Pressure drop in flow meters is the decrease in pressure that happens as the media passes through the flow meter. It is the difference in pressure that the line has before the meter inlet and after the meter outlet. Coriolis flow meters, especially those with dual tube measuring technology, have an inherent restriction in the design. The flow coming from the pipe into the meter inlet is split into two small tubes that have a significantly smaller pipe diameter than the process piping.
Because of this decrease in tube size, the pressure of the flow decreases while it is moving through the meter. Other flow meter technologies, like magnetic and ultrasonic flow meters do not have this sort of reduction/obstruction in their principles of operation and there is minimal flow restriction as the flow body of the meter is very close in diameter to the process piping. This explains why other technologies do not create as much of a drop in pressure.
This general principle is referred to as the CV value, which quantifies the fact that a smaller line requires more pressure to produce the same flow rate as a larger line.
Coriolis meters are known for having the potential for significant pressure drop in comparison with other flow meter technology types. This difference in flow tube size between the external piping and the internal meter tubes is why you will sometimes hear the suggestion being made that you need to “size up” for a Coriolis flow meter to decrease the pressure drop. A bigger size meter will have bigger flow tubes. Bigger tubes create less constriction. Less constriction creates less pressure drop. Less drop in pressure means higher flow rates downstream.
It is also worth noting that the higher the flow rate, the more pressure drop will occur across the meter. In essence, pressure drop is exponentially worse at higher flows than lower flows which is why you will see some data tables in flow meter datasheets that show the pressure drop in a graph and it appears as a curve and not a straight line in the graph.
Coriolis flow meters are also more susceptible to an increase in pressure drop when the liquid is more viscous in nature. High viscosity media decreases the ability for the liquid to move through the decreased pipe size. An analogy would be trying to blow honey through a straw as opposed to water. Which one can move through the straw with less force (pressure) applied to it? Obviously the less viscous one, the water. So, not only do you have to accommodate for pressure drop by the nature of the measurement principle, but you also must compensate for it in reference to the media as well.
Pressure drop matters because it affects the potential flow rate on the downstream side of the meter. With a significant enough pressure drop from the flow meter, there may not be enough pressure left to create adequate flow rates for the process.