Just to put a close on this thread, for anyone else trying to use the Mitsubishi Sika Vortex Flow Sensors as a pulse input for an Emon project. Push Pull, PNP and NPN didn’t work for me, and looking directly for a pulse on the pin didn’t work. @gck303 informs me that he put an oscilloscope on the pin, and also got nothing. So I think the the pulse pin (Pin 4; ‘Do not connect’) really doesn’t work.
Which leaves me with a voltage sensing solution- which for me will be the Emon Heat pump board…
At least, that’s one avenue closed. Thanks to Trystan, Brian and Robert for chipping in.