Ok, so i began testing the flowmeters to find out this:
When i fill the first 10 liter canister and pour it into another canister i noticed that the flow value in the beginning is different from one in the end. For example: if canister is full the flow rate is 100 l/h, after half of it is poured into the second canister the flow rate decreased to 90l/h for convenience.
So this is what i do:
1) I'll measure the flow rate when the first canister is full
2) Pour 5 liters out of it and put the hose back into the first canister and measure the flow rate for the water left inside it
3) to minimize the accuracy of the experiment I'll repeat it 5 times.

any opinion is welcome