How does a viscometer work? | Labomat
How does a viscometer work?
How does a viscometer work?
How does a Brookfield viscometer work?
The Brookfield viscometer is of the rotary type.
It measures the force required to rotate an immersed element (the mobile) in a fluid. The mobile is driven by a motor via a calibrated spring. The torsion of this spring is indicated by the position of a pointer on a circular dial or by a digital display. By using different speeds and different mobiles, a variety of measurement ranges can be traversed, thus allowing great versatility of the device. For a given viscosity, the viscous resistance, or resistance to flow, indicated by the degree of compression of the internal spring, is proportional to the speed of rotation of the needle. It is related to the size and shape of the mobile (its geometry). The resistance increases when the size of the mobile increases and / or when the speed of rotation increases. It follows that for a given mobile geometry and a given speed of rotation, an increase in viscosity will be indicated by an increase in the compression of the internal spring. For all viscometer models, the minimum range is obtained by using the largest mobile at the highest speed. The maximum range is obtained by using the smallest of the mobiles at the lowest speed. The measurements carried out with the same mobile at different speeds are used to detect and evaluate the rheological properties of the product tested. These properties and techniques are the subject of chapters 4 and 5. The viscometer is made up of several mechanical subassemblies. See Figure 3-1 for a schematic view of the main components of the analog viscometer. The drive motor and multi-speed transmission are located on the top of the unit inside the housing on which the metal identification plate is attached. The case contains a calibrated beryllium-copper spring, one end of which is connected to the pivot axis and the other directly to the dial. The dial is driven by the motor and in turn it drives the pivot axis through the spring
Schematic diagram of a Brookfield rotary viscometer
Brookfield analog viscometer Brookfield digital viscometer
Under the main housing is the pivot cup, through which protrudes the lower end of the pivot pin. The diamond tip inside the pivot cup rotates with the dial or transducer. The pivot axis is carried on this support by the pivot point. The end at the bottom of the pivot axis includes the mobile link on which the viscometer wheels are fixed
For digital devices, the synchronous motor has been replaced by a stepping motor. A torque transducer transforms the torsion of the spring into electrical tension which can then be converted into viscosity by the electronic card.
Viscosity measurement techniques
As with any precision device, proper measurement techniques improve the efficiency of the Brookfield viscometer. A step-by-step procedure is given in the user manual supplied with each device. Here we present advice and recommendations collected by our clients through 85 years of experience. They provide the solid foundation for a good viscosity measurement method and a starting point for exploring more advanced techniques.
What parameters should be specified when taking a viscosity measurement?
7 parameters must be specified in the method:
- Device type (LV, RV, HA or HB)
- Sample temperature
- Measurement time (the viscosity of the products often depends on the stirring time and therefore measurement)
- Beaker used & presence of the caliper (normally you should use the low size 600 ml beaker and the caliper, if this is not possible, specify that you have not put the caliper and use such reference of the beaker)
- Product history (a product which has just been stirred will not necessarily have the same viscosity as a product which has been at rest for several minutes or hours ...)
The mobile and the stirrup
Carefully inspect each mobile before use. If it appears corroded or damaged to the point of having changed its dimensions, it could be the cause of an incorrect viscosity measurement.
As all mobiles feature a glossy polished finish when new, any knock marks or apparent damage can be an indication for cell phone replacement. If you have particular problems with classic 300 series stainless steel mobiles, be aware that the same mobiles are available in 316 series stainless steel or in a Teflon coated version. Other materials can also be used.
When you attach the mobile to the viscometer, remember that it has a left-hand thread and that it must be firmly screwed to the axis. Slightly raise the coupling pin when you attach the mobile so as not to damage the diamond point or the pivot support. Once the mobile is fixed, be careful not to strike the mobile against the walls of the measuring container as this may distort the alignment of the axis.
We advise you to immerse and position the mobile before fixing it on the viscometer.
