Measuring viscosity with a viscometer involves selecting the right instrument for your fluid, preparing your sample, and converting raw readings (time or torque) into viscosity values. The two most common approaches are rotational viscometers, which measure the resistance a spinning disc encounters in your fluid, and capillary (glass tube) viscometers, which time how long a fluid takes to flow between two marks. Each method has a distinct procedure, but both ultimately give you a number in centipoise (cP) for dynamic viscosity or centistokes (cSt) for kinematic viscosity.
Rotational Viscometer: Step by Step
A rotational viscometer works by dipping a metal spindle into your sample and spinning it at a set speed. The instrument measures how much torque (rotational resistance) the fluid exerts on the spindle, then converts that into a viscosity reading. This is the most common type for lab and industrial use because it handles a wide range of fluids, from thin solvents to thick pastes.
Start by pouring your sample into the appropriate container, making sure it’s at the target temperature. Temperature has a large effect on viscosity, so even a few degrees off can skew your results. Attach the spindle to the viscometer’s coupling by threading it on (most use a left-hand thread, so you turn it clockwise to tighten). Lower the spindle into the sample to the immersion line marked on its shaft.
Set your rotation speed and start the motor. The viscometer will display a torque percentage and a viscosity value. For a valid measurement, your torque reading needs to fall between 10% and 100%. Below 10%, the instrument isn’t sensing enough resistance to give an accurate number. Above 100%, the spindle is meeting too much resistance and the reading maxes out.
If you don’t know the approximate viscosity of your sample, start with the smallest spindle available. A smaller spindle displaces less fluid and encounters less drag, so it’s suited for thicker samples. If the torque reads below 10%, switch to the next larger spindle and test again. If it reads above 100%, go back to a smaller one. Keep swapping until you land in that 10% to 100% range. Always turn the motor off before removing or changing a spindle.
Capillary Viscometer: Step by Step
Capillary viscometers, like the Ostwald or Cannon-Fenske type, measure kinematic viscosity by timing how long a fluid takes to flow through a narrow glass tube under gravity. They’re simple, inexpensive, and extremely precise for thin, free-flowing liquids like oils, solvents, and fuel.
Fill the viscometer with your sample following the manufacturer’s instructions (each design has a specific fill level). Place it in a temperature-controlled water bath and let it equilibrate. Then draw the liquid up past the upper timing mark. Release it and start your stopwatch as the meniscus passes the upper mark. Stop the watch when it reaches the lower mark. This elapsed time is called the efflux time.
To get kinematic viscosity, multiply the efflux time (in seconds) by the viscometer constant, a number specific to that particular tube and printed on its certificate or documentation:
Kinematic viscosity (cSt) = efflux time (s) × viscometer constant
If you need dynamic viscosity instead, multiply the kinematic viscosity by the fluid’s density:
Dynamic viscosity (cP) = kinematic viscosity (cSt) × density (g/cm³)
Calibrating With a Reference Liquid
If your capillary viscometer doesn’t have a known constant, you can calibrate it using a reference liquid. Water is the most common choice, with a well-documented viscosity of 0.8007 cP at 30 °C. Run water through the viscometer first and record its flow time and mass. Then run your unknown sample under the same conditions. The viscosity of your sample relates to the reference by comparing the ratio of their flow times and masses:
Sample viscosity = reference viscosity × (sample mass × sample time) / (reference mass × reference time)
This ratio method cancels out the tube’s geometry, so you don’t need to know the viscometer constant at all.
Handling Non-Newtonian Fluids
Not all fluids behave the same way at every speed. Water, honey, and motor oil have a fixed viscosity regardless of how fast you stir them. These are Newtonian fluids. But many real-world materials, including paints, blood, yogurt, ketchup, and polymer solutions, change viscosity depending on how much force you apply. These are non-Newtonian fluids, and they require extra care when measuring.
The key variable is shear rate, which in practical terms corresponds to how fast your spindle rotates or how quickly the fluid flows. A traditional viscometer gives you a single-point measurement at one speed, which can be misleading for a non-Newtonian fluid. To get the full picture, you need to measure viscosity at multiple spindle speeds (or shear rates) and plot the results. This creates a flow curve that shows how the fluid’s resistance changes with increasing force.
If you’re working with a rotational viscometer, run your measurement at several different speeds and record the viscosity at each one. If the values change significantly across speeds, your fluid is non-Newtonian, and you should report viscosity alongside the specific shear rate or spindle speed used. Without that context, the number is incomplete.
Viscosity Units and Conversions
Viscosity comes in two flavors: dynamic and kinematic. Dynamic viscosity measures a fluid’s internal resistance to flow when an external force is applied. Kinematic viscosity measures the same thing but accounts for the fluid’s density, essentially describing how fast the fluid flows under gravity alone.
- Dynamic viscosity is reported in pascal-seconds (Pa·s) in SI units, or in poise (P) and centipoise (cP) in older systems. 1 poise equals 0.1 Pa·s. Water at room temperature has a dynamic viscosity of about 1 cP.
- Kinematic viscosity is reported in square millimeters per second (mm²/s) or in stokes (St) and centistokes (cSt). 1 stoke equals 100 mm²/s.
To convert between them, use this relationship:
Kinematic viscosity = dynamic viscosity / density
When using CGS units, that’s stokes = poise / (g/cm³). This conversion is why capillary viscometers, which naturally measure kinematic viscosity, require you to know the fluid’s density if you want dynamic viscosity.
Cleaning and Avoiding Errors
Cross-contamination is one of the most common sources of error in viscosity testing. Even small amounts of a previous sample, cleaning solvent, or dust on a spindle can throw off your reading. Remove the spindle from the viscometer before cleaning it, and make sure the coupling surfaces (where the spindle attaches to the instrument) stay clean and smooth. Any dirt or residue there can cause the spindle to wobble, which introduces mechanical error into the torque measurement.
For capillary viscometers, residue inside the tube is equally problematic. Rinse thoroughly with an appropriate solvent, then let the tube dry completely before your next measurement. If you’re working with viscosity standard fluids for calibration, be especially careful not to mix standards of different viscosities or introduce solvent into the standard.
Temperature control matters more than most people expect. A few degrees of drift can change viscosity by 5% to 10% or more, depending on the fluid. Use a water bath for capillary viscometers, and let rotational samples equilibrate to your target temperature before starting. Record the measurement temperature alongside every viscosity value, since the number is meaningless without it.