RE: Ultra sonic sensor calibration - the speed of sound is proportional to the square root of the temperature in Kelvin. You will need to take that into account.
RE: Measuring pressure cheaply - weld a disk on the end of a pipe. Seal the pipe if you want the air that is in it to be your reference. Glue a strain gage to the disk. The strain will change with the chaging air pressure.
1. Water pressure. Actually differential h2o pressure with absolute pressure of 40 to 60 psi would be great.
2. Air pressure. Absolute air pressure between 30 - 50 psi.
Having a tough time finding either of these sensor that are afforable. Say around 5 bucks ... less would be better. I've found some in the 35 to 100's of dollars. I'd like to have digital output for input to an Arduino.
There much be a cheap way to measure pressure. Medical devices are very low pressures but I would think the auto industry would have devices in the ranges above. I just can't find them.
@David: ....but this xufeifei is posting stuff all over EET...
Bummer -- I'll see what I can do about that.
Mom is great -- I'm at my brother's house -- she will be round here in about 30 minutes -- we'll take a stroll to the shops at the bvottom of the road -- tomorrow I'm going wo her writing club with her -- it's great being over here and not working 12 hours a day :-)
@Crusty: Why not take a leaf out of the radar technology and have a spinning ultrasonic dector turret on the robot. This could be turned by a stepper motor, so you would know where it was pointing relevant to the robots fixed chassis...
That's certainly a possibility -- but in the fullness of time I hope to have my Pixy machine vision sensor mounted on top of the robot ... I'll think about it...