Viscosity is one of the most critical factors that directly influence the flow properties of a liquid. High viscosity fluids are more prone to deformation by stress and show resistance in flow. On the contrary, low viscosity fluids flow relatively quickly and have fewer stress deformation chances. The two main methods of measuring viscosity are dynamic viscosity and kinematic viscosity.
Viscosity is defined as the internal friction of a liquid. It represents the amount of electrical resistance existing between molecules. The fluid’s dynamic viscosity ratio to the fluid’s density at the same temperature is termed Kinematic viscosity. The unit is (m²)/s and is calculated by ν=μ/ρ
The force required to produce a unit flow rate using a unit area of a liquid layer per unit distance is termed Dynamic viscosity. The unit is pa.s and is calculated by μ=τ/(du/dy). Both viscosity types are considered indicators that assess lubricating oil viscosity. A direct relationship exists between dynamic viscosity and low-temperature fluidity, but kinematic viscosity shares an inverse association with lubricating oil viscosity.
Why measure kinematic viscosity?
A kinematic viscometer is widely used in viscosity measurement experiments. Refined oil is prepared by bleeding and stirring base oil and additives in lubricating oil. Varying grades of lubricating oils have different kinematic viscosities. Therefore, kinematic viscosity is a critical factor in lubricant categorization.
How to choose a fully automatic kinematic viscometer?
As kinematic viscosity is an essential technical property for oil and equipment lubrication management, what needs to be considered before choosing a fully automatic kinematic viscometer?
- The device should be chosen based on sample nature and offers a suitable viscosity tube. One major thing about an automatic viscometer is that it provides the user with various viscosity tubes which can be selected based on the requirement. Simply change the viscosity tube per the sample. The instrument has two optional sensors:
- Thermal sensor viscosity tube: suitable for opaque and transparent non-conductive samples, such as lubricating oil samples.
- Optical sensor viscosity tube: suitable for transparent and conductive fluids such as polymer samples
The instrument offers two sensor options for the homogenous nature of the sample, one is 100 times the normal viscosity tube, and the other is 20 times the Duplo viscosity tube. The 100 times viscosity tube is suitable for homogenous samples and has two different detections areas, namely Clower and Cupper, are present. Clower is for high viscosity sample detection, and Cupper is for low viscosity sample detection. The 20x Duplo viscosity tube is suitable for uneven samples with poor repeatability and uncertainty during repeated sampling. Its two detecting zones have the same viscosity tube constant, so one test yields two results.
- Choose the appropriate number of oil baths and viscosity tubes. The fully automatic viscometer run is run continuously for 24 hours, 365 days without interruption.
- The Single bath and single tube detect one temperature at once, and the maximum detection speed is 6 samples/hour.
- The Single bath and double tubes detect one temperature at the same time, and the maximum detection speed is 12 samples/hour.
- The Double bath and double tube detect two temperatures simultaneously, and the maximum detection speed is 12 samples/hour.
- The Two-bath and four-tube detect two temperatures simultaneously, and the maximum detection speed is 24 samples/hour.
3) Choose the device based on the temperature range. The temperature range of the automatic viscometer is 15–150℃, 0.01℃@15–100℃, 0.03℃@150℃
Sample preheating system:
The system’s function is to increase the temperature of the sample from ambient to the required temperature before injection. This is usually done with the sample exhibiting poor fluidity at a temperature which makes injection difficult.
Cooling Circulation System:
An external cooling circulation system is necessary to cool the constant temperature bath if the ambient temperature is higher than the detection temperature or the difference between them is less than 8°C. It reduces the degree of inaccuracy caused by ambient temperature. The automatic kinematic viscometer features a circulating cooling system joint that can be readily connected to an external cooling circulator,
What are the factors that affect the determination of kinematic viscosity?
1) Gauge factor:
A stopwatch is used to closely monitor and record the time the sample takes to flow through the viscometer. It’s essential to use a high-quality stopwatch to avoid experimental errors.
