1. Look at following article:
https://support.unitronics.com/index.php?/selfhelp/view-article/calculate-totalizing-or-integral
2. Expansion module you use IO-D16A3-RO16 is relatively "slow", as it ise serial type of communication with PLC.
But if you read sequential values at fixed time interval, let's say, 1 sec, it will work for you.
2. Other way is to use "Frequency Measurement" option:
This can give you estimated frequensy, proportional to flow, at selected time interval.
You can use MI or DW for both, High Speed Counter and for Frequency Measurement.