Good morning,
possibly following could solve your problem:
use a unit for constant cycle time (i.e. 100ms cycle time) and a fast counter.
The counter adds all pulses to the total volume. Store the counter value in a shift register which shifts every cycle (means, that every 200ms a value of the counter is stored). Sub the actual value from one of the outputs of the shift register (i.e. if you use the 5th output, then you have exactly 1 sec between the two values).
With arithmetic units you can convert the result to an applicable value (i.e. Ltr./min or gal./hr.).
For each batch you store the counter value at beginning and at end of the batch in a marker (the difference is the total volume of the batch or the difference of the start-value and the counter value is the actual volume before ending the batch).
Sorry for the bad english, but I hope you could understand the principle of my solution.
If not: Don't hesitate to ask specific questions.
Kindest regards
boppel