- Coefficient of viscometer:
Frequent Calibration of the capillary viscometer is a must to avoid errors.
2) Factors of test operation:
- Temperature accuracy of the test bath:
A constant temperature test is one of the essential conditions when measuring the kinematic viscosity of oil products. All the necessary guidelines for the test must be followed appropriately as the oil’s viscosity downgrades with an increase in temperature and vice versa, or else the results will have a significant degree of errors. The temperature control accuracy of the constant temperature bath must reach±1ºC according to GB/T265, and the temperature control accuracy of the bath in ASTMD445 is ±0.5ºC.
Time for oil to flow through the viscometer:
Sample flow time must lie within the specified range. The standard flow time is supposed to be > 200S. If, in any case, the flow rate of the oil exceeds the specified range, turbulency will be observed. Hence, making it difficult to calculate the viscosity using the Bersel equation. If the flow rate is very high, the reading error will be high too. A slow flow rate makes it difficult to maintain a constant temperature.
Other influencing factors
Air bubbles in the sample can affect the volume of the oil and result in the formation of a gas plug after entering the capillary. Consequently, the flow resistance of the oil will increase, and the results will be high.
The test oil sample may or may not contain impurities and moisture. If the capillary viscometer is not cleaned timely, the sample flow will be affected, and the result will be high. Because the viscometer’s washing solvent isn’t dry, it will dilute the sample, resulting in lower findings.
How to make portable kinematic viscometer measurements more accurate?
Viscosity measurement using a portable kinematic viscometer is going to give varying results. How to make the measurement more accurate?
- The performance indicators must be up to the standards. The device should be checked frequently to ensure measurement performance is within the specifications and whether the coefficient error falls under the given limits.
- The temperature of the liquid sample must be observed. If the temperature error is as small as 0.5 °C, the viscosity value of the liquid exceeds 5%. Therefore, the effect of temperature on viscosity is significant. The temperature should not exceed 0.1 °C for accurate measurement.
- The selection of the measuring container is important. Different rotors must correspond to the outer cylinders, or the measuring result would be severely distorted. When the single-cylinder rotational viscometer is concerned, the diameter of the outer cylinder must be infinite.
- The rotor selection must be made carefully. Adjust the speed such that the displayed value lies within 20–90 grids. Such instruments primarily use a dial and pointer for readings; therefore, the errors are summed up to 0.5 grids. The relative error is considered to be small (5 grids) when it’s 10%, and to reduce the relative error even by 1%, the readings must be 50 bars. If the indicator value is greater than 90 grids, the torque created by the hairspring is too great, which could cause creep. Therefore, the rotor and speed must be properly set.
How to confirm the thermal equilibrium time with a fully automatic kinematic viscometer?
The time taken by a viscometer to heat up after the oil sample is sucked into the viscometer tube is termed thermal equilibrium time. It ensures that the oil sample has achieved test temperature, and an average oil hardly takes 120–150 seconds to reach the required temperature. As discussed previously, viscosity and temperature have an inverse relation. Therefore, an increase in viscosity results in loss of oil fluidity. The smallest temperature variation can result in large changes in viscosity. Because the oil sample is not fully heated to the test temperature, shorter thermal equilibration times still result in high repeatability, but accuracy diminishes. The unnecessary detection time will be extended if the thermal equilibration time is designed too long.
The following steps can do optimization of thermal equilibrium:
1) Fill the sample pan with 5 identical oil samples;
2) Set the thermal equilibrium time as 900 seconds, 450 seconds, 225 seconds, 150 seconds, or 120 seconds.
3) Start the oil sample queue and wait for the device to finish analyzing all of the samples.
4) Determine the thermal equilibrium time at which the viscosity increase.
4) Find the thermal equilibrium time at which the viscosity value increases. This is because the oil sample is not entirely heated to the prescribed temperature. The viscosity of the oil sample at a low temperature is lower than that of the oil sample at a perfectly constant temperature. If the oil had sufficient time to heat up to the required temperature, the viscosity would remain unchanged.