Reading flow from sontex 531 wired to emontx

Hello all, this is my first topic and post in this forum,
I got ready setting up my new emonpi with emontx connected wirelessly,
I’m trying to monitor my heat pump using the emontx v3.4, I managed to connect the opencollector wire of the multical 531 (exit 17-18) to the emontx and I would loke to read the flow/h (m3/h) to do the math and calculating COP.
The pulse counter started to show input but I cannot figure out how to transform the pulses into flow/time unit! (L/min or l/h)
My sontex releases 26.5 pulses per Liter
Any help is greatly appreciated