Ask around your local drag racers and sports car racers as both SCCA and NHRA use ultrasound to measure tubing thicknesses and any track official @ any sanctioned NHRA strip can get you the name of the local outfit that does tech for local drag cars and/or sports car tech. You could also ask the guys @ the local chassis fabrication shops that build racing roll bars and chassis. The cost should be almost pocket change unless you've got a storage yard of tubes needing measuring. OR, you could resort to the old timey method and drill a 3/16th dia. hole in the tube, deburr the inside of hole and measure with calipers then weld and file.
_________________ Don't make me go get my flying monkeys-
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