I am trying to design a probe to automatically detect diameter of the borewell pipe as well as length of the plastic casing inside the borewell.
I am using four proximity sensors arranged at right angles to measure the diameter of the PVC casing and using stepper motor to lower the probe inside the borewell.
Based on number of rotations by the stepper motor I can control and measure the depth of the PVC casing.
But I need to detect the PVC pipe to accurately measure the length of the PVC casing as the proximity sensors calculate the diameter even when there is no PVC casing.
Upon searching Google I figured plastic spectroscopy is used in detecting pladtics in recycling plants to segregate plastic from other waste. Can this be used to detect PVC pipe in the borewell? If yes can someone guide me through the process and if not then are there any alternative method.
Please help

 

Looking via NIR camera the wall is well transparent for hot or cold pipes, but not those having empty or stopped-flow.