Capillary Viscometers: Precise Measurement of Liquid Viscosity

A capillary viscometer is an instrument used to measure the viscosity, or thickness, of a liquid by measuring how long it takes the liquid to flow through a small-diameter tube, or capillary. The flow, or efflux, time is directly proportional to the liquid's kinematic viscosity, and may be converted directly to viscosity by use of a conversion factor unique to each instrument. Viscosity is generally temperature dependent, so the capillary viscometer is usually used in a controlled-temperature water bath set to a specific temperature.

Viscosity may be thought of as the internal friction of a liquid, or its tendency to resist flowing. Viscosity is thus an important property of fluids. It is of critical concern in lubricants, paints, beverages, and in any case where a liquid must be transferred, stirred, or manipulated.

A capillary viscometer may take any of several designs, but most common is the U-shaped or Ostwald viscometer, of which the Cannon-Fenske and Ubbelohde types are typical. A Cannon-Fenske capillary viscometer is a U-shaped piece of glass tubing bearing two glass bulbs or chambers on one arm, separated by a calibrated length of capillary tubing. Another bulb is low on the other arm, to which the sample is charged. The sample and the capillary viscometer are then suspended in a fixed-temperature water bath and allowed to come to thermal equilibrium.

Once thermal equilibrium is reached, the sample is drawn up into the upper chamber, and the test begins. The test sample is allowed to flow from the upper chamber to the lower through the capillary, and the efflux time, or time it takes to traverse the length of the capillary, is measured. Capillary viscometers come with a manufacturer-supplied conversion factor which allows calculation of the kinematic viscosity directly from the efflux time.

For high-viscosity liquids, solutions of these substances in suitable solvent may be used and measurements of the viscosities of the solutions at various concentrations determined. Construction of a graph plotting per cent concentration versus viscosity allows the viscosity of the pure liquid to be found by extrapolating the graph to zero solvent concentration.

In the Standard International (SI) system, viscosity is measured in units of Pa*s, or Pascal seconds. This is rather high for most liquids, though, and it is more common to measure viscosities in units of the mPa*s, or milli-Pascal*second, which is one thousandth of a Pascal*second. In the cgs sysytem, still used by the American Society of Testing and Materials, the unit of viscosity is the poise, or more usually, the centipoise (cP), which is a hundredth of a poise. Both the SI system and the cgs system are based on the metric system, so conversion between them is simple, and 1 cP is equal to 1 mPa*s.