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Most all the same considerations are needed. In both cases we have temerature, pressure, flow and volume.

I just did not know about this procedures you are mentioning and its really interesting to know about them. I'll make some time and check the links. Thanks.Quote

Re: The compressibility factor (Z) of air - the curve is similar to the deviations you have found. It would only take some simple mathematics or use values from a look up table to correct for this.

I did not know about that either and that was the reason for my question. On the other hand, neither the graph nor the compensation is required to find the volume. That's why I did not put much effort on solving that. Thanks for that info also. (another subject to study)Quote

Approximating 3 decimal digits might be enough for the display, but its how the numbers are determined that can influence the effective precision in the results. Certain equations can have a precision reducing effect.

That is absolutely true; but I'm not approximating until the results are send to the display. All calculations are done with the floats without any approximations. On the other hand, none of the results shown are used to calculate the volume with the only exception of the TC which is measured by Arduino and I did not placed any approximations in the Arduino code. In the Processing code, there aren't any approximations in calculating the volume either and its just a simple linear equation linking the TC and the volume as explained above. That's why such repeatability is obtained, which is way better than solving the exponential equation as there would be more measured variables involved, each of them contributing with errors to the results. If you try to solve the exponential equation with the values shown, there will be significant errors not corresponding to the volumes shown, because I did not do it that way. I realized that was a tough proposition. You can find the equation I used though, by simply plotting the Volumes vs. TCs values shown in Excel, (insert graph, add trend line-show equation in the options). I left the other values there because I wanted to calculate the Standard Deviations for them and observe "behavior" of those variables and also to help explain the method. Same as with the Graph; but none of that is really necessary. The screen is probably saturated with information an operator does not need and will just create confusion; but I need it for now, while I get this right.Quote

From your link to the sensor, its pressure range is 1 psi to 150 psi (60 mbar to 10 bar) (69 hPa to 10,342 hPa). The datasheet notes High resolution (min. 0.03 %FSS), so this represents 3.1 hPa resolution.

1psi-150psi is the possible Full Scale Span (FSS) of the sensors in that Series (SSC). In other words, in that particular Series you can find sensors with FSS inside that range; but there are many different ones in there with different FSS and not all of them are 150psi FSS. The FSS for the one I'm using is 250mbar (250MG in the part number) and I believe (could be wrong though) from the table in page 2, that its resolution is 0.03% of its FSS, in this case = 0.075 mbar and not 3.1 hPa.Quote

I'm not sure how you have pre-conditioned the signal levels into the 10-bit ADC. Perhaps resolution could be improved as follows. If ADC code 0 represents the lowest pressure you'll ever measure, say 40 hPa, and ADC code 1023 represents the highest pressure you'll ever measure, say 140 hpa, then the resolution becomes only 0.09765625 hPa per step. You may only need an op-amp or 2 to achieve this.

Perhaps there is another way. The sensor does not provide that output. It is a linear analog sensor providing a voltage output already amplified from certain value above 0V to a little less than 5V (I don't remember exactly; but its in the datasheet somewhere) therefore the range is not 1023 different steps in the ADC as it looks; but less. That output corresponds to Pr values from 0 mbar (referenced to the Atmosphere as it is a gauge sensor and not absolute) to its FSS in this case 250mbar. That's why I chose it, as it requires no additional electronics, just power and ready to go. On the other hand it is a high quality device (check the recommended applications in the datasheet)Cont...