, computers.
My Problem is, i don't know how many Oil, Water, ... im using in my system.
Then i have create a primitiv flowsensor. This Circuit show's you how many fluid you use per second, minute or hour (if you have a very slow flow ^^)
For better Display i use the nixie LED mod but you don't need it.
Screenshots
But now we look at the circuit.
Numbers and Areas
At the "blue" area the parameters for load und unload the storage tank.
The "red" area is a RS-Flip-Flop to start or stop measure.
The "green" area count the ingame ticks.
The "yellow" area calculate the ticks to realtime (seconds, minutes or hours)
The "white/gray" area calculate the used fluid over time X.
The "turquoise" area reset the tick-counter for a new measure.
And the last one converts the data. Thats not necessary, but looks good
With the parameters you can configure the fill-stop of the tank. I set it to 1000. If the tank is filled, the rs-flip-flop switch the state and set the input-pump off. The output-pump works now and the tick-counter starts with work. Now you use the fluid and its flow out. The white/gray area calculate the diff. fluid value (1000-value) and divide it with the ticks. Ingame you have 60 ticks per secound. Also, you must divide the ticks width 60 to get the used seconds. This makes the combinator "7".
Combinator 13 width constant combinator 10 get the "-1" value if the tank is filling.
At constant combinator 11 are all times saves width the signals "s =second; m=minutes; h=hours". If you edit the signal on combinator 12 you can change the calulation. But the combinator 11 is not necessary! You can take the value directly into the combinator 12.
OK, the wires are very messy. Also i have tryed to draw a diagram for you.
The Picture has a high resolution, click on it and you can zoom.
Diagram
If you have questions, write it and im trying to answer that 
Have fun and get big factorio
