Calibrated with clear markings, the capillary viscometers support visual viscosity measurements of liquids. The direct or reverse flow styles have respectively positioned reservoirs to calculate the time it takes a specific fluid quantity to flow through the capillary. Since the driving force is gravity and cannot be varied, the standardized glass tubes produce more reliable results. Common Ostwald and Ubbelohde capillary viscometers are inexpensive alternatives to digital versions when determining material properties.