How to choose the right rotational viscometer?

In the world of laboratory testing, viscometers are fundamental tools for assessing fluid flow behavior. Viscosity measurement results provide data support for product quality control, process scale-up, raw material acceptance, and formulation development. Rotational viscometers are widely used because they can give comparable measurements for both Newtonian and non-Newtonian fluids. If the selection is not improper, the comparability and reliability of subsequent measurement data will be reduced. The following is an explanation from seven aspects: sample characteristics, model range, temperature control, rotor and speed combination, standard use, data comparison principles, and maintenance points.

1. Preliminary screening according to sample characteristics

The physical and chemical properties of the sample directly determine the available measurement methods. First, the uniformity of the sample is evaluated: a standard rotor can be selected for a single-phase transparent liquid; Suspensions containing particles or settling should consider a spiral rotor or the addition of a rotor guard. Second, the thixotropic or shear thinning behavior of the sample is evaluated – the viscosity of such a sample can change with measurement time or speed, so the rotor speed and reading time in the measurement program need to be fixed.

The amount of sample is also a practical constraint. Conventional rotor requires 400~600 mL sample volume; If the sample is only available at a single time of 8~13 mL, a small sample adapter can be used. For ultra-low viscosity samples below 15 cP (e.g., some solvents or light oils), where the readings are low on standard rotor measurements, an ultra-low viscosity adapter is recommended that increases the measurement torque at low viscosity by changing the rotor geometry. In addition, samples containing volatile components (e.g., organic solvent-based adhesives) should be equipped with solvent shields to prevent the viscosity increase caused by component volatilization during the measurement process.

For samples that are prone to oxidation or hygroscopicity at high temperatures, such as certain resins or syrups, a nitrogen purge interface can be used to form an inert gas shield on the surface of the sample.

2. Select the model and range according to the viscosity range

Rotary viscometers are divided into four main models according to spring torque, covering low, medium and high viscosity ranges respectively. The principle of correct selection of the model is to estimate the viscosity value of the sample at the measurement temperature so that it falls within the range of 10%~90% of the selected model. If the viscosity of the sample is close to the lower limit of the model range, the measured torque percentage is low (less than 10%), and the reading error is large; If the range exceeds 90% and is close to full torque, the instrument may be in a high load state for a long time, which may affect the spring accuracy.

It should be noted that the ranges in the table above are obtained using the most sensitive rotor (usually rotor No. 1) in combination with the lowest speed. The actual measurement range will be extended to the lower viscosity area as the number of rotors increases and the rotation speed decreases. Conversely, larger rotors or higher speeds can be used to measure higher viscosity samples. There is an overlap between the ranges, for example, the low-range part of the RV type and the high-range part of the LV type overlap, and both models can be selected within this range, but the LV type has higher accuracy in the low viscosity section.

For samples with unknown viscosity, it is recommended to use the "rotor exploratory method": start with the smallest rotor, start at the lowest speed, observe the torque percentage, if it is less than 10%, switch to a smaller rotor or reduce the speed until the torque enters the range of 10%~90%.

3. Temperature control method and accuracy requirements

Viscosity is nonlinearly related to temperature. Under conventional laboratory conditions, a temperature fluctuation of 1°C can cause a change in fluid viscosity ranging from 2%~10% (depending on the sample type). Therefore, temperature control is a link in viscosity measurement. When selecting a thermostat, determine the desired temperature range:

For the usual temperature zone of -20~95°C, it is recommended to use a circulating thermostatic bath with a jacketed sample cup. The temperature control accuracy of the bath should be within ±0.1°C. The thermostatic liquid (water or water-glycol mixture) is circulated through the jacket, allowing the sample to maintain a uniform temperature during the measurement. This method is suitable for the analysis of most oils, resins, coatings, food and other samples.

For the high temperature zone of 100~250°C, the circulating water bath cannot reach this temperature range, and an electric heater can be selected. The device directly heats the sample cup by means of a heating rod or heating jacket and is equipped with a temperature sensor for feedback control. When using, pay attention to the high-temperature stability of the sample and avoid decomposition or cross-linking reactions.

For ultra-low viscosity samples (<15 cP) or where tight temperature control is required, the Peltier temperature control system is also available, which is faster to ramp up and down and does not require circulating liquid, but is more expensive. If only the rapid comparison measurement near the room temperature is carried out, and the laboratory ambient temperature is stable within the range of 23°C±1°C, the external temperature control equipment can be temporarily not configured, but the actual temperature at the time of measurement needs to be recorded.

In addition to considering the temperature range, the selection of temperature control devices also needs to pay attention to whether the sample cup is suitable for the rotor type used. Low-volume sample adapters typically come with a jacket or electric heating interface that can be connected directly to external temperature control devices.

4. Selection strategy of rotor and speed

The same viscometer can be configured with multiple rotors (4 commonly and 6 or more in some models) as well as multiple speeds (from 0.3 rpm to 100 rpm). The combination of different rotors and speeds determines the actual range of the instrument. Follow these steps when choosing:

Step 1: Estimate the viscosity of the sample. Approximate values can be obtained by consulting the literature, previous test data, or simple outflow methods (e.g., coating-4 cups). Step 2: Check the "Rotor/Speed Combination Range Table" in the instrument manual to find the combination that makes the expected viscosity value fall within the range of 30%~70% of the range. The measurement linearity of this interval is good. Step 3: For non-Newtonian fluids (thixotropic, pseudoplastic, or expansive), the measurement conditions need to be fixed. Because the viscosity values measured at different speeds are different. In the quality inspection standard, it is usually stated that "using a size X rotor, at Y rpm, the reading after Z minutes is measured". For the R&D phase, viscosity values at multiple speeds can be further measured and rheological curves can be plotted to understand the non-Newtonian properties of the sample.

