Gravimetric Capillary is the most common technique used to measure kinematic viscosity. It is temperature controlled, usually 40 ºC for single grade oils, and both 40 and 100 ºC for multi-grade oils. The relation between viscosity and time determines the measurements using capillary viscometers.
The more viscous an oil, the longer it will take to flow through a capillary under the influence of gravity alone. Several standardized capillaries are used today. Most lab instruments use ‘tubes’ or glass capillaries. A more recent advancement for field measure of kinematic viscosity uses a split aluminum cell capillary.
The instruments are engineered to operate as either reverse-flow capillaries or direct-flow capillaries. The sample reservoir is located below the measuring marks in direct flow capillaries, while the reservoir is situated above the measuring marks for reverse-flow capillaries.
Reverse-flow capillaries enable the testing of opaque liquids and some can have a third measuring mark. With three measuring marks, there are two subsequent flow times and improved measurement repeatability. The different types of capillary tubes can be seen in Figure 1.
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Figure 1. Common glass capillary types of kinematic viscometer. Glass Capillary Types (1) Ostwald (2) Ubbelohde (3) Cannon-Fenske (4) Houillon (Modified Zeitfuchs crossarm)
Manual Constant Temperature Bath Systems
There is a very precise temperature-controlled bath in these systems where the direct flow capillaries are immersed. An oil sample, generally 10 ml, is suctioned into the tube until it reaches the start point. The suction is released and the oil flows by gravity via the controlled capillary section of the tube. Two or three marks are visible on the tube.
An operator monitors the meniscus of the oil as it crosses the start point. At this point, the operator notes the time taken by the oil to cross the final mark. The tubes are selected in such a way that the test takes at least 200 seconds to complete. This makes manual timekeeping easier. ASTM D 445 was originally written for the manual method and is the method for kinematic viscosity.
The main benefit of the manual system is that it is considerably inexpensive compared to automatic versions. It is also fairly accurate due to the requirement of minimum 200 seconds test time. To maintain the test time requirement, different tubes are required for oils of different viscosity ranges and it is fairly easy to change the tubes in such manual systems.
The disadvantages of manual systems are that the tests are labor intensive and slow, and after the tests, the tubes have to be cleaned manually.
Automated modified Ubbelohde method
An automated modified Ubbelohde method is a common system used in laboratories. A 10 ml bottle is placed in a small carousel rack. Like in the manual method, the system draws oil up to the tubes, but all of the tasks in this case are controlled by a computer program.
In this system, an operator is not required to monitor and time the oil flow. The automation allows the system to maintain the accuracy of the manual system but eliminates labor for timing and tube cleaning. Systems can be fitted with dual solvent option for sooty oil samples that are hard to clean.
The drawback of such systems is that it is still slow - generally 12 samples per hour can be obtained with a 10 position carousel. The two tubes in the system are usually fixed in place so they are less flexible. A considerable amount of solvent is necessary to clean the tubes (up to 15 ml per sample) and 5 ml of oil sample per measurement.
Direct Flow Capillaries
As they are more suitable for opaque fluids, these systems are preferred for in-service condition monitoring, and the lab versions have higher flexibility and throughput. “Hele-Shaw” technique or “Houillon” are the common names for this method. ASTM D7279 is the ASTM method that describes this approach.
A common question for anyone considering obtaining a viscometer is how this method compares to ASTM D 445, a far more popular viscosity method. ASTM D 7279 has excellent repeatability, and a standard offset (detailed in the method) is the only requirement to obtain identical ASTM D445 results.
For most users who are focused on the trend change, laboratory instruments designed using this technique exceed machine condition monitoring requirements and have excellent accuracy. Measurements are taken using this technique by pipetting a small sample of oil (between 0.6 to 1.6 ml) and directly introducing it into the tube that is heated to the desired temperature. To minimize cross contamination, disposable pipette tips are used.
Conclusion
The high throughput of direct flow systems is one of their key advantages; with each bath holding up to 4 tubes and all measurement in parallel, 25 to 45 samples per hour can be easily achieved. Tubes are cleaned automatically, and dual solvent options are available for hard-to-clean sooty samples. A picture of Spectro Scientific SpectroVisc Q310 dual bath automatic viscometer is shown in Figure 2.
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Figure 2. Spectro Scientific SpectroVisc Q310 dual bath kinematic viscometer
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This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.
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