Measurement of liquid viscosity

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Measurement of liquid viscosity Viscosity is an important physical property of fluids. Viscosity measurement is closely related to industrial technologies such as petroleum, chemicals, and metallurgy; it is also widely used in fields such as biology and medicine. Methods for measuring liquid viscosity include the capillary method, rotational method, falling ball method, vibration methods (torsional oscillation method and vertical vibration method), plate method, etc. In this experiment, the rotational method was used to measure the viscosity of salt lake brine and pure water. [Experimental objectives] 1. To understand the principle of measuring liquid viscosity using the rotation method ； 2. Master the measurement of liquid viscosity using a rotational viscometer. [Experimental principle] Viscosity is divided into dynamic viscosity and kinematic viscosity; generally, dynamic viscosity is simply referred to as viscosity. When a fluid flows, there are velocity differences between flow layers, and motion is transmitted layer by layer. When there is a velocity difference between adjacent flow layers, the faster flow layer attempts to accelerate the slower one, while the slower flow layer tries to slow down the faster one. This interaction intensifies as the velocity difference between the strata increases. This property of fluids is known as viscosity, and the force of interaction between fluid layers is called internal friction or viscous (drag) force. Dynamic viscosity is a physical quantity used to indicate the degree of viscosity of a fluid; it is defined as the ratio of shear stress to shear rate in a steady laminar flow (1-1). The unit of dynamic viscosity is pascal-second, denoted as . In practical applications, kinematic viscosity is often measured directly, and it is defined as the ratio of dynamic viscosity to the density of the fluid (1-2). The unit of kinematic viscosity is square meter per second, denoted as , (GB3102). 3) It is also used in actual work. The rotating cylinder method apparatus consists of two concentric cylinders with different radii. As shown in Figure 1, the outer cylinder is a hollow cylinder. The liquid with the viscosity to be measured is filled between the inner and outer cylinders. When an external force causes one of the cylinders to rotate at a constant speed while the other remains stationary, a velocity gradient is created within the liquid over the radial distance between the two cylinders, and thus internal friction is generated in the liquid. Due to internal friction, a shear stress is applied to the rotating cylinder; by measuring this stress, the viscosity of the liquid can be calculated. If the radii of the inner and outer cylinders of a coaxial configuration are r and R respectively, the immersion depth of the inner cylinder is h, and the angular velocity of the rotating cylinder is ω, assuming that the fluid flows in a laminar manner, that both cylinders are infinitely long, and that there is no sliding at the interface between the fluid and the cylinders, then the torque exerted on the stationary cylinder due to the internal friction within the fluid, when one of the cylinders rotates while the other remains stationary, can be derived as given by equation (1-3) or (1-4). As can be seen from equation (1-4), when the space between the two cylinders contains the fluid to be tested, and if the torque M, the immersion depth h, the angular velocity ω, as well as the radii r and R of the inner and outer cylinders can be determined, it is possible to calculate the viscosity of the fluid. Figure 1: Schematic diagram of the rotating cylinder method. Figure 2: Structural principle of the rotational viscometer. Experimental instruments and apparatus: As shown in Figure 2, when measurement is carried out using the rotational method, a synchronous motor rotates at a constant speed; it is connected to a scale disc, and through a hairspring and shaft, it drives the rotor to rotate. If the rotor is not subjected to fluid resistance, the hairspring, pointer, and dial rotate at the same speed, and the reading indicated by the pointer on the dial is “O”. Conversely, if the rotor is subjected to the viscous resistance of the liquid, a torque is generated in the spring, which counteracts this viscous resistance until equilibrium is reached; at this point, the pointer connected to the spring indicates a certain reading on the scale dial (i.e., the torsional angle of the spring). Multiplying the reading by a specific coefficient gives the viscosity of the liquid (mPa.s). The experimental setup is shown in Figures 3 and 4. Figures 3 and 4 1. Main technical specifications of the NDJ-1 rotational viscometer: Viscosity measurement range: Measurement error: (for Newtonian liquids) Rotors available: Five rotors numbered 0, 1, 2, 3, and 4. Rotation speeds: Four options – 60 r/min, 30 r/min, 12 r/min, and 6 r/min. Power supply: AC220V±10%, 50HZ. 2. The rotational viscometer used in experiments comes equipped with 5 rotors; the relevant data are shown in Tables 1 and 2. It is possible to select an appropriate rotation speed and rotor based on the approximate viscosity range of the liquid being measured. To achieve higher measurement accuracy, it is best to read values within the range of 20–80; otherwise, it is necessary to change the rotor or the rotation speed. Table 1 Table 2 Experimental steps] 1. Prepare the liquid to be tested (brine and pure water), place it in a container with a diameter of at least 70 mm and a height of at least 130 mm, or in a straight-tube container, and accurately control the temperature of the liquid to be tested. 