor was running, as it was at 0 pressure or thereabouts and it’s impossible to obtain an accurate reading especially with pressure fluctuations. But if the compressor was running, take it from the compressor discharge.

 

[estresao]

What pressure are you feeding into your compressor? When I was in the nitrox making business, I sampled lp air, hp compressor suction and HP compressor discharge. The lp air was to be able to calibrate the analyzer.

We did not measure hp compressor suction when the hp compressor was running, as it was at 0 pressure or thereabouts and it’s impossible to obtain an accurate reading especially with pressure fluctuations. But if the compressor was running, take it from the compressor discharge.

My compressor is a small Bauer PE100-TW which do 100 L/m. The inlet of compressor's diameter is 20mm and the static mixer and stick inlet is 40mm.

I've tried both, setting the cell at the 40mm section and at the 20mm section and at some point was able to get some accurate reference value, I mean, not exactly the % in the tank but a reasonable value which required a little offset. After some fills you get used to that and know what value you want in the stick analyser to have the expected value in the tanks.

I know there will be always some differences due the humidity difference between my ambient air and the filtered (and dried) gas produced.

I'd like to have continuous gas sampling downstream, but my compressor only have one outlet. When filling double tanks I can sample in the other post of the manifold, but when filling single tanks I just cannot.

 

[estresao]

Still thinking....

Because the flow inside the stick is obtained by suction from the compressor, if I'm not wrong the O2 cell, wherever is located is going to be negative pressure, which is not good.....

But.... what if I take gas samples using, for example, a fish tank pump to feed a bypass where the cell is located and send back the gas to the stick?

 

[TravisD]

If you have much/any vacuum in your stick - or rather at the intake of the compressor, that's the problem that needs to be solved. It's not good for the compressor. But yeah, O2 sensors read PP02, not F02, so pressure matters, which is why you calibrate with the compressor running before adding the O2 into the stream.