The mass flow meter method gives a more accurate addition rate. Using scales, the spiking rate is measured as the difference in two scale readings at two different times. This time interval will depend on the pumping rate and the weight on the scale. The spiking rate is then adjusted up or down depending on the target. In addition, if the pump rate is left on a constant setting, the spiking rate will slowly decrease as the back pressure due to the liquid level in the drum falls. Changing rates also presents a problem.
On the other hand, the mass flow meter takes instantaneous readings. It is connected to a computer, which not only saves the data, but also adjusts the pumping rate at 1-second intervals, so a constant (+ 1.0% maximum) pumping rate is achieved at all times. In addition the spiking rate can be changed accurately over a short time (<30 seconds) with no blind adjustments.