Rotor geometry also needs to be noted: cylindrical rotors are suitable for low-viscosity liquids, disc-shaped rotors (such as #2~#4转子 in the LV series) are suitable for medium-viscosity samples, and paddle-type rotors are suitable for high-viscosity pastes. For samples containing particles, the gap between the rotor and the sample cup wall should be greater than 5 times the particle size, otherwise the particles may get stuck in the gap, causing the reading to jump.

5. Selection of viscosity standards

Viscosity standards are reference substances for verifying the accuracy of instruments and transmitting measurement values. When selecting, three conditions need to be matched: the viscosity range of the model, the conventional measurement temperature, and the expansion uncertainty of the nominal value of the standard liquid. For example, for LV viscometers commonly measured in the range of 1~100 cP at 25°C, silicone oil-based standards with nominal values of 5 cP, 10 cP, and 50 cP can be selected.

Standards are divided into Newtonian fluid type (silicone oil, mineral oil) and aqueous solution type (sucrose solution, etc.) according to the matrix. Silicone oil-based standards have good stability and are not volatile, and are suitable for the calibration of most rotary viscometers. The aqueous solution standard solution needs to be freshly prepared and pay attention to mildew prevention. The expiration date of the standard solution is usually 6~12 months after opening, but it is recommended to use it within 3 months after opening in actual use.

Usage steps: Inject the standard solution into the sample cup or measuring vessel, maintain a constant temperature to the specified temperature (±within 0.1°C), and measure according to the rotor and speed combination required by the standard liquid certificate. If the relative deviation between the measured value and the nominal value is within the range of the nominal accuracy of the instrument (such as ±1%), the instrument is normal; If the deviation exceeds the allowable range, the rotor should be cleaned, recalibrated or contacted for maintenance. Note: Shake well before each use of the standard solution (avoid bubbles from vigorous shaking) and record the actual temperature.

Standard storage: After opening, the cap of the standard liquid bottle should be tightened and placed in a cool and dry place. Avoid long-term exposure to air to prevent dust from falling or solvent volatilization that can lead to increased viscosity. Mixing standards of different nominal values is not recommended.

6. Differences in selection between R&D and quality control scenarios

Laboratory quality control (QC) focuses on fast, repeatable pass/no-go judgments. In QC scenarios, you can choose a basic viscometer, which has a fixed speed and torque display. A common practice is to establish standard operating procedures for a product, specifying the model, rotor number, speed, temperature and qualified range. QC personnel follow the operating procedures without frequent adjustment of parameters.

Product development (R&D) scenarios require more flexible instruments. The touch screen viscometer can store multiple sets of measurement programs, support automatic gradient speed testing, and display the viscosity curve directly. R&D personnel often need to compare the rheological behavior of different formulations and different shear rates, so it is recommended to choose a data output viscometer with continuous speed change function to facilitate subsequent analysis.

If data comparison is required between different factories or upstream and downstream suppliers, special attention should be paid to the selection of the same model (at least the same torque level), the same rotor number, the same speed setting and the same temperature control conditions. For example, both sides use RV models, No. 4 rotor, 20 rpm, 25°C constant temperature. If the model is different (one uses LV, the other uses RV), even if the same sample is measured, due to the difference in instrument spring torque and rotor geometry, the results lack conversion formulas, and it is not suitable to compare them directly. In this case, third-party standards can be used for conversion, but the steps are more cumbersome.

7. Calculation and representation of measurement results and data recording

The measurement of a rotary viscometer is calculated as a percentage of torque. The general calculation formula is:
Viscosity (cP) = Torque Percentage × Rotor Factor × Speed Correction Factor
The torque percentage is directly displayed by the instrument, and the rotor factor and speed correction factor can be found in the instrument manual. Some of the new viscometers display the final viscosity value directly on the screen, eliminating the need for manual calculations.

The following information should be included when recording data: sample name and batch, measurement temperature, relative humidity (for hygroscopic samples), rotor model, rotational speed, torque percentage (if available), final viscosity value and units, measurement date and operator. For non-Newtonian fluids, it is recommended to supplement the time required to record the stabilization of the reading. Each report should indicate the viscosity standard used and the date of last calibration.

8. Maintenance and precautions

Viscometers are precision mechanical-electronic instruments with major maintenance points such as rotors, jewel bearings and springs. After each use, the rotor should be cleaned with a suitable solvent (which does not react with the sample and is easy to volatilize) to avoid dry solidification of the sample. Do not use metal brushes or hard objects to scratch the rotor surface during cleaning, so as not to change the geometric size and surface roughness of the rotor. The rotor is dried with a soft cloth or special dust-free paper and stored in the rotor box to prevent collision and deformation.

The instrument host avoids severe vibration. When replacing the rotor, the viscometer lifting arm should be lifted first, and the rotor should be gently screwed into the connecting rod, and it should not be pushed in. After the rotor is installed, gently pluck the rotor with your fingers to check if the rotation is smooth. If there is a stuttering or friction sound, it may be a bearing or jewel bearing offset, and it needs to be checked by a professional. Once the measurement is complete, remove the rotor or long-term suspension may cause asymmetric wear of the bearing.

It is recommended that the viscometer be calibrated once a year, which should be performed by a qualified institution. In daily use, it can be quickly verified with standard solution every month, and the verification data can be recorded. If the verification deviation continues to exceed the accuracy range of the instrument, maintenance should be arranged.