Flow Rate Sensor Recommendation?

[MedicineMan]

I am planning on plumbing a UV sterilizer off of (not through) the return line from my sump. The flow rate in the return line is much greater than what is optimal for the sterilizer, so I will install gate valves to control the flow rate of the water coming into the sterilizer. However, I would like to be able to accurately measure the incoming flow rate to the sterilizer to make sure it's optimal. I have been trying to find a flow rate sensor that is well constructed, accurate, and reasonably priced (a relative thing, I know).

I don't have a controller system, nor do I plan on getting one, so I'm not looking for anything connected to a controller. Does anyone out there have any experience with flow rate sensors that are accurate and well built?

Thanks for any and all input!