Viscosity is a measure of a fluid's resistance to flow. It is one of the most important properties of fluids and plays an important role in the petroleum industry.
The viscosity of crude oil affects our ability to pump it out of the ground; the viscosity and volatility of fuel affects how easily it atomizes in the fuel injectors; and the viscosity of a lubricant affects its ability to protect the engine.
It is an important measurement characteristic in food, paint, polymer coatings and other industries where convection flow is a critical product or use characteristic.
Small changes in viscosity can have a huge impact on the properties of petroleum fluids. The importance of kinematic viscosity measurement led the petroleum industry to develop an accurate method, first published in 1937, ASTM D445, Test Method for Kinematic Viscosity of Transparent and Opaque Liquids.
The term "kinematics" simply means that measurements are made while the fluid is flowing under the influence of gravity. It is determined by measuring the time required for a volume of fluid to flow by gravity through a calibrated glass capillary viscometer.
Although it sounds simple, achieving the high precision and precision required by the industry is an extremely difficult task. There are many factors that affect the accuracy of this test method.
From an economic standpoint, a one percent error in product viscosity that results in a blend adjustment can easily add a cent per gallon to the product cost. For a major lubricant manufacturer, this could result in lost revenue of $1 million or more per year.
temperature
As the saying goes, "The most important factor affecting a home's value is location, location, location." When it comes to viscosity measurements, "the most important factor affecting the quality of viscosity measurements is temperature, temperature, temperature."
Temperature control is the most important parameter to obtain accurate and precise kinematic viscosity measurements. This is especially true for petroleum products because their rate of viscosity change per unit temperature is significantly greater than for other products.
Therefore, small changes in temperature can have a large impact on the viscosity of a fluid. The most commonly measured bath temperatures of 40°C and 100°C must be controlled within +/- 0.02°C. This is a very tight window and great care must be taken to achieve this control.
There are many factors that affect temperature:
thermometer. When measuring viscosity, a specified thermometer or other temperature sensing device with specified accuracy and meeting the requirements of the test method must be used.
Reading a thermometer accurately to 0.01°C can be difficult, so using a magnifying glass is recommended. The thermometer must be immersed in the bath to the correct depth. Must be calibrated to +/- 0.02°C at least annually. In addition, the freezing point of the thermometer should be determined every six months and a correction factor applied.
The procedure for calibrating a thermometer is complex and is described in ASTM E77, Test Methods for Inspection and Verification of Thermometers. Because temperature control is such an important parameter, it is recommended to use two thermometers per bath.
The bath temperature is uniform and stable. Bath temperature uniformity and stability are other important parameters. The entire length of the viscometer must be maintained at the proper temperature. The type of circulator used, the age of the bath fluid, and the viscosity of the bath fluid affect temperature uniformity.
The viscosity of the circulator and bath fluid needs to be balanced to provide a uniform temperature throughout the bath. Bath fluids need to be changed completely before they start to change color, as discoloration usually indicates that the fluid has oxidized and the viscosity may have increased.
The bath should not be located close to ventilation openings, such as in a fume hood, as this often results in excessive temperature gradients in the bath. If a second viscometer is added to the test bath while another adjacent viscometer is being used for measurements, the temperature stability will be negatively affected.
illumination. While in the bath, the sample in the tube needs to have adequate illumination to ensure consistent visual inspection of the meniscus across the timeline. It is important to note that this lighting does not affect bath temperature control and stability.
These temperature requirements should not be taken lightly, as slight changes in temperature can cause large changes in the viscosity of some types of petroleum products.
Viscometer
All viscometers require fully annealed low expansion borosilicate glass for construction. There are many factors to consider when selecting the size of viscometer required for sample analysis. First, the viscometer must be calibrated to calculate the viscosity.
This procedure is described in ASTM D446, Specifications and Operating Instructions for Glass Capillary Kinematic Viscometer. Viscosity standards are used to determine viscometer constants. Although a single standard is sufficient to obtain a constant, using two standards covering the capillary range will produce a more reliable calibration constant.
Viscosity standards have an expiration date and should not be used beyond that date. It is possible to purchase a calibrated viscometer that has a constant at the appropriate temperature. Because for some types of viscometers, the viscometer constant changes with temperature, it is important to calculate the viscometer constant for the temperature of the analyzed sample. Viscometer constants should be verified at least annually.
The viscometer must be sized so that the flow time for manual determination is at least 200 seconds; otherwise eye-hand coordination will be a significant factor. The operator waits for the meniscus of the fluid to pass the timing line and measures the flow time using a clock or stopwatch. A flow time greater than 200 seconds is required to eliminate possible operator variation.
