Hey Dan,
I'm mostly just guessing here, but I assume you are measuring how much fluid is going through a 'flowmeter'.
Somehow, somewhere, there must be a device which the PLC can query to get what the current flow is. Saffa would (I assume) be asking what sort of device is being used to get the flowrate.
The flowrate multiplied by the time should give one the amount of fluid which passed in that time - assuming the flow is constant and the pump was not shut off.
One could, I assume, write an interrupt routine that is triggered by the real time clock, every, say, ten seconds and in the interrupt routine if the pump has been running continuous read the flow rate, multiply it by ten seconds and add the amount of fluid passed to a memory location for storing the amount of fluid passed. Then every hour or every day take that number and pass it on as the amount passed per hour or day - resetting the memory location as necessary.
Poet.