this could explain the spike (air is hot at the end of inflation in the bottle) and when you stop it is cooling and the pressure decrease ...the slight difference between the deflation too (strong deflation at the begining causing a drop of the temperature ...)
Have you thought about differential methods or using acoustic resonance to probevolume (the elastic constant of a container of gas depends on the volume of gas).
Referring to the operation I can't find a convincing explanation for these 2 facts:- ...-The slight difference during the first half of deflation between the ideal exponential decay curve (pink) and the real pressure measured (top of the blue area). After that they are almost identical.
I do see some similarities when comparing this with the testing of diaphragm gas meters by using a bell prover
In both cases here, measuring temperature and pressure with high precision and accuracy is important.
However, I think there is a problem with precision ... it could be mathematical, poor matching of sensor range to ADC range, or perhaps higher ADC resolution is required.
Perhaps the compressibility factor (Z) of air is playing a role here.
I'm trying to measure the volume of the object not the gas. I have no idea about these tests you are mentioning or the diaphragm gas meters; but I will search as it seems to be very interesting. If you have a link to a website, please post it.
That I don't see very well; but I'll check your suggestions carefully.-I'm rounding the results just for display on the screen; but calculations are done without rounding with full floats before displaying the results. I'll check anyway, there maybe some cases where its not. I though approximating 3 decimal digits was enough for display for now.
The AD conversion I'm using which includes averaging among other things, is resolving more than the 1024 normal steps the Arduino ADC can resolve (using it). I've been working on achieving this for a while now (you can check some of my previous Projects). That creates the 0.05hPa steps in the 0-250mbar range which will otherwise won't be possible to achieve and constitute a very good observation from you which I haven't noticed.
As example, deflation starts by time 20 000 milliseconds after the inflation is stopped at 150hPa (that's what I set it for). Inflation is stopped by finding when the Pr surpasses that threshold and there is a small error involved there. Since time elapsed is the trigger, perhaps the resolution in the differentiation of time originated by the loop duration is creating the impression of the 0.25hPa.