Basic knowledge of viscometer classification and application

Viscosimeter (Viscosimeter) An instrument used to measure the viscosity of fluids (liquids and gases). Viscosity is a physical quantity that expresses the internal friction that occurs inside the fluid when the fluid is flowing. It is the ability of the fluid to resist deformation and is an important indicator for identifying certain finished or semi-finished products. Viscosity varies from fluid to fluid and varies with temperature. There are mainly three types of capillary viscometers, rotational viscometers and falling ball viscometers.

Definition Instruments used to measure fluid viscosity Representative capillary viscometer, rotational viscometer, etc.

Applications Viscosity measurement of real fluids

Involved subject Mechanics

Viscosity concept

When a liquid flows, the property of internal friction between its molecules is called the viscosity of the liquid. The viscosity is expressed by viscosity, which is used to characterize the resistance factor related to the properties of the liquid. The viscosity of insulating oil is the same concept as the viscosity of general liquid, which is the internal friction of liquid, that is, when the insulating oil moves in relative laminar flow under the action of external force. The property of insulating oil molecules to generate internal frictional resistance. The greater the internal friction of the insulating oil, the greater the viscosity, the more difficult it is to flow, and the poorer the heat dissipation performance.

There are many ways to express viscosity, which can be roughly divided into two categories: The viscosity measured directly according to the definition of viscosity is called “absolute viscosity”, such as dynamic viscosity, kinematic viscosity, etc. If the measured viscosity is compared with a liquid of known viscosity under certain conditions, it is called “relative viscosity” or “conditional viscosity”, such as Engel’s viscosity.

According to the measurement method, the viscosity is generally divided into three types: dynamic viscosity, kinematic viscosity and Engel’s viscosity.

A viscometer is a physical property analysis instrument for measuring fluid viscosity. Viscosity is a physical characteristic of a fluid substance, which reflects the internal friction between molecules when the fluid is subjected to an external force. The viscosity of a substance is closely related to its chemical composition. In industrial production and scientific research, the composition or quality of substances is often monitored by measuring viscosity. For example, in the production process of polymer materials, the viscometer can be used to monitor the viscosity of the synthesis reaction product and automatically control the reaction end point. Others such as petroleum cracking, lubricating oil blending, automatic control of the production process of certain foods and drugs, monitoring of crude oil pipeline transportation process, quality inspection of various petroleum products and paints, etc., all need viscosity measurement.

Instructions

1. The machine must be kept level
2. Avoid air bubbles when the rotor is placed in the sample, otherwise the measured viscosity value will decrease. The way to avoid it is to place the rotor obliquely into the sample, and then install the rotor. The rotor cannot touch the wall and bottom of the cup. The sample to be measured must not pass the prescribed scale.
3. When measuring different samples, the rotor must be kept clean and dry. If there are other samples or water remaining on the rotor after cleaning, it will affect the accuracy of the measurement.
4. The maximum acidity (PH) cannot exceed 2. If the acidity is too high, a special rotor should be selected. When using ULA, the sample volume should be determined (only 16ml)
5. Select the viscosity standard liquid according to the measured viscosity range, and verify the instrument before each use of the viscometer or rheometer, or periodically calibrate to ensure the accuracy of the measurement. Newtonian fluids in various viscosity ranges can be provided Silicone oil or oil standard product with high precision, the accuracy is ±1%, and the recommended service life of viscosity standard liquid is one year from opening.
6. When connecting the rotor, use your left hand to gently lift and pinch the mandrel (on the host), and rotate the rotor with your right hand. This operation is to protect the mandrel and hairspring in the fuselage, which can prolong the service life of the instrument
7. The value should be selected when the value is relatively stable, otherwise there will be a large error in the obtained value
8. When choosing a rotor, it depends on the viscosity of the sample to be measured and which rotor has the closest measurement range, so choose the number.

Main category

1. According to the operation mode: capillary type, rotary type and vibration type. Capillary Viscometer:

The working principle of the capillary viscometer is to judge the viscosity of the sample by the time it takes for the sample to flow through the container. The absolute value of the measured value is called dynamic viscosity, which is widely used in petrochemical fields.