2. Install the protective frame on the instrument (turn it to the right to install, turn it to the left to remove). 3. Screw the selected rotor onto the connecting screw (turn it to the left to install, and to the right to remove). Rotate the elevation knob to lower the instrument slowly, allowing the rotor to gradually sink into the liquid being tested until the rotor’s level marker is level with the liquid surface; then adjust the instrument to be horizontal. Press the pointer control lever to turn on the motor switch, rotate the speed control knob to increase the desired speed until it aligns with the speed indicator, then release the pointer control lever to allow the rotor to spin in the liquid. After several rotations (usually 20–30 seconds), wait for the pointer to stabilize (or take a reading after the specified time). Press the pointer control lever (Note: ① Do not use too much force ； ②When the rotation speed is low, it is possible to avoid using the control lever and read the value directly; fix the reading, then turn off the motor so that the pointer stays within the reading window, allowing the value to be read. 4. When the indicated value is too high or too low, the rotor and speed can be adjusted to ensure that the reading falls within the range of 20 to 80 marks. 5. To test the attachment using a size 0 rotor and a low-viscosity fluid, follow the steps below. (See Figure 4) ① Install rotor No. 0 on the connecting screw (rotate it to the left to install it). ② The sleeve is fitted over the cylindrical part at the bottom of the instrument and tightened with the sleeve fixing screw. ③ Fill the fixed sleeve (with a bottom) with 20–25 ml of the liquid to be tested. ④ Slide the outer test cylinder into the fixing sleeve and tighten it with the test cylinder fixing screw. Testing can be carried out after controlling the temperature of the liquid being tested. ⑤ When using an open-bottom external test tube, the procedure is essentially the same; once the external test tube and the rotor are submerged in the liquid, the groove ring on the external test tube serves as the liquid level line. 6. Selection of range, coefficient, rotor, and speed: ① First, estimate the viscosity range of the liquid to be measured, and then select an appropriate rotor and speed based on the range table. ② When the approximate viscosity of the liquid being tested cannot be estimated, it should be assumed to be high, and rotors ranging from small to large as well as speeds ranging from slow to fast should be tried. The principle is that for liquids with high viscosity, a smaller rotor and a lower rotation speed should be used; for liquids with low viscosity, a larger rotor and a higher rotation speed should be used. ③ Coefficient: The reading indicated by the pointer on the dial at the time of measurement must be multiplied by a specific coefficient from the coefficient table in order to obtain the measured absolute viscosity (MPa·s). That is, η = k.α, where: η = absolute viscosity, k = coefficient, and α = the reading indicated by the pointer (deflection angle). Notes: 1. Please read the manual carefully before use and follow the operating procedures strictly. 2. The instrument must be measured within the specified frequency, voltage, and tolerance ranges; otherwise, it will affect the measurement accuracy, and corrections must be made using the appropriate correction formulas if necessary. 3. Care should be taken when loading and unloading the rotor. When installing or removing it, lift the connecting screws slightly while performing the operation; do not use excessive force, and avoid applying lateral forces to the rotor to prevent it from bending. 4. The motor must not be started without pressing the pointer control lever, and the speed must not be changed while the motor is running. 5. Turn on the motor start switch; if the motor fails to start promptly, turn the switch off immediately and try turning it on again. 6. When lifting or lowering the instrument, support it with your hand to prevent it from falling due to its own weight. 7. After installing the No. 0 rotor, do not “rotate” it without liquid to avoid damaging the shaft tip. 8. The measuring container and rotor should be cleaned after use and kept clean and dry.