My solution is that I can measure the rpm of the Steam Turbine Generator on one of my power plants at about a 1 second interval. The steam turbine turns at 3600 RPM, synchronous with the grid. so I just divide by 60, and voila' real time grid frequency!
You did not mention to measure RPM of a turbine... Power plant ? What power plant ?
It is for a Power Generating Company ? Just confuse here... and curious.
My code is design for the included schematic, to measure the AC frequency of coming in at my place, like you see in my picture set-up. But, it may be possible to use the AC freq code. As long you have a good TTL signal, but it will be a different circuit.
To measure the RPM of a turbine, you need the proper circuit and setup to get a proper TTL signal, so the Arduino can use in the program. I did a frequency counter code, using the 1 second window to count the pulses. The accuracy of it is +/- 1 Hz. Look for the member Nick Gammon's code. He did a better one.