After printing the new venturi tube with internal conical sections I have removed the support material and attempted to use it. There are some design issues which need attention. I made the pressure port pieces too small and have had to improvise with a couple of plastic M4 nuts glued on. To be honest that seems to work quite well so I might do that again as its easier that printing connection pieces.
I have guessed at the new internal dimensions within the firmware code as I am unsure of how the formula applies to the conical sections and I cannot find any information as to how to proceed. Rather than go through all the calculations again I have updated the firmware from my earlier post and performed a quick test.
For documentation purposes I have set the code with the internal dimensions:
A1 = 0.01455 m^3
A2 = 0.001145 m^3
Here are the results:
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| Raw Data from the ADC - Gain setting 1, positive = inhalation, negative = exhalation |
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| Differential Pressure - much improved sensitivity |
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The results seem to be much better than the last attempt. The graphs clearly show inhalation and exhalation and the sensitivity is much improved. I suspect that with a calibrated pump the accuracy could be improved further.
As I don't have a calibrated air pump my intention is to improvise with a balloon and a syringe body...
I have a 25 ml syringe body. I wish I had a bigger volume one but they are very expensive for some reason. I will fill a balloon with air from the syringe until I have a litre of air. I will then release the balloon air through the tube and monitor the results on the volumetric flow graph. If I get 1000 m/s I have a litre per second of air flow. I would settle for something close. Another method would be to obtain a calibrated air flow meter and use that to compare to what I have made...however I would still need a uniform volume of air to test and compare with.
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| The latest Venturi Tube design with face mask and tubing |
If we compare the results from the previous venturi tube (Mark 3) we can see that some considerable improvement has been achieved:
| Venturi Mark 3 | Venturi Mark 4 | |
| Peak Raw Data Bits | 850 | 2900 |
| Peak Differential Pressure | 103 Pa | 342 Pa |
| Peak Volumetric Flow | 0.06 m^3/s | 0.03 m^3/s |
| Peak Velocity of Flow | 7.1 m/s | 21 m/s |
The tube's sensitivity has definitely improved. We need to assess the volumetric flow measurement more closely hence the need for a calibrated air flow. Once we have that I believe it will be possible to use this design of tube for accurate measurement of air flow.
Once that has been sorted I need to add the BME280 Temperature, pressure and humidity sensor which arrived recently.
I have also just bought a 3.5 inch serial display...lets hope I can make it work well and it can be upscaled when necessary.
That's all for now - Langster!
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