Compact UltraSonic FlowMeters

Yes. Pick up the phone and call the manufacturer. Tell whoever answers the phone that you have no clue about flow meters and ask them to help you gain some knowledge.

Or, go to their website and read the troubleshooting section.

Or, hook one up, run some water through it and see if it works.

Also order some more meters to last until you get someone to teach you how they work.

"These flowmeters failed so often that we are running out of stock."

After a statement like that, I really think what you should be asking is what is a good replacement for them that is far more reliable, or what is the probable cause of the failure and can we correct this. But you ask how to determine if they are toast. Flow meters are not low cost disposable items.

However, before asking any of the above, a lot more information would be necessary.

There are in-line ultrasonic flow meters and clamp-on ultrasonic flowmeters.

There are device failures and then there are faults like inability to get a reliable reading where the flowmeter is functioning, but it can produce a valid reading.

It isn't clear which of the above is the case here. But in any case, the technology for cooling water should be transit time, not doppler.

My experience shows that clamp-on transmit time ultrasonic flowmeters are subject to diminished signal strength due to process conditions - aeration, suspended solids or bubbles or pipe condition. Rust and scale on the inside of the pipe can seriously affect the ability of a clamp-on to get sufficient signal strength. Bubbles/aeration bounce the signal, which doppler needs, but freaks out transit time. Or the transfer of the signal from transducer to pipe is marginal - loose transducer, washed out transfer compound, or an insufficiently smooth pipe surface.

The manufacturer should be able to tell you how to test the transducers and the electronics to confirm their basic performance.

Why are they needed, Uncle? What are they used for? Flowrates, yes, but why?