A true flow switch is best. A pinched line can fool a pressure switch. Placement of the switch can help. If you put it after any place that might pinch you are OK, but if you are too far down the line there may not be a lot of pressure to measure. Some flow switches have a low side and high side, so you measure the pressure across your laser. A blocked line would not show any pressure across the laser. This can get a little complicated to hook up though.
A flow switch right after the laser is best. Many of the Chinese lasers use a cheap flow sensor. It is basically a magnet in a plastic cylinder. The flow causes the magnet to move to a reed switch. I can get these in quantity for about $10, but I would need to order a bunch of them and they are not adjustable, so you need to know the exact flow. Some have springs and some relay on gravity to return the magnet when flow drops.
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