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Viscosity Using Ostwald Viscometer: A Detailed Guide

Viscosity Using Ostwald Viscometer: Learn how to accurately measure viscosity with an Ostwald viscometer, including detailed steps for sample preparation, calibration, and factors affecting measurements.

Viscosity Using Ostwald Viscometer: A Detailed Guide
Viscosity Using Ostwald Viscometer: A Detailed Guide

Viscosity Using Ostwald Viscometer: A Detailed Guide

Overview of Viscosity

Viscosity is one of the fundamental properties of liquids that indicate their internal resistance to flow. It is a way of quantifying the density or the thickness of a fluid or technically, the amount by which a fluid resists deformation at a given rate. 

In other words, viscosity is the flow characteristic of a fluid that determines how fast or slow it flows; the greater the viscosity, the slower a fluid moves. This property is very important in natural and artificial processes, since it has an impact on everything from the blood flow in our bodies to the manufacturing of Skin formulations.

In many industries, it is important to be able to measure and control material viscosity in order to produce quality products, efficient equipment and optimal performance. 

in Pharmaceutics, viscosity of liquid medications affects their absorption and efficacy. In the food industry, viscosity plays a crucial role for the mouthfeel and texture of products. Likewise, in the oil and petrochemical industries, viscosity is responsible for extraction process, refining and transportation of products.

Principles of Viscosity Using Ostwald Viscometer

In the evaluation of the properties of fluids, viscosity measurement is a key parameter and it has a major role in science and industry. It gives the information about the behavior of fluids under different conditions, which is crucial for pharmaceutical sciences, lubrication, materials science and other disciplines.

Definition of Viscosity

Viscosity is the measure of internal resistance to flow of fluid. It is a measure of how viscous the fluid is in terms of its resistance to deformation for a particular rate. As it were, viscosity means how thick or thin a liquid is, or how well the fluid pours. The example is honey, which is more viscous than water because it flows slower and has higher resistance to movement.

The Ostwald viscometer is a contraption that is used to determine the viscosity of liquid substances and relies on the principle of capillary flow. The principle that this instrument uses in determining the viscosity of a fluid is laminar flow through a capillary tube.

Working Principle

The capillary used in this case is so designed that the fluid whose viscosity is to be measured drains through it under influence of gravity. The measurement of the time needed for a given quantity of the fluid to pass through the capillary, enables us to know the viscosity. 

The efflux time represents the time flowing from one end of the tube to the other and is directly proportional to the viscosity of the fluid.

Calculation of Viscosity Using Ostwald Viscometer

The dynamic viscosity is determined by a reference fluid with known viscosity, commonly water found at an understood temperature level. The formula used is based on relative density of the test fluid and reference fluid, as also their efflux times, from which viscosity of the former can be calculated.

Viscosity Calculation Formula:

Ostwald Viscometer: Design and Operation

The Ostwald viscometer was named after the German chemist Friedrich Wilhelm Ostwald, who invented a special liquid measuring device that allows you to determine accurately the viscosity of liquids. The principle of capillary flow is the basis of its operation since it determines the viscosity by observing how long it takes a liquid to flow through a narrow capillary tube under the effect of gravity.

Description of Ostwald Viscometer

An Ostwald viscometer has an elegant simplicity of design, wherein a tube of U-shape with two bulbs is depicted by a glass.

Capillary Tube: A small-bore uniform tube joining two bulbs together. The measurement process itself involves the flow of liquid through this tube.

The first bulb provides the inlet for the liquid, and the other serves as an outlet by collecting the liquid after it has passed through the capillary.

Markings: At certain points along the tube, which are usually indicated on the upper and lower parts of the capillary section, there is a mark identifying the volume of liquid used for measurement.

Reservoir: A bigger bulb or region where the liquid is pumped and harvested during the measuring act.


Filling: It was then ensured that the liquid in test was introduced into a bulb until the capillary tube and the adjacent bulb is filled with no air bubble.

Measurement: In most cases, the viscometer sits in a water bath with the temperature held constant since viscosity is affected by temperature.

Timing: The liquid is then allowed to drain back down through the Capillary tube by gravity. The efflux time is measured by using a stop watch the duration of time that the liquid takes to pass between two marked points on the capillary tube.

Calculation: The efflux time is then used, in conjunction with the known density of the medium and reference values for a calibration liquid (typically water), to calculate the viscosity of the test liquid.

The basic operation principle is that the efflux time, which is the time taken to travel from point A to point B with the liquid, is proportional to the apparent viscosity. 

By comparing this time with the time in which liquid of known viscosity would flow through the same distance, the viscosity of the test liquid can be obtained.

Preparation for Viscosity Using Ostwald Viscometer

Acquiring accurate results requires careful sample preparation and calibration because the Ostwald viscometer is a popular choice for many due to its simplicity and precision.

Sample Preparation

Proper sampling is a crucial requirement for precise viscosity measurements.

Sample Selection: Select a test sample that is representative of the liquid whose viscosity you want to determine. Make sure that there are no particles and bubbles left on the sample to ensure the accuracy of measurements, as these can greatly impact accuracy.

Temperature Control: This is because viscosity is temperature-sensitive and therefore in order to find the sample’s viscosity, one has to first bring the sample to a certain, constant temperature before measurement. This is usually done by incubating the sample in a water bath with a regulated temperature for long enough to reach thermodynamic equilibrium.

Degassing: Eliminate any air bubbles that might have formed in the liquid, for they pose as impediments to flow through the capillary tube. This can be achieved using a gentle shake or the use of a vacuum degasser.

Volume Measurement: Make sure to get a large enough sample volume that the Ostwald viscometer, including reservoirs and capillary tube, will be filled without introducing air bubbles.

