A fluid's viscosity, its resistance to flowing, is usually expressed as the ratio of its shear velocity of motion to rotational torque. It's difficult to measure because it's so dependent on temperature and shear rate. Older methods timed how long fluids drained from a cup; this is flawed because this method is hard to automate, requires large volumes, and is subject to experimenter interpretation. Current methods put fluid to be tested between a rotor and a stator, and then measure how much force it takes to spin the rotor through the fluid. However, the moving parts are subject to gumming and abrasion, and gyroscopic effects also color results.
Now a new type of sensor from Biode Inc., Westbrook, Maine, eliminates moving parts, using sound waves to measure viscosity. Called the ViSmart, the sensor is either immersed in the liquid to be tested, or the liquid is placed on its sensing surface. The liquid's viscosity determines how much of the liquid is actually coupled (in other words, linked hydrodynamically) to the sensing surface — and that's exactly what the sensor leverages to measure viscosity. The greater the viscosity, the thicker the layer of entrained fluid, and the more power it takes to force acoustic waves through the sample.
ViSmart is essentially a quartz-crystal wave resonator. When it contacts liquid, shear waves penetrate into the adjacent fluid to a depth d, transferring power from the sensor to the sample. The square of the power loss is proportional to the product of frequency, density, and viscosity.
Because viscosity is so affected by temperature, the sensor also includes a temperature chip; results are adjusted for that variable.
Three flavors of viscosity
There are three ways to express viscosity. When shear rate and temperature are equal, specific gravity allows conversion from one to another.
- Absolute viscosity
Also called dynamic viscosity, this is the most common unit used to quantify fluid thickness — expressed as the ratio of a fluid's shear velocity of motion to rotational torque. It's measured as the tangential force per unit area needed to move one horizontal plane with respect to another (separated by the fluid being investigated) at unit velocity. Dynamic viscosity units are N sec/m2, Pa˙sec, or kg/m˙sec where 1 Pa˙sec = 1 N˙sec/m2. The unit for this is the centipoises. One Poise (P) = dyne sec/cm2 = g/cm˙sec = 1/10 Pa˙sec.
- Kinematic viscosity
The ratio of absolute viscosity to density, kinematic viscosity is a quantity involving no forces. Its units are centistokes (cS) and it's equal to absolute viscosity divided by specific gravity.
- Acoustic viscosity
This newer expression of viscosity is based on the transfer of acoustic shear wave energy from a quartz crystal having a characteristic impedance. The square of the power loss is proportional to the product of frequency, density, and viscosity. Since the frequency is known, the sensor measures viscosity multiplied by density. Acoustic sensors measure viscosity in units of centipose multiplied by specific gravity. Thus, the digital output of the acoustic wave sensor can be automatically displayed in centipose units if the user knows the specific gravity (density) of fluid.