The caliper supplied with certain models protects the mobile and influences the measurement when checking the calibration with mobiles 1 and 2. The caliper must always be in position during measurements. If it becomes necessary or beneficial to use the viscometer without the caliper, this should be stated on the measurement report. Note: Calipers are only supplied with LV and RV models. The HA and HB models as well as the cone / plane models do not require a caliper. The caliper is also not used when the viscometer is supplemented with accessories.
Selection of mobile and speed
When you need to take a viscosity measurement according to a precise procedure or specification, use the rover and speed indicated after verifying that you are using the correct model of viscometer.
When you need to take a particular measurement, the best method of choosing speed and rover is trial and error. Its goal is to obtain a value on the display between 10 and 100 %. Remember that the precision is better when the value approaches 100 (see section 3.3.7). If the value is greater than 100, select a slower speed and / or a smaller rover. Conversely, if the reading is less than 10, select a higher speed or rover. If an approximate value for the viscosity of the fluid is known, a faster method of determining the correct rover / speed combination is to use the conversion chart (ruler of the rover / speed / model multipliers) supplied with each analog model or the AUTORANGE button on digital models. The goal is to select a mobile / speed combination whose min / max range frames the estimated value. For each mobile / speed combination, the maximum measurable value is equal to the conversion factor multiplied by 100. This maximum is also called “full scale range” (FSR in English). For the digital models which have the AUTORANGE key, select a moving part and a speed then press and hold this key down to display the FSR value in cP (or mPaS) on the display.
The recommended minimum measurable value is equal to the conversion factor multiplied by 10. For example, the spindle # 2 of LV viscometers at a speed of 12 RPM has a conversion factor equal to 25. The maximum measurable viscosity in this spindle configuration / speed is 25 X 100, or 2500 cP, the minimum measurable viscosity would be 25 X 10, or 250 cP.
Thus, if the viscosity of a product to be analyzed is estimated at 4000 cP, another mobile / speed combination must be chosen to perform the measurement. With a little experience, a quick glance at the conversion slider may be enough to select a suitable rover / speed combination. When multiple measurements are to be taken, the same rover / speed combination should be used for each measurement.
When measurements need to be taken at different speeds, select a moving body giving results in the min / max range for each speed selected. This can lead to measurements of less than 10%, which may be acceptable if one recognizes the low accuracy of such measurements. 3.3.4 Vessel size For measurements with standard Brookfield viscometers, we recommend the use of a vessel 83 mm in diameter or larger. The usual container is the Griffin 600 ml low size beaker.
Using smaller vessels will result in higher viscosity readings, especially with mobiles 1 and 2.
When using smaller vessels, the easiest method is to mention the vessel size on the measurement report and ignore the likely effects on the calibration. If the same type of container is used for subsequent testing, there is no correlation problem to expect. It is also possible to recalibrate the viscometer to compensate for the choice of using smaller vessels. It is also possible to consider the use of the SSA low volume sample accessory.
The sample should be free of any trapped air bubbles. Air can be released by lightly tapping the container on the table or by using an appropriate agitation or pumping system. The sample should be at a constant and uniform temperature. This can be checked by testing the temperature at different places in the container. Make sure that the temperatures of the product, the mobile and the caliper are at the same stabilized temperature before taking your measurement. The temperature homogenization can often be obtained by stirring the product before measurement. Make sure, however, that this prior agitation does not affect the viscosity of the product (see section 4.7.5). The conversion factors used to calculate viscosity from the readings obtained on the viscometer are independent of temperature. Thermostatic baths can be used to maintain the desired temperature. High temperature measurements (up to 300 ° C) may require the use of the Thermosel accessory.
Homogenization of the sample is also very important, especially for dispersions, where sedimentation is common. In most cases, simple agitation before measuring will restore dispersion.
The mobile must be submerged up to the middle of the mark engraved on the shaft. Failure to comply with this immersion depth may cause incorrect measurements. In some cases, the product can change its rheological structure when the mobile is immersed. To avoid this phenomenon, we recommend inserting the mobile at a place on the surface that will not be used for the measurement. The mobile will then be moved horizontally to the center of the container. This must be done before attaching the mobile to the viscometer.