A capillary viscometer, usually a Saybolt viscometer, is a common viscometer. Its working principle is: the sample container (including the outflow capillary) is filled with the sample to be tested, placed in a constant temperature bath, and the height of the liquid column is h. Open the cock, the sample begins to flow to the liquid receiver, and at the same time start counting the time until the sample liquid level reaches the scale line. The more viscous the sample, the longer this time. Therefore, this time directly reflects the viscosity of the sample.

Rotational viscometer: The common rotational viscometer is the cone and plate viscometer. It mainly includes a flat plate and a cone plate. The motor drives the plate to rotate at a constant speed through the variable gear, and the sample to be tested is kept between the two plates by capillary action, and the cone-plate is driven to rotate by the friction between the sample molecules. Under the action of the torsion spring in the torque detector, the cone plate rotates at a certain angle and stops rotating. At this time, the torque applied by the torsion spring is related to the molecular internal friction (ie viscosity) of the sample to be tested: the greater the viscosity of the sample, the greater the torque. There is a variable capacitor inside the torque detector, and its moving piece rotates with the cone plate to change its own capacitance value. The torsion spring torque reflected by this capacitance change is the viscosity of the tested sample, which is displayed by the meter.

The measurement principle of the rotary viscometer: a rotor is driven by a spring plate to continuously rotate in the fluid, and the viscosity is judged by the degree of twist of the spring. It should be noted that the viscosity range required to be measured by a rotary viscometer is related to the size and shape of the viscometer rotor and the rotational speed. Rotational viscometers are the most commonly used viscometers in laboratories.

Vibration viscometer: The working principle of this viscometer is: when the object in the fluid vibrates, it will be hindered by the fluid, and the size of this effect is related to the viscosity of the fluid. Commonly used vibrating viscometers are ultrasonic viscometers, and there is a shrapnel in the detector. When excited by the pulse current, the shrapnel produces mechanical vibrations in the ultrasonic range. When the shrapnel is immersed in the tested sample, the amplitude of the shrapnel is related to the viscosity and density of the sample. In the case of known density, the viscosity value can be obtained from the measured amplitude data.

The vibration sensor of the vibratory viscometer emits a certain frequency, and the viscosity is converted by the change of the vibration amplitude or the vibration amplitude of the sensor is kept consistent by changing the change of the driving force, and the viscosity is calculated by calculating the change of the driving force. Due to the different shapes and vibration modes of vibration sensors, there are several types of vibrating viscometers.

The falling ball viscometer is based on the Hoeppler measurement principle, which is simple and accurate for dynamic viscosity measurement of transparent Newtonian fluids. The core idea is to measure the time it takes for a falling ball to fall through a sample filling tube tilted at a working angle under the action of gravity.

The Ford cup viscometer is manufactured in accordance with the American Society for Testing and Materials paint and raw material standards, and is used to measure the viscosities of inks, coatings, paints, etc. The viscosity of the sample is measured by measuring the time required for a certain volume of the sample in the aluminum cup to flow out from the small hole at the bottom. It is widely used in some countries in Europe and North America. The Ford cup is refined from a high-quality aluminum cup with a capacity of 100ml.

2. According to the working mode: offline viscometer (sampling test), online viscometer (24 hours continuous measurement), portable viscometer.

3. According to the type of measurement product:

Solder paste viscometer: The solder paste viscometer adopts a coaxial double cylinder rotary viscometer with a screw pump sensor, which is mainly used to measure solder paste, film thickness paste, adhesive, solder paste anti-solder paint, liquid anti- Solder paint, other inks, pastes, etc. According to the airtightness of the measurement part, it has the ability to adjust the temperature, and it can be connected to a personal computer for automatic measurement and data reading for automatic calculation.

Portable viscometer: The portable viscometer adopts coaxial double cylinder type, and the helical linear sliding method pump sensor maintains a fixed flow, and can measure the viscosity of the viscous body accurately according to the change of the connection stirring viscosity.

