Show
measuring cup image by Antonio Oquias from Fotolia.com
There are scientific ways to measure viscosity with a viscometer; but, a do-it-yourselfer's kitchen has most of the tools necessary to obtain a simple measurement of a motor oil's viscosity. Although, it involves some knowledge of algebra, a basic experiment can measure the time it takes for a round sphere to drop from the surface of a liquid like oil, to the bottom of a glass measuring cup or other cylinder. Once he figures that number (it's measured in centimeters per second), he can plug in the other variables into a formula to gauge the viscosity of oil at room temperature.
Weigh a clean empty measuring cup (a cylinder) on the gram scale. Note the weight on the paper.
Put about half a bottle of regular motor oil into the cylinder. Weigh the oil-filled measuring cup. Subtract the figure in Step 1 from this figure. This is the weight of the oil.
Note the height of the oil in the cup. This height should be expressed in centimeters. Note this height on the paper. (The reading on the cup also indicates the volume of the oil in milliliters).
Ask someone to stand ready with the stopwatch.
Pick up one of the clean glass marbles and position it very carefully over the liquid so that it barely touches the surface of the oil.
Simultaneously release the marble and start the stopwatch. Stop the timer the instant the marble touches the bottom of the cup. This is the velocity. The velocity is the distance that the marble ball sank (in centimeters) divided by the time it took to reach the bottom of the cup (in seconds). Note this time on the paper. Repeat Steps 4 and 5 a few times and average the answers.
Measure the density of the marble. This is the weight per unit volume, measured in grams per centimeters cubed (g/cm^3). To do this, put the marble on the gram scale and note the reading.
Measure the marble's volume by pouring some water (fill it roughly to its halfway point) into the other cup. Set down the cup and let the water settle. Note the water level. It is measured in ml, which is equivalent to cm^3.
Place the other glass marble into the water-filled measuring cup and let it sink to the bottom. Note the height of the displaced water by writing the number down.
Subtract the figure in Step 7 from the figure in Step 8. This difference equals the volume of the marble (in c/^3).
Measure the density of the oil. You already have the weight and volume of the oil. The density equals the weight divided by the volume.
Measure the radius of the marble. Put the marble on a flat, horizontal table. Using a ruler, measure the diameter of the marble. The radius is half the diameter.
Insert these numbers into the formula n=2(??)ga^2/9v to find the viscosity of the oil. The answer is expressed in terms of units of poise (g/cm times s). Here is a list of important variables: ?? = density of the sphere?density of the oil (in g/cm^3) g = acceleration due to gravity (980 cm/s^2) a = radius of the sphere (in cm) v = average velocity of the falling marble (in cm/s)
Viscosity the most important physical property of a lubricating oil, as determines how easily the oil circulates and its load carrying capability. Maintaining the correct balance between low viscosity for ease of circulation and high viscosity for load carrying is important for the application of any type of lubricant. Oil provides other benefits in addition to lubrication, so it is important that it is able to flow under any condition. During use, the oil viscosity is influenced by contaminants such as water, oxidation, fuel entering the oil, and soot. This makes viscosity measurement an important test for the oil used in a mechanical system. Kinematic viscosity (the resistance to flow under gravity) is the conventional method to monitor machine condition. The following parameters influence the viscosity of oil:
Kinematic viscosity, ν, indicates the flow behavior of a substance under the influence of Earth’s gravity. It is calculated by dividing the dynamic viscosity by density (ρ), which is defined as mass per volume. Kinematic viscosity is widely established due to historical reasons: Gravity is constant anywhere on Earth and as a driving force, it does not need any elaborate technical equipment. The SI unit is either mm2/s or m2/s, where 1 m2/s = 1,000,000 mm2/s. The SI units can be derived from the kinematic viscosity equation: The other common units used are centistokes (cSt) or stokes (St), where 1 St = 100 cSt Measuring Kinematic ViscosityGravimetric CapillaryThe most common method for measuring kinematic viscosity is the use of a gravimetric capillary (Figure 1) that is usually temperature controlled at 40°C and 100°C for multigrade oils, and only 40°C for single grade oils. Generally, measurements made using capillary viscometers rely on the relation between time and viscosity. The more viscous an oil, then the longer it takes to flow via a capillary under the influence of gravity alone. Currently, there are several standardized capillaries in use. Most the laboratory instruments use glass capillaries or “tubes”. A more recent development for field measure of kinematic viscosity utilizes a split aluminum cell capillary (Figure 2). Figure 1. Glass capillary types. 