^{6}Although we will measure time, and differentials of time, as positive quantities, the mass flowmeter should be
configured to show a negative flow rate (W) when propane flows from the tank to the building. This way, the integrand (the
product “inside” the integration symbol; W dt) will be a negative quantity, and thus the integral over a positive time interval
(from 0 to x) will likewise be a negative quantity.