Viscosity controller: The viscosity controller can well manage the viscosity of various fluids such as coatings, inks, adhesives, food, medicines, oils, etc.; it adopts a coaxial double cylinder type; Fixed flow, stable numerical value can be obtained; Viscosity of viscous body which changes continuously by stirring viscosity can be measured accurately.

Stormer viscometer: Stormer viscometer is a test instrument used to measure the viscosity of paint and other coatings expressed by KU value. The design basis of the instrument is ASTM standard and GB9269-88 standard. Tianjin Jingke Material Testing Machine Factory is the largest enterprise specializing in the production of paint, coating, ink equipment material testing machines and special equipment for coating industry laboratories. The viscometer adopts a single-chip microcomputer, and the operator can directly read the KU value of the test sample from the instrument without looking up the table.

Main application

In actual engineering and industrial production, it is often necessary to detect the viscosity of the fluid online to ensure the best process operating environment and product quality, thereby improving production efficiency. Through the online measurement of the liquid viscosity in the process, the data of the rheological behavior of the liquid can be obtained, which has important guiding value for predicting the process control of the product process, the conveyability and the operability of the product during use. The properties of the liquid are often related to other properties of the product such as color, density, stability, changes in solid content and molecular weight, and the most convenient and sensitive way to detect these properties is to detect the viscosity of the liquid online. During the production process according to On-line viscosity detection within the scope of process technology requirements can minimize the scrap rate of products and the downtime of production lines.

Precautions

According to its measurement principle, in order to obtain accurate and reliable measurement data, the following points must be paid attention to:

1. The performance indicators of the instrument must meet the requirements of the National Metrology Verification Regulations. The instruments in use should be checked periodically, and if necessary (the instrument is frequently used or in a qualified critical state), an intermediate self-inspection should be carried out to ensure that its measurement performance is qualified and the coefficient error is within the allowable range, otherwise accurate data cannot be obtained.

Second, pay special attention to the temperature of the measured liquid. Many users ignore this point and think it doesn’t matter if the temperature is slightly different. Our experiments have proved that: when the temperature deviation is 0.5°C, the viscosity value deviation of some liquids exceeds 5%. The temperature deviation has a great influence on the viscosity, and the viscosity decreases when the temperature rises. Therefore, special attention should be paid to keep the temperature of the measured liquid constant near the specified temperature point, and it is best not to exceed 0.1°C for accurate measurement.

3. Selection of measuring container (outer cylinder). For the dual-cylinder rotational viscometer, it is necessary to read the instrument manual carefully. Different rotors (inner cylinders) match the corresponding outer cylinders, otherwise the measurement results will deviate greatly. For a single-cylinder rotary viscometer, in principle, the outer cylinder radius is required to be infinite, and the actual measurement requires that the inner diameter of the outer cylinder, that is, the measuring container, is not less than a certain size. For example, the NDJ-1 rotational viscometer produced by Shanghai Tianping Instrument Factory requires that the diameter of the measuring beaker or straight container is not less than 70mm. Experiments have proved that, especially when the No. 1 rotor is used, if the inner diameter of the container is too small, it will cause a large measurement error.
4. Correctly select the rotor or adjust the speed so that the indication value is between 20 and 90 divisions. This type of instrument adopts dial plus pointer to read, and its stability and reading deviation are combined to have 0.5 divisions. If the reading is too small, such as around 5 divisions, the relative error caused is more than 10%. If you choose a suitable rotor or speed to use If the reading is at 50 divisions, then the relative error can be reduced to 1%. If the indication value is above 90 grids, the torque generated by the hairspring will be too large, which will easily cause creep and damage the hairspring, so the rotor and speed must be correctly selected.
5. Frequency correction. The nominal frequency of domestic instruments is 50 Hz, and the power supply frequency in China is also 50 Hz, and the variability is less than 0.5% when tested with a frequency meter, so frequency correction is not required for general measurement. However, for some instruments in Japan, Europe and the United States, the nominal frequency is 60Hz, and the frequency must be corrected, otherwise there will be a 20% error. The correction formula is: actual viscosity = indicated viscosity × nominal frequency ÷ actual frequency
6. The depth of the rotor immersed in the liquid and the influence of air bubbles. Rotational viscometers have strict requirements on the depth of the rotor immersed in the liquid, and must be operated in accordance with the instructions (some double-cylinder instruments have strict requirements on the amount of liquid to be tested, which must be measured with a graduated cylinder). When the rotor is immersed in the liquid, there are often air bubbles. Most of them will float up and disappear after a period of time after the rotor rotates. Sometimes the air bubbles attached to the lower part of the rotor cannot be eliminated. The existence of air bubbles will bring large deviations to the measurement data, so tilt Slowly dipping the rotor is an effective method.
7. Cleaning of the rotor. The measuring rotor (including the outer cylinder) should be clean and free of dirt. Generally, it should be cleaned in time after measurement, especially after measuring paint and adhesives. Pay attention to the cleaning method, soak it in a suitable organic solvent, and never use a metal knife to scrape hard, because serious scratches on the rotor surface will cause deviations in measurement results.
8. Other issues requiring attention.
9. Most instruments need to adjust the level, pay attention to the level problem at any time after replacing the rotor and adjusting the height of the rotor and during the measurement process, otherwise it will cause reading deviation or even fail to read.
10. Some instruments need to be equipped with a protective frame, read the instructions carefully and install according to the regulations, otherwise it will cause reading deviation.
11. Determine whether it is an approximate Newtonian fluid. For non-Newtonian fluid, the rotor, speed and rotation time should be specified after selection, so as not to misunderstand that the instrument is not accurate. To sum up, although the rotational viscometer is simple in structure and easy to use, if it is not used correctly, a qualified instrument cannot obtain accurate measurement results, which will affect product quality.

