** calculation of mass-flow from volumic-flow **

Hello,

I have a serious calculation problem and I'd need some help or guidance.
I measure the consumption of a tractor with 2 volumic flowmeters, turbine
type.
I get a fuel consumption in l/h.
I have a balance (AVL balance) that gives me the exact mass-flow in kg/h.
How can I calculate the mass-flow from my measure volumic-flow, taking in
account the fuel caracteristics and the temperature conditions ?
At the end, I want to compare the calculated mass-flow with the one given by
the balance, and see if I'm close to it.

greetings,
thank you in advance, 

--------------------
Fred