Now if you have seen my last demonstration, (link here) you will see I used another bellows unit as my test lung then. I had no idea of what weight to place on the bellows, so I guessed and put some wrenches on them. Since then I have acquired a proper test lung. It is a Puritan-Bennett 0612. I have also mounted the ventilator on an old chair stand to make it easier to work on and move around with the heavy weights.
Once we started using a real test lung, we found out that the wrench on the bellows lid would not do for weight. One pound per square inch is about 27 inches of water. The Ziplock bag I was using was about 100 square inches so it would need 100 pounds on the bellows. I don’t think the poor bag would stand that very long. I needed a replacement that was tough and readily available. I settled on a 2 liter peritoneal dialysis bag. I tested this by standing on it, so I knew it would withstand at least 200 pounds without bursting. I then tried a destructive test by jumping on it. It still did not burst. Case closed. The uninflated surface area of this bag is about 50 square inches. I put about 30 pounds of weight on the bellows .
The manometer is constructed almost exactly to the original plan. I decided to go with a small diameter tube to keep the total compliance of the system low. The internal diameter of this tubing is 0.17 inches (3/16 nominal), this is about 44mm, and adds about 3 c.c.s of compliance at 20 inches of pressure. This is a negligible amount for a 500cc tidal volume (0.6%). This size of tubing (3/16ths) is about as small as you can go with water. If you go with a smaller tubing diameter you can end up with sections of air between water sections that are difficult to purge. This small diameter will not let me use my floating magnet trick for a sensor. I added food grade coloring to make it easier to see the level.
You can see it runs fairly well. As near as I can calculate, it is running a tidal volume of about 500 cc. I checked this by inflating the test lung to this size and placing it in water and comparing it to the pre inflated displacement. The rate is about 18 breaths per minute. This works out to a minute volume of about 9 liters. Not bad. I think I am getting close to a truly useful device for use in a pandemic.
If we wanted to change these parameters we could:
- widen the bellows settings to increase the tidal volume or
- narrow the bellows settings to decrease the tidal volume
- increase the weight on the bellows to reduce the inspiration time or
- decrease the weight on the bellows to increase the inspiration time
- increase the cycle time of the PLC to increase the expiration time or
- reduce the cycle time of the PLC to reduce the expiration time
Things I learned:
- The peritoneal dialysis bag will work OK. I am a lot less worried about reliability with this bag.
- The bellows should be built more sturdy.
- The manometer principle is valid for peak inspiratory pressure.
- I will have to think about the best way to handle the manometer level sensing. In order to sense the level accurately, I will need sensors for fluid in tubing or use conductive sensors.
- I will have to improve the sensing position adjusters. i may use mechanical switches instead of magnetic switches.
- I may try adding the maximum weight on the bellows for a safe maximum pressure and adjust the inspiration time with a manually operated valve that lets me control the flow rate out of the bellows.
- I need better regulation of the input air pressure.
- Once I get all the bugs worked out I will rebuild it to a more useful form factor.
- I will have to do a calibration of the system using other pressure sensing equipment, then I can mark the levels on the manometer.