Hippy, before I consider your post #73 code, let me express a substantial doubt: when I record the least significant byte (bit 0 to 7) of the output, it swings from 10 to 200 without moving. We can apply any smoothing, average or other tools but it will not be possible to have a meaningful data from that byte.
The least significant byte is the noise the linked article talks about. We can make that meaningful, apply smoothing, but only by averaging all 24-bits. That's a pain to do on a PICAXE.
But that noise is like someone with very shaky hands manually adjusting a watch which doesn't tick to keep up with what the actual time is; the seconds hand will go back and forth and all over the place even if they are keeping hour and minute time correctly.
But, by removing the seconds hand we won't see that noise, even though it's still there. There might be a little noise in the minutes every now and again but not that much. That's what we are achieving by truncating to 16-bits. It therefore doesn't matter that there is noise.
Another analogy is an audio amp wound-up to full, maximum resolution, "11" as the joke goes. There will be lots of hiss. Turn the volume down a little and there will be less hiss.
By making the volume lower, reducing the range of volume, we effectively average out the noise. We are reducing 24-bits, actually 23-bits because of our subtract $300000 trick, to 16-bits. The pressure gets compressed, but so does the noise -
The only consequence of doing it this way is that we no longer have altitude resolution of +/-2mm, only +/-250mm, and that's not so bad when we don't actually need to have +/-2mm for the end application.
My conclusion is that something is wrong in reading the pressure raw data from the sensor.
Can you have a new look to the data sheet to ensure that the ADC reading collect the right sensor raw data?
It looks right to me; the numbers match what I think we would be expecting to see.
The best way to confirm it would be to get some readings and look at the results. At ground level, up at 1 metre, up at 10 metres, at 20 metres; the more the merrier, and the higher the better.
If the drop in readings going from ground level to 10 metres is somewhat the same as the drop going from 10 metres to 20 then that would seem to confirm it.
Knowing what those readings were and at what altitude would also make it easier to determine how to convert pressure to an actual altitude output.
As regards the datasheet; D1 is pressure, D2 is temperature, the ADC command we use is $48, which means read D1 (pressure) with 4096 oversampling. That all checks out.
