Ultrasonic Distance Sensor

Requirements

• 1mm accuracy optimal, 3 mm is absolute max
• Open source version of http://www.cnccontrols.com/thcontrol.asp - 20-250 mm range for pointing to metal, with no obstructions

Selection

• Maxbotics - 1 mm resolution options, not water protected - 12" minimum distance - $40 - [1]. Adafruit has it.
• Microptik - 3 mm resolution - http://www.micropik.com/PDF/HCSR04.pdf - 2-400 cm range - services@elecfreaks.com. Sample Arduino sketch - [2]
• HC-SR04 - appears to get 3 mm accuracy - [3]

Comparisons

• http://www.robotshop.com/en/compare-infrared-ultrasonic-distance-sensors.html

Sourcing

• 4-30 cm - $99 - [4]
• 1-100 inch - $25 - [5]
• $4 Sparkfun - 6"-10' - [6]
• $6 - .5 cm accuracy - [7]