1. Ostwald (direct) 2. Ubbelohde (direct) 3. Cannon-Fenske (direct) 4. Houillon (Modified Zeitfuchs crossarm) reverse flow These instruments are designed to work either as direct-flow capillaries or as reverse-flow capillaries. For the direct-flow capillaries, the sample reservoir is positioned below the measuring marks. For the reverse-flow capillaries, it is located above the marks. Opaque liquids can be tested using the reverse-flow capillaries, and some can have a third measuring mark. Having three measuring marks provides two subsequent flow times and improves the measurement repeatability. Figure 2. Split aluminum cell capillary Common Types of Kinematic ViscometerManual Constant Temperature Bath SystemsIn these systems, the direct-flow capillaries are immersed in a highly accurate temperature controlled bath. A sample of oil, usually 10 ml, is suctioned into the tube until it reaches the start point. Then, the suction is released to make the oil flow by gravity through the controlled capillary section of the tube. Two or three marks are visible on the tube. An operator observes the oil meniscus as it passes the start point and then measures the time taken for the oil to pass the final mark. The tubes are chosen so that the test takes at least 200 seconds to complete, making it easier for manual timekeeping. ASTM D 445 is the technique for kinematic viscosity and was initially written for the manual method. Figure 3. Cannon Instrument Company constant temperature bath The advantages and disadvantages of the manual constant temperature bath systems are listed below: Advantages
Disadvantages
Automated Modified Ubbelohde MethodRelated StoriesAn automated modified Ubbelohde method is a common system used by laboratories. Here, a 10 ml bottle is placed in a small carousel rack. Like the manual method, the system draws oil up to the tubes. However in this method, a computer program controls all of the tasks, removing the requirement for an operator to monitor and time the oil flow. Figure 4. Automated Ubbelohde viscometer The advantages and disadvantages of the automated modified Ubbelohde method are as follows: Advantages
Disadvantages
Direct Flow CapillariesThese systems are highly favored for in-service condition monitoring, as they are more appropriate for opaque fluids. The lab versions exhibit higher throughput and flexibility. “Houillon” or “Hele-Shaw” are the common names for this method, and the ASTM that describes it is ASTM D7279. A common question posed by someone who is considering buying a viscometer is how does this method compare to ASTM D445, which is a widely acknowledged viscosity method. As ASTM D7279 exhibits exceptional repeatability, all that is needed to acquire results similar to ASTM D445 is a standard offset. For users who focus on changing trends, laboratory instruments designed using this method have exceptional accuracy and surpass machine condition monitoring needs. Figure 5. Spectro Scientific SpectroVisc Q300 kinematic temperature bath viscometer To make the measurement using this method, a small oil sample (between 0.6 - 1.6 ml) is pipetted and directly introduced into the tube, which is heated to the desired temperature. To reduce cross contamination, disposable pipette tips are employed. The following are the advantages and disadvantages of the direct flow capillaries: Advantages
Disadvantages
Portable, Solvent Free Direct Flow Capillary ViscometerWhen a kinematic viscosity result is required for field or mobile applications, a new generation of viscometers based on the Hele-Shaw split cell capillary design can be employed. A single heated aluminum block with a machined capillary ensures temperature controlled viscosity at 40°C without the need to use solvents for cleaning. As with lab systems, a 60 µl sample is pipetted and introduced into a temperature controlled cell, which is usually set at 40°C. Once the process is finished, the device directly displays the kinematic viscosity on the screen. After the test, the operator vigorously cleans the plates using a cleaning pad, and the cell can be warmed to test the next sample. The following are the advantages and disadvantages of the portable, solvent free direct flow capillary viscometer: Advantages
Disadvantages
ConclusionViscosity is a critical property of a fluid, and monitoring viscosity is highly essential to oil analysis. Kinematic viscosity measurement techniques for used oils should be investigated, and the differences between the methods should be considered. It is important that the details of viscosity measurements are understood so that accurate lubrication decisions can be made. While choosing an onsite viscometer, a user should not expect the laboratory’s kinematic viscometer to be in complete agreement with the onsite instrument, especially for field systems. Instead the conditions, the method, and the user environment should be considered. The user should review whether the solvents are difficult to obtain or maintain, and whether the equipment will be used routinely. It is advisable to baseline the new oil with the same viscometer that is used with the in-service oil. This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific. For more information on this source, please visit AMETEK Spectro Scientific.
Please use one of the following formats to cite this article in your essay, paper or report:
|