In this process you need to look at how fast the rate of flow can change. If you rate of change of flow can be faster than the sample rate you specified then the integration process could be inaccurate.

Secondly if you had enough memory to store all the sample values for some period of time then you could actually implement a moving sum process such that you can accumulate a legitimate value faster than at once per hour.

Lastly a comment,,,what is the resolution of the flow sensor. Is it enough that you could actually scale its value down and instead of using it at kg/hour maybe you could consider its value as (kg/60) / minute. This could have the effect of requiring the loop to just require 60 additions instead of 3600.

Michael Karas