Ostwald Viscometer: Parts, Diagram, Uses & Viscosity Formula

Ostwald Viscometer: Parts, Diagram, Uses & Viscosity Formula

What is an Ostwald viscometer?

An Ostwald viscometer is a U-shaped glass instrument used to measure the viscosity of a liquid. It works by timing how long a fixed volume of liquid takes to flow, under gravity, through a narrow capillary tube between two etched marks. Comparing this flow time (and the liquid’s density) against a reference liquid of known viscosity — almost always water — gives the viscosity of the test liquid.

It belongs to the same family of comparative-method glassware as the stalagmometer: both instruments use the same instrument for a reference and a test liquid so that the instrument’s own physical constants cancel out of the final calculation, leaving only quantities you actually measure.

You’ll typically meet an Ostwald viscometer in an undergraduate physical chemistry lab, and in polymer science, where it is used to estimate the relative molecular mass of a polymer solution from its intrinsic viscosity.

Parts of an Ostwald viscometer (labelled diagram)

An Ostwald viscometer has six main structural parts: a plain suction limb, and a measuring limb made up of a measuring bulb (with two timing marks), a capillary tube, and a reservoir bulb. The diagram below labels each one.

• Suction tube: The plain open limb used to draw liquid up through the capillary into the measuring bulb, past the upper mark.
• Upper mark (a): A fixed etched line at the top of the measuring bulb — timing starts the moment the liquid meniscus passes this mark on the way down.
• Lower mark (b): A second fixed etched line near the bottom of the measuring bulb — timing stops here. The fixed volume between the two marks is the same every time, for every liquid.
• Measuring bulb: The bulb between the two marks, whose fixed volume drains through the capillary during each timed run.
• Capillary tube: The fine, narrow bore that liquid must pass through, providing the flow resistance the timing depends on.
• Reservoir bulb: The larger bulb at the base of the measuring limb that holds the liquid sample and feeds the capillary.

How an Ostwald viscometer works

Liquid is drawn up past the upper mark, then released to flow back down under gravity through the capillary. The rate of that flow is described approximately by the Hagen–Poiseuille equation, which relates flow rate to viscosity, capillary geometry and the driving pressure:

In the idealised form above, a small kinetic-energy correction is left out — it accounts for the energy used accelerating the liquid into the capillary, rather than overcoming viscous drag. For classroom purposes this correction is small enough to ignore when flow times are reasonably long (typically well over 100–200 seconds) and the two liquids being compared are not too dissimilar, which is why the instrument is normally used comparatively rather than to calculate absolute viscosity directly.

Viscosity formula

Since the capillary radius, length and measured volume are fixed properties of the same instrument, they cancel out when you compare a test liquid against a reference liquid measured with that same viscometer:

Water is commonly used as the reference liquid because its viscosity and density are well-established over a range of temperatures — approximately 1.00 cP (centipoise) at 20°C. Use the tabulated value of water’s viscosity and density at your actual working temperature; 1.00 cP and 1.00 g/mL are convenient approximate values often used for basic classroom calculations near room temperature.

How to determine viscosity: step-by-step

1. Clean the viscometer. Rinse thoroughly with distilled water, then with a small amount of the liquid you’re about to test.
2. Fill with the reference liquid (water). Use suction to draw water up the measuring limb past the upper mark, then release.
3. Time the flow. Start a stopwatch the instant the meniscus passes the upper mark (a), and stop it the instant it passes the lower mark (b). Record the flow time (t₁). Repeat 2-3 times and average.
4. Empty and rinse, then fill with the test liquid. Draw the test liquid up past the same upper mark.
5. Time the flow again. Record the flow time (t₂) for the test liquid between the same two marks. Repeat and average, as before.
6. Note the densities. Look up or measure (using a specific-gravity bottle or pycnometer) the density of the test liquid (ρ₂); use the tabulated density of water (ρ₁) at your working temperature.
7. Apply the formula. Substitute t₁, t₂, ρ₁, ρ₂ and the known η₁ of water into η₂ = η₁ × (ρ₂t₂)/(ρ₁t₁) to find the viscosity of the test liquid.

Observation table

Common uses of an Ostwald viscometer

• Undergraduate physical chemistry practicals: The standard experiment to determine the relative viscosity of a liquid compared with water.
• Polymer science: Estimating the relative molecular mass of a polymer solution from its intrinsic viscosity.
• Quality-control applications: Viscosity measurements can help compare formulations or flag batch-to-batch differences in liquid products when used under controlled, repeatable conditions.
• Comparing liquids: Any situation needing a quick, low-cost comparison of viscosity between two or more liquids without electronic rheometry equipment.

Ostwald viscometer vs stalagmometer vs burette

These three are all calibrated glass instruments that work by comparing a test liquid against a reference under identical conditions — but each measures something different:

If you’ve already measured surface tension, see our stalagmometer guide for the drop-count method it uses instead of timed flow.

Buying guide: what to look for

For school and college physical chemistry labs, look for:

• Borosilicate glass construction for chemical resistance and thermal stability
• Clearly etched, permanent upper and lower timing marks
• A capillary bore suited to the viscosity range you’ll typically measure
• Bulk/institutional availability for setting up multiple lab benches at once

Common errors & accuracy tips

• Not cleaning between liquids: Residue from the previous liquid changes the effective flow resistance of the next reading. Always rinse with the new liquid before the real measurement.
• Holding the viscometer at an angle: The instrument must be held (or clamped) perfectly vertical — any tilt changes the driving pressure and skews the flow time.
• Timing only once: A single trial can be off by a second or more. Repeat 2-3 times per liquid and average.
• Using an outdated density or reference-viscosity value: Both water’s density and viscosity change with temperature — use values for your actual working temperature, not just a generic textbook figure.
• Bubbles in the capillary: Air bubbles disrupt the flow and give an inconsistent time. Fill slowly and check the capillary is bubble-free before timing.

Related products

People also search for

Frequently asked questions