6 Varied Technologies To Measure Viscosity
Data on a material’s viscosity gives manufacturers valuable knowledge of its characteristics to help predict pumpability and pourability, performance in specific operations, and ease of handling.
There are different techniques for measuring viscosity, each suited to particular materials and conditions. Instruments vary from simple, like counting the seconds for a liquid to drain off a device, to sophisticated automatic recording and controlling equipment. The most popular measurement tool is called a Viscometer (several types exist).
In the 1800’s, the earliest measurements of viscosity were taken with capillary tubes. Here are a few different techniques used today.
1. Capillary Viscometer
This traditional way of measuring viscosity used capillary tubes to measure the time it took for a volume of liquid to traverse the length of the tube. These devices are still in use and are called Ostwald or Ubbelohde Viscometers.
2. Zahn Cup
This is a small container with a handle and a small hole in the bottom. The time it takes to empty the cup of the liquid, through the hole, forms the basis of the measurement of viscosity. The Zahn cup is still frequently used in the paint industry.
3. Falling Sphere Viscometer
In this method, a sphere of known density is dropped into a fluid sample, and the user records the time it takes for the sphere to fall to a specified point. This kind of measurement is most often used on ships to monitor the quality of the fuel being pumped into the engine. A falling piston Viscometer uses a similar technique.
4. Vibrational Viscometer
This method measures the damping of an oscillating electromechanical resonator immersed in liquid. It is often used to take continuous readings, in cases of a stream or batch, to ensure consistency.
5. Rotational Viscometer
This technique measures the torque required to turn an object inside a liquid. From that reading, the fluid’s viscosity is calculated. This method is most popular for quality control. These Viscometers can run through an entire process, logging data continuously to provide a record of the change in viscosity over time. When compared with historical data from previous rounds, it can help reduce the margin of error and retain consistency. This is a popular method for many applications. However, it comes with its difficulties. Torque can be hard to measure. Because the spindle rotates, wires attached to a torque sensor wind and snap easily. The option to use slip rings exists but is expensive, hard to set up and prone to wear and tear.
6. Resonant Viscosity Sensors
More recently, a wireless technique was developed using rotary torque transducers. This does not need a physical connection to the rotating shaft. It uses a radio frequency (RF) link to send power to the sensing element and to receive torque-reading signals back.
Some sensors have no moving parts and, due to the high frequency of the vibration, are immune to the vibration effects of the environment. High-temperature electronics and advanced packaging techniques allow for a wide operating temperature range. These sensors do not need to be calibrated by the end user.