I've been continuing to experiment with the pressure sensor from the previous post. I had a problem for a while where the pressure sensor results were varying by +/- 50mV which basically made the pressure sensor unusable for my purposes as I was looking for better than 5mV accuracy. I did some more troubleshooting and discovered that my power supply was causing the variance. I put an additional voltage regulator before the circuit and that brought the accuracy down to 1-2mV, which is much more usable.
I've also found another Freescale pressure sensor (MP3V5004G) that has about double the accuracy of the sensor I'm currently using (MPX5010). I'm going to try out that sensor in the near future to see if it improves my results.
Now that I'm getting decent results I'm going to build a prototype meter using an arduino and start experimenting with a bit of software filtering and averaging to see if I can get a more accurate result. Eventually I'll also attach an LCD display to the arduino and make a hand-held meter for measuring specific gravity.
Here are some of my latest experimental results with the MPX5010 and solutions of varying density:
hydrometer reading of solution -> sensor reading
(with h = 6 inches)
1.000 -> 882mV
1.028 -> 896 mV
1.062 -> 933mV
1.100 -> 961 mV
(with h = 8 inches)
1.000 -> 1.105V
1.028 -> 1.130V
1.062 -> 1.148V
1.100 -> 1.177V