Repeatability and precision
Brookfield viscometers have a guaranteed accuracy of + -1% relative to the full scale value of the moving / speed combination used. The repeatability of the measurement is given at + -0.2 %.
Brookfield analog viscometers have a needle directly connected to the calibrated coil spring. The reading is taken on a graduated scale from 0 to 100
The precision is therefore one graduation out of 100, including 1 % Full Scale Range FSR.
It is always this value of 1% of the full scale that must be taken into account to indicate a measurement accuracy.
- The full scale is calculated as follows: FSR = TK * SMC * 10,000 / RPM
- TK = Spring constant (for an RV: TK = 1)
- SMC = Mobile constant (The constants of each mobile are specified in the appendices of all the user manuals)
- RPM = Rotation speed (in revolutions per minute)
The full scale therefore depends on 3 parameters: the type of sensor (LV, RV, HA or HB), the mobile and the speed.
The full scale value is displayed on the viscometer (either when you choose a rover or a speed or using the green band at the bottom of the screen when you take measurements, or by pressing the Autorange key)
This is why it is advisable to adjust your choice of mobile and speed in order to have a % the highest possible (90%-> 1.1% relative error, 50%-> 2%, 25%-> 4%, 20%-> 5%, 10%-> 10%)
To increase the % torsion: increase the size of the mobile or the speed of rotation. When choosing the setting, you should ask yourself the right questions: how is my product used? E
In fact, there are Newtonian products whose viscosity does not depend on agitation, but most of the products are non-Newtonian and often shear-thinning: the more they are agitated, the more their viscosity decreases.
If you want to measure the product at rest: favor a large mobile and a low speed. If you want to measure the product in agitation: favor a small mobile and a high speed.
Get a viscosity reading
Before handling the viscometer, make sure that it is securely attached to the tripod and that it is properly level. Select a mobile / speed combination and attach the mobile to the viscometer.
Activate the rotation of the mobile and let it run until a constant measurement stabilizes on the display. Be aware that some overruns may be observed following the speed acceleration taken by the mobile when starting. Several procedures can be used in order to obtain a satisfactory reading.
In some cases, it is sometimes necessary to wait up to 5 minutes in order to achieve an apparent balance. Usually, it is sufficient to wait for a stable reading to appear on the screen for a few moments. A more reproducible procedure consists in defining a certain number of rotations of the mobile before taking the measurement. The time required to complete a certain number of revolutions varies significantly depending on the speed used. An alternative technique therefore consists in letting the mobile turn for a certain time rather than counting the number of turns. In some cases, a balance may not be found and the measurement will continue to oscillate between two values. This is often due to the presence in the fluid of an elastic component. If the measurement increases or decreases steadily, the fluid is probably a time dependent product, and requires special techniques in order to be measured effectively. See section 4.5. The percentage of torsion of the internal spring displayed on the screen by the device can vary from 0.1 or 0.2 % even after equilibrium is reached. In this case, just use the median value as an acceptable value.
Larger fluctuations may indicate particular conditions described above. When a valid percent twist measurement has been obtained, multiply that value by the conversion factor for the mobile / speed combination used. Note regarding conversion factors and measurement ranges: Both factor and range can be used to obtain a viscosity value (in centipoise) from the spring torsion percentage value. To obtain the viscosity measurement in centipoise, it suffices to multiply the measurement of % read on the display by the conversion factor.
For some Brookfield accessories, the measuring range is shown in place of the conversion factors. It is equal to the conversion factor multiplied by 100. So, to calculate the viscosity, just divide the indicated measuring range by 100, then multiply the reading by % by this value.
Users are often concerned with the accuracy and good working condition of their viscometer. Here are some simple tests to verify its mechanical performance:
(A) Variations in the frequency of the mains power supply may cause the mobile to rotate at an incorrect speed. Voltage variations have no effect as long as the variation is not greater than + - 10% value indicated on the viscometer identification plate and that the frequency remains constant. The other symptoms indicating a defective power supply are: difficulty of the motor when starting, jerky rotation of the mobile, very significant