Apply Teflon tape to the male end to prevent leaks. Another method is to drill a hole in the pipe that is slightly smaller than the diameter of the male threaded end of the flow meter. Because the PVC is softer than the metal end of the meter, when you apply pressure and screw it into the pipe it will self-tap. These installations usually leak. You can use this method with copper plumbing because although the copper is soft, it is less likely to strip when you screw in the flow meter.
Some meters fit loosely in the hole sealed with a gasket and are held in place with pipe clamps.
Flow meters are not always accurate and should be used as a reference in conjunction with other observations you make about a system.