Calibration of Ostwald Viscometer

Ostwald viscometer calibration is very important for obtaining an accurate result on the viscosity of a fluid and entails the use of a known fluid such as distilled water under certain temperature.

Filling: Tilt the viscometer with the calibration fluid and ensure that it is filled up, and there are no air bubbles remaining inside. This may involve the use of a funnel with filling as well as syringe, depending on the design of the viscometer.

Temperature Equilibration: Put the viscometer in a preheated bath which is set to the temperature of measurement. Ensure that the thermal equilibrium between the viscometer’s calibration fluid and the bath is achieved in such a manner that enough time has been provided for it.

Timing Measurement: Time the elapsed time for the calibration fluid to flow from the first point to the second point on the viscometer. Repeat this measurement several times to avoid inconsistencies and inaccuracies.

Calculation of Calibration Constant: Use the viscosity of the calibration fluid as a known value and the measured efflux time to calculate the calibration coefficient for knowing the viscometer.

Repetition with Test Sample: Following calibration, take the measurement again when applying the test sample under identical temperature conditions and method of measurement. Then, the viscosity can be determined by multiplying the slope of the calibration constant.

Procedure for Viscosity Using Ostwald Viscometer

Measuring the viscosity of a liquid accurately is a critical task in many scientific and industrial settings. The Ostwald viscometer, with its simple design and operation, offers a reliable method for this measurement. Here’s a step-by-step guide to the measurement process, along with tips to enhance accuracy.

Step-by-Step Measurement Process

  1. Preparation and Calibration:
    • Start with the preparation of your sample and the calibration of your Ostwald viscometer, as previously outlined. Ensure the viscometer is clean, dry, and free from any residues.
  2. Filling the Viscometer:
    • Use a syringe or a pipette to carefully fill the viscometer with the liquid sample, avoiding the introduction of air bubbles. Fill until the liquid reaches above the upper mark of the capillary tube.
  3. Temperature Equilibration:
    • Place the viscometer in a temperature-controlled water bath. The temperature should be maintained consistently throughout the experiment. Allow sufficient time for the sample to reach thermal equilibrium with the bath.
  4. Measuring Efflux Time:
    • Allow the liquid to flow down until the upper meniscus aligns with the upper mark of the capillary. Start your stopwatch as it passes this mark.
    • Stop timing when the upper meniscus reaches the lower mark. Record this time, known as the efflux time.
  5. Repeat for Accuracy:
    • Conduct the measurement several times to ensure consistency. Use the average of these readings for a more accurate result.
  6. Calculation:
    • Calculate the viscosity of your sample using the formula provided, incorporating the efflux time, the density of the liquid, and the calibration constant obtained from a known standard.

Tips for Accurate Viscosity Using Ostwald Viscometer

  1. Avoid Air Bubbles: When filling the viscometer, ensure no air bubbles are present, as they can obstruct the flow and lead to inaccurate measurements.
  2. Consistent Temperature: Maintain a consistent temperature throughout the experiment. Even slight fluctuations can significantly affect the viscosity.
  3. Use Fresh Samples: Always use fresh samples to avoid changes in viscosity due to evaporation or contamination.
  4. Proper Alignment: Ensure the viscometer is vertically aligned in the water bath. Any tilt can affect the flow rate and thus the efflux time.
  5. Multiple Measurements: Take multiple measurements to account for any variability in the experiment. The average of these measurements will provide a more reliable value.
  6. Record Environmental Conditions: Note down any environmental conditions that might affect the measurement, such as atmospheric pressure and humidity, especially if comparing results across different days or locations.
  7. Clean and Dry: Ensure the viscometer is thoroughly cleaned and dried between different samples to prevent cross-contamination or residue interference.

Calculation of Viscosity Using Ostwald Viscometer

Example Calculation

Suppose we want to calculate the viscosity of an oil sample using water as the reference liquid. We have the following data:

  • μref​ (viscosity of water at 25°C) = 0.89 mPa·s,
  • ρtest​ (density of oil) = 0.80 g/cm³,
  • ρref​ (density of water) = 1.00 g/cm³,
  • ttest​ (efflux time for oil) = 100 s,
  • tref​ (efflux time for water) = 30 s.

Thus, the viscosity of the oil sample is 2.37 mPa·s at 25°C.

Factors Influencing Viscosity Using Ostwald Viscometer

Various factors may significantly influence viscosity measurements, and temperature is one of the key factors along with the concentration and composition of the sample.

Temperature Effects: Normally, when the temperature of liquids increases, viscosity decreases due to increased thermal energy which reduces the forces of attraction that oppose flow. On the other hand, the viscosity of gases increases with r as higher thermal energy leads to a greater transfer of momentum between molecules.

Thermal Equilibrium: It is absolutely important to establish thermal equilibrium between the sample and its surroundings before measuring it. Temperature variations may result to temperature viscosity readings.

Sample Concentration and Composition

Concentration: Mixtures or solutions can be much affected by the concentration of the solute in concern, which will determine how viscous they become. Increased viscosities are generally associated with increasing concentrations since more interactions between the molecules or particles in a solution are expected.

Composition: The sample’s chemical nature, such as the presence of additives, impurities, or the molecular weight of polymers in a solution is also an aspect that affects viscosity. The multicomponent nature of the synthesis can cause complications in which several components interact in a way that changes the flow characteristics of the mixture.

The precise viscosity measurement requires a good understanding and control of such factors as they directly affect the internal resistance to flow thus influencing the measured results.


In summary, the measurement of viscosity using a Ostwald viscometer constitutes the backbone of the areas of pharmacology, materials science and several other scientific fields, providing a reliable and simple means of determining the resistance to flow in liquids. Essential for acquiring reliable viscosity measurements are the precise preparation of samples, correct calibration of the viscometer as well as good knowledge of the variables which affect viscosity.

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