Capillary viscometers measure the viscosity of liquids by measuring the flow time through a small tube. Viscosity is important in many industries, and capillary viscometers come in various designs. The efflux time is converted to viscosity using a unique conversion factor. Viscosity is measured in units of Pa*s or mPa*s, or in the cgs system, the poise or centipoise. High viscosity liquids can be measured by using solutions in a suitable solvent and constructing a graph to find the viscosity of the pure liquid.
A capillary viscometer is an instrument used to measure the viscosity, or thickness, of a liquid by measuring how long it takes for the liquid to flow through a small diameter tube, or capillary. The flow time, or efflux, is directly proportional to the kinematic viscosity of the liquid and can be converted directly to viscosity through the use of a conversion factor unique to each instrument. Viscosity is generally temperature dependent, so the capillary viscometer is usually used in a temperature-controlled water bath set to a specific temperature.
Viscosity can be thought of as the internal friction of a liquid or its tendency to resist flow. Viscosity is therefore an important property of fluids. It is of paramount importance in lubricants, paints, beverages and anywhere else a liquid needs to be transferred, agitated or manipulated.
A capillary viscometer can take one of several designs, but the 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 tube carrying two glass bulbs or chambers on one arm, separated by a calibrated length of capillary tube. Another bulb is low on the other arm, to which the sample is loaded. The sample and capillary viscometer are then suspended in a fixed temperature water bath and allowed to reach thermal equilibrium.
Once thermal equilibrium is reached, the sample is drawn into the upper chamber and the test begins. The test sample is allowed to flow from the upper to the lower chamber through the capillary and the efflux time, or time required to traverse the length of the capillary, is measured. Capillary viscometers are equipped with a conversion factor provided by the manufacturer which allows the kinematic viscosity to be calculated directly from the efflux time.
For high viscosity liquids solutions of these substances in a suitable solvent can be used and measurements of the viscosity of the solutions at various concentrations can be determined. The construction of a graph that reports the percentage concentration as a function of the viscosity allows to find the viscosity of the pure liquid by extrapolating the graph to the zero concentration of the solvent.
In the International Standard (SI) system, viscosity is measured in units of Pa*s or Pascal seconds. This is quite high for most liquids, however, and it is more common to measure viscosities in units of mPa*s, or milli-Pascal*second, which is one-thousandth of a Pascal*second. In the cgs system, still used by the American Society of Testing and Materials, the unit of viscosity is the poise, or more commonly the centipoise (cP), which is one-hundredth of a poise. Both the SI system and the cgs system are based on the metric system, so converting between them is simple and 1 cP equals 1 mPa*s.
Protect your devices with Threat Protection by NordVPN