Obviously, a clean viscometer tube must be used for analysis, but what exactly does "clean" mean? The tube must be free of dust or other particles, and the liquid should clearly wet the glass surface.
Typically, it is sufficient to rinse several times with a sample solvent such as naphtha, followed by a dry solvent such as acetone, and then purge with a dry, dust-free gas such as air or nitrogen. The solvent must also dry without leaving any residue. This is especially of concern if commercial or utility grade solvents are used.
Residues of previous samples coated on the inside of the viscometer can be removed with a Chromic Acid 1 cleaning solution or a strong oxidizing cleaning solution that does not contain chromium. 2
Never use alkaline liquid cleaners or any cleaning solutions with a pH greater than 8.0 as they will actually change the size of the capillary by dissolving the glass walls, thus changing the calibration constants.
These types of fluids are commonly found throughout analytical laboratories and can be used for other applications, but if used to clean test tubes they will require recalibration of the test tubes. Another way to significantly change the minimum size viscometer calibration constant is to soak the test tube in boiling water overnight. The tube may be clean but will need to be recalibrated before use.
The tube must also be dry before use. It is recommended to use dry, dust-free air or nitrogen.
When suspended in a constant temperature bath, the pipe should be in the specified vertical position, free of vibration, and at the specified immersion depth. Follow the manufacturer's instructions and the instructions in the test method.
timing device
The timing device must be accurate to within +/- 0.07% of reading and must be able to read within a resolution range of 2,000 seconds or 0.1 seconds for one part for a flow time of 200 seconds. Care must be taken when using electronic timing devices as alternating current may not provide the required accuracy.
Sample processing
If particles are visible, a filtered sample should be collected. This is especially important when analyzing used oil. Because the sample should be free of air, it should be allowed to solidify to dissipate entrained air, if any. Just as corrosive cleaning fluids will change the calibration constants, so will corrosive (high pH) samples.
As the pH of the sample increases, the change in the constant increases faster. The smaller the capillary, the faster the constant changes. After analyzing a caustic sample, it may be necessary to recalibrate the test tube, although very caustic samples are not typically found in the petroleum industry.
Samples must be equilibrated at the required testing temperature. It may take up to 30 minutes, some materials may take longer.
Two flow time measurements should be obtained for each sample. If two determinations of kinematic viscosity calculated from flow time measurements are consistent with the specified measurability limits for the type of sample being tested, the average of the two determinations should be reported.
If inconsistencies are determined twice within the specified determinability range, the measurement must be repeated after the possible causes have been investigated and corrected. A dirty viscometer, incorrect bath temperature, or equilibration time may be to blame.
Automatic viscometer
Automated viscometers have been used in the petroleum industry for more than 30 years to determine kinematic viscosity D445. They simulate the physical conditions, operations, or processes of manual equipment and must meet the accuracy specified by the method.
All requirements except one of the above requirements apply equally to automated instruments. For automated instruments, flow time requirements exceeding 200 seconds were once relaxed. ASTM Subcommittee D02.07 Flow Characteristics, which is responsible for this test method, is currently determining the minimum allowable flow time for these automated instruments.
If the flow time requirement is indeed relaxed, two additional requirements will come into play. First, the calculation of kinematic viscosity from tube constants and flow times may require an additional term, the kinematic energy correction factor. This factor takes into account the energy exerted to accelerate the fluid as it flows from the reservoir to the capillary tube.
For longer flow times, this correction is negligible. Second, the timing device needs to maintain resolution and detection sensitivity of 1 part in 2000.
QC
Quality control samples should be used periodically to monitor the measurement system and verify that the results are within the accuracy of the test method.
Any sample that is similar in composition to those typically analyzed should be used. ASTM D6299, Standard Practice for Application of Statistical Quality Assurance Techniques to Evaluate Analytical Measurement System Performance, provides detailed information on quality control systems.
generalize
The test method for the kinematic viscosity of transparent and opaque liquids is one of the most critical tests in the petroleum industry. It has been used since 1937, but the method is constantly being improved. The technique seems simple, but many factors affect the accuracy of the measurement. Meeting all requirements for performing a test method will allow the laboratory to meet or exceed the specified accuracy.
Author's Notes
Chromic acid is hazardous to health and must be handled with caution. It is toxic, a recognized carcinogen, highly corrosive and potentially dangerous when in contact with organic materials.
Strongly oxidizing cleaning solutions that do not contain chromium are also highly corrosive and potentially dangerous when in contact with organic materials.
