The accurate measurement of flows in microfluidic channels is important for commercial and research applications. We compare the accuracy of flow measurement techniques over a wide range flows. Flow measurements made using holographic microparticle tracking velocimetry (μPTV) and a gravimetric flow standard over the range of 0.5100 nL/s agree within 0.25%, well within the uncertainty of the two flow systems. Two commercial thermal flow sensors were used as the intermediaries (transfer standards) between the two flow measurement systems. The gravimetric flow standard was used to calibrate the thermal flow sensors by measuring the rate of change of the mass of liquid in a beaker on a micro-balance as it fills. The holographic μPTV flow measurements were made in a rectangular channel and the flow was seeded with 1 μm diameter polystyrene spheres. The volumetric flow was calculated using the HagenPouiseille solution for a rectangular channel. The uncertainty of both flow measurement systems is given. For the gravimetric standard, relative uncertainty increased for decreasing flows due to surface tension forces between the pipette carrying the flow and the free surface of the liquid in the beaker. The uncertainty of the holographic μPTV measurements did not vary significantly over the measured flow range, and thus comparatively are especially useful at low flow velocities.
Experiments in Fluids