Viscometers tend to be a “buy once and forget it” type of purchase, particularly for labs supporting the manufacture of common fluidbased products such as foods, paints, and drugs.
For this reason, Marv Kaiser, global product manager at Cole-Parmer (Vernon Hills, IL), urges purchasers to “do their homework”—to consider their purchase in light of viscosity ranges that interest them. “What fluids are you using? Knowing your material’s viscosity range can help narrow down what type of viscometer you will need.” This will ensure, at least from an operational perspective, that instrument and fluids are compatible. “Otherwise, the viscometer can deliver inaccurate results and cause your instrument to malfunction.” Buyers can refer to online viscosity value charts to help match applications to instruments. With viscosity ranges and sample types in mind, purchasers next need to navigate among viscometer choices. Following is a partial list:
Falling ball viscometers measure viscosities of both liquids and gases based on the time it takes for gravity to pull a sphere through a tube. “This design allows these instruments to work in wide ranges and with reproducible results,” Kaiser says.
Viscosity cups use gravity to allow a fluid to flow through an orifice at a specified temperature. The fluid’s flow over time is used to calculate viscosity.
Consistometers are metal troughs with graduations that measure viscous materials as they flow at an incline under their own weight. Consistometers are primarily used to measure paint viscosity and also work well for syrups, jellies, sauces, and cosmetics.
Used for test methods conforming to ASTM guidelines, glass capillary viscometers (also known as Ostwald viscometers) are used with viscosity baths to maintain stable analysis temperatures. The instruments’ low cost and established accuracy make them a desirable choice for measuring kinematic viscosity in applications such as asphalts, lubricants and fuels, paints, and polymers.
Tuning fork vibration viscometers measure viscosity by detecting the driving electric current needed to resonate two sensor plates at a constant frequency. In contrast to most viscometer designs, the digital display of the reading eliminates inconsistencies from user variations in operation.
Rotational viscometers, which accommodate very wide viscosity ranges, are considered the most versatile type of viscometer. Advanced units include software that produces graphs and charts in addition to logging data. Rotational units allow users to keep track of a fluid’s specific gravity.
First things first
Given this wide range of viscometer choices—and this list is by no means exhaustive—John Welter, general manager at PCE Americas (Jupiter, FL), suggests putting first things first, namely cost, because “knowing your required viscosity range is only part of the buying decision process.”
“What is your budget limit? Can you get approval for the top-quality version with all the bells and whistles, or do you just need a basic model that gives you the viscosity with no other options?”
The next consideration is how the instrument will fit into your workspace and workflows. Welter continues: “A viscometer that goes into a QA lab would normally be a tabletop model. If you are mixing small batches in a production area or working in the field, a portable device allows you to verify viscosity during mixing and make adjustments on the fly. If your product is piped around the plant, then an inline sensor may be ideal since it can communicate directly to the process control engineers.
The last factors to consider are the many physical variables related to viscometry measurements, particularly those that may change, either from experiment to experiment or, during production, from batch to batch. Temperature is a key variable, as its effect on viscosity may be linear or nonlinear. “Try to list the production issues you have experienced in the past, and note whether they have affected the accurate measurement of viscosity,” Welter tells Lab Manager. “The last thing you want is to learn that your viscometer will not work for your application due to known production variables that were not relayed to the supplier during the presale process.”