Here is a gas blending system for those of you that don’t want to build a permanent station at home. This was my last winter project, when I made a movable continuous blending station for Trimix for my small Coltri compressor.
I want to use a simple rotameter-based system for blending because we have been using something similar for years in our diving club filling station. Based on experience, the rotameter is a very/sufficiently accurate system for mixing gas without continuous O2/He monitoring.
Sports Divers' Association of Kuopio (Finland) continuous blending filling station. It’s one of most professional filling stations that I know of personally.
Benefits for continuous blending
Design:
Firstly, we have to find the correct rotameters based on compressor flow. I use standard rotameters for air and based on my compressor, Coltri flow 80l/min.
It’s good to select the full scale when using the rotameter as it helps to make an accurate blending. For my case, after some studies and tests, I selected a scale for O2 rotameter to be 1-10l/min and He 0.7 to 7l/min. For example, 30% nitrox needs to add 5l/min O2 (notice gas density since my rotameters are designed for air, rotameter scale just numbers). I use a short static mixer tube, but the system also works without one.
For regulators to O2/He tanks, I use normal O2 welding regulators (at least what I ordered works well also for He).
I use SolidWorks for all designs and try to make the models look realistic as well. Good design saves a lot of time during the final assembly!
Testing:
For intermediate pressure, 3 bar is enough for short hoses and my small flow compressor. After the leak test, everything is ready for the first blend.
Now for the moment of truth… I first started nitrox blending so that I could see how the rotameter works. I made the first test using 1, 2 ,..5 l/m. I filled my 3l tank for a few minutes and tested the results, from which I also made a graph. For Trimix, I tested several different combinations and using the above data, I drew a figure on how the rotameter scale responds to real life.
My backyard filling station
From tank to tank... This is a cool feature…
I made an adapter for my old Mares first stage and connected it to the case inlet. Now I can move gas from the tank to the tank. It’s also possible to boost 100% helium or add He or O2 to old mixing… For example, if I have some old bail-out gas, now it is easy to move it to another tank and add some extra gas.
Project closing:
After the first test, I changed the O2 rotameter to a smaller scale. I also made some labels so that the device looks more professional.
Overall, I’m happy with the result. The blending setup is fast to build in the backyard and the system is very stable and accurate.
There is a small issue though, which I don’t yet know the root cause of. My old Coltri 2nd stage valve started to leak after the first test. After the service kit changes, everything works again. I've heard that it can happen by boosting pure helium above, but I only made 20/30. Anyway, the compressor works again, and it is serviced now for the first time since 2010. However my compressor flow is now better than before, when I made a filling graph, have to make new when I have any extra time and most probably update the helium rotameter to a step bigger
Note:
If you are interested in making a continuous blending system, you need to understand and accept the risks of O2. An advanced gas blender certificate is needed with rich experience. If you follow the blending starting and ending procedure and don’t try to make too high concentrated gas, this is an excellent system for gas blending boosting.
Arto
I want to use a simple rotameter-based system for blending because we have been using something similar for years in our diving club filling station. Based on experience, the rotameter is a very/sufficiently accurate system for mixing gas without continuous O2/He monitoring.
Sports Divers' Association of Kuopio (Finland) continuous blending filling station. It’s one of most professional filling stations that I know of personally.
Benefits for continuous blending
- It’s Fast
- All equipment can be put into one case
- Possibility to use all gas from tanks
- No continuous O2 or He motoring / wearing O2 sensors
- Possibility of moving gas from tank to tank
- Very cheap hardware (nitrox blending ~100 USD)
- O2 max ~40%
- Can be a little dangerous in some cases if pure oxygen gets into the oil-lubricated compressor
- Some compressor may dislike high concentrated % helium
Design:
Firstly, we have to find the correct rotameters based on compressor flow. I use standard rotameters for air and based on my compressor, Coltri flow 80l/min.
It’s good to select the full scale when using the rotameter as it helps to make an accurate blending. For my case, after some studies and tests, I selected a scale for O2 rotameter to be 1-10l/min and He 0.7 to 7l/min. For example, 30% nitrox needs to add 5l/min O2 (notice gas density since my rotameters are designed for air, rotameter scale just numbers). I use a short static mixer tube, but the system also works without one.
For regulators to O2/He tanks, I use normal O2 welding regulators (at least what I ordered works well also for He).
I use SolidWorks for all designs and try to make the models look realistic as well. Good design saves a lot of time during the final assembly!
Testing:
For intermediate pressure, 3 bar is enough for short hoses and my small flow compressor. After the leak test, everything is ready for the first blend.
Now for the moment of truth… I first started nitrox blending so that I could see how the rotameter works. I made the first test using 1, 2 ,..5 l/m. I filled my 3l tank for a few minutes and tested the results, from which I also made a graph. For Trimix, I tested several different combinations and using the above data, I drew a figure on how the rotameter scale responds to real life.
My backyard filling station
From tank to tank... This is a cool feature…
I made an adapter for my old Mares first stage and connected it to the case inlet. Now I can move gas from the tank to the tank. It’s also possible to boost 100% helium or add He or O2 to old mixing… For example, if I have some old bail-out gas, now it is easy to move it to another tank and add some extra gas.
Project closing:
After the first test, I changed the O2 rotameter to a smaller scale. I also made some labels so that the device looks more professional.
Overall, I’m happy with the result. The blending setup is fast to build in the backyard and the system is very stable and accurate.
There is a small issue though, which I don’t yet know the root cause of. My old Coltri 2nd stage valve started to leak after the first test. After the service kit changes, everything works again. I've heard that it can happen by boosting pure helium above, but I only made 20/30. Anyway, the compressor works again, and it is serviced now for the first time since 2010. However my compressor flow is now better than before, when I made a filling graph, have to make new when I have any extra time and most probably update the helium rotameter to a step bigger
Note:
If you are interested in making a continuous blending system, you need to understand and accept the risks of O2. An advanced gas blender certificate is needed with rich experience. If you follow the blending starting and ending procedure and don’t try to make too high concentrated gas, this is an excellent system for gas blending boosting.
Arto