Installation attention

1. Care should be taken when loading and unloading the rotor. When loading and unloading, the connecting screw should be lifted slightly for operation. Do not use too much force or force the rotor laterally to avoid bending the rotor.
2. Please do not place the viscometer with the rotor on its side or upside down.
3. Keep the connection end face and thread of the connecting screw and rotor clean, otherwise it will affect the shaking degree of the rotor.
4. When lifting the viscometer, hold it with your hands to prevent the viscometer from falling due to its own weight.
5. After replacing the rotor, please input the new rotor number in time. After each use, the replaced rotor should be cleaned (wipe clean) in time and put back into the rotor rack. Please do not leave the rotor on the instrument for cleaning.
6. When changing the measured liquid, please clean (wipe clean) the rotor and the rotor protection frame in time to avoid measurement errors caused by the confusion of the measured liquid.
7. The instrument and the rotor are one-to-one matching, please do not confuse several instruments and rotors.
8. Please do not disassemble and adjust the instrument parts at will.
9. When moving and transporting the instrument, install the yellow cap on the connecting screw, tighten the screw, and put it in the packing box.
10. After installing the rotor, please do not rotate it for a long time without liquid, so as not to damage the shaft tip.
11. Many of the suspensions, emulsions, polymers and other high-viscosity liquids are “non-Newtonian liquids”. Inconsistency with the results measured under the time is normal and not an instrument error. For the determination of non-Newtonian liquids, the rotor, rotational speed and time should generally be specified.
12. Doing the following points will help to measure more accurate values:

Precisely control the temperature of the measured liquid.

Immerse the rotor in the liquid to be measured for a long enough time to make the temperature of the two consistent.

Maintain the homogeneity of the liquid.

When measuring, place the rotor in the center of the container, and be sure to install the rotor protection frame.

Ensure the cleanliness and shaking of the rotor.

When the high speed measurement immediately changes to low speed, the machine should be turned off, or the measurement time at low speed should be a little longer, so as to overcome the error caused by the rotational inertia of the liquid.

Use No. 1 rotor when measuring low viscosity, and No. 4 rotor when measuring high viscosity.

When the viscosity is measured at low speed, the measurement time is relatively longer.

During the measurement process, due to the need to change the rotor, the liquid to be measured, etc., after changing the position of the viscometer by rotating the lifting chuck, the horizontal position of the viscometer should be checked and adjusted in time.