Monitoring sewers 24/7 – what’s your experience?
Palmer-Bowlus flumes have a solid reputation. They've been used in LA for over six decades with very few issues. They work particularly well in round bottom channels. If that's your installation point, then it's a good choice.
Otherwise you can't go wrong with Parshall flumes. They're used just about everywhere and come in over 20 sizes so just about every flow rate you can imagine can be monitored. Throat sizes for the Parshall flumes go up to 50 feet.
As for the ultrasonic transducers, they're great for common level applications. You've got to watch the manufacturer's specs on your preferred product, however, because each one has a slightly different beam angle and you need a solid signal if you're going to accurately calculate the results you want to track.
Hopefully the other infrastructure is in place already for flow monitoring: on-site flow verification, process controls, level alarming, etc. Either flume option is good – most go with Parshall because there is a little more added flexibility. If it works for your installation, however, Palmer-Bowlus tend to have lower construction expenses and still does a great job.