Capacitive sensor torch height controls are primarily used for oxy-fuel cutting applications only, however if the majority of your cutting is on materials thinner than about 5mm, then you can use capacitive height control with plasma. With plasma, the capacitive sensor seems to get affected more by splatter, and most of the capacitive THC systems do not have the ability to control a separate pierce and cutting height. Hypertherm produces a capacitive sensor that could be used with plasma.. it is designed for industrial high production use, and is probably not the lowest cost solution, but would likely be the most reliable. Here is a link:
http://www.hypertherm.com/en/Product...sensor_ohc.jsp
Jim Colt