The sound of viscosity

Kerem Durdag
Chief Operating Officer,
BiODE Inc.
Westbrook, Main

Though viscosity is the degree to which a fluid resists flow, it is more specifically a measure of a fluid's molecular friction. It can be a useful property to quantify because it gives an indirect measurement of a liquid's consistency in both molecular weight and molecular-weight distribution.

Viscosity can give other insights into materials and properties that can be valuable in manufacturing processes. Trouble is, almost all methods for measuring viscosity were developed originally for laboratory analysis. They were never intended for making measurements quickly. They certainly are not amenable to the sort of real-time environment that characterizes modern-day manufacturing or processing. So it has not been practical to use viscosity measurements for, say, adjusting liquid properties on the fly, or for tweaking recipes as processing environments change.

This situation is changing, however, with the advent of new measurement techniques that use solid-state acousticwave sensors to gauge viscosity.

These sensors basically use a quartz crystal to beam an acoustic signal into the fluid of interest and gauge how the liquid affects it. The square of the power loss of the acoustic wave passing through the fluid is proportional to the product of the signal frequency, fluid density, and fluid viscosity. Because frequency is known, the sensor's signal can be analyzed for viscosity X density or cP X S.G.

In operation, the sensor portion of the device consists of a quartz-crystal resonator that is in uniform motion at a specific frequency and amplitude. The upper surface of the resonator is in direct contact with the liquid under test while the lower surface has electrodes that are hermetically sealed from any fluid contact. The resonator establishes a standing wave (actually a shear wave) through its thickness. The wave pattern interacts with the fluid on the upper surface and the electrodes on the lower surface. As the shear wave penetrates the surface of the fluid touching the resonator, a thin layer of fluid is set in motion absorbing power from the wave. Electrodes monitor the loss of power from the shear wave, which is in proportion to the viscosity of the liquid. The amount of fluid set in motion by the resonator is a layer only a few microns thick. Thus, the acoustic signal doesn't affect the bulk of the liquid.

Many of the advantages this technique provides stem from its solidstate operation. An acoustic viscometer, the term for a device that measures viscosity, has no moving parts. So it is highly immune to shock and vibration. Commercial units, for example, can withstand 30-g impacts. The quartzcrystal transducer operates at a relatively high frequency, so viscosity measurements will be accurate regardless of whether the liquid is static, or in laminar or turbulent flow. This quality makes these devices candidates for gauging the viscosity of liquids flowing at high rates. Of course, many commercial processes involve just such high-flow-rate scenarios. Acoustic-wave sensors deployed in such applications can measure viscosities ranging from 0 to well beyond 10,000 centiPoise (cP) with ±3% repeatability over a temperature range of 20 to 135°C.

OLDER TECHNOLOGY
A review of older viscometer technology helps illustrate how solid-state acoustic sensors can lead to more efficient liquid processing. Regardless of the technology used, it has been difficult to get accurate viscosity measurements. Temperature, pressure, shear rate, and other factors all affect viscosity. Most viscometers, moreover, could only measure samples extracted from production. The idea of measuring the viscosity of liquid in its native production environment has been more or less unheard of. Tests made on samples ran the risk of missing significant production events such as viscosity changes caused by the addition of chemicals.

The simplest and perhaps most common method of measuring viscosity is the viscosity cup. A known volume of liquid is placed in a special cup. An orifice in the bottom of the cup is opened so the liquid can drain. The combination of cup volume and orifice size yield a value known as the cup factor. The time it takes to empty the cup along with the cup factor determines the viscosity of the sample. Another simple test is to measure the amount of time it takes a ball-shaped object to fall a given distance through a liquid.

The instrument of choice used to measure viscosity in most analytical labs is the rotational viscometer. Here a rotating disk on the end of a spindle sits inside a cylinder at a known distance from the cylinder wall. The liquid sample covers the disk. The liquid between

the rotating disk and stationary cylinder exerts a viscous drag on the rotating disk. The relationship between the torque required to turn the disk and the speed of the spindle determines the liquid's intrinsic viscosity measured in milliPascals/second (mPa/sec) or cP.

While accurate and well suited for off-line static measurements, rotational viscometers are highly susceptible to sample flow conditions or movement in the measurement platform. Engineers adapting rotational viscometers to in-line process measurement must also take their size into consideration.

There are several approaches to measure viscosity in-line. One inserts a capillary tube of known and controlled geometry into the flow stream to measure pressure drop under a known flow rate. This measures kinematic viscosity in units of millimeter2/second or centiStokes (cS), the ratio of viscosity to density. Problems develop when the capillary tube becomes clogged or the flow rate through it varies. It is both difficult and expensive to keep the flow constant. The task usually requires positive-displacement pumps.

Another in-line viscosity measurement employs a piston and cylinder to measure kinematic viscosity in cS. Fluid is diverted into a cylinder where air pressure or magnetic coils control the rise and fall of a piston. When air raises the piston, the piston is then allowed to fall by gravity. The fall time measures the viscosity. Magnetic coils use a constant magnetic force to drive the piston through the liquid. The two-way travel time of the piston indicates fluid viscosity. Both techniques risk the piston jamming from particulates in the flow-stream.

Though cS and cP each measure different aspects of viscosity, the measurements are related through the specific gravity (S.G.) of the liquid: cS = cP/S.G.

MAKE CONTACT:

BiODE Inc., (207) 856-6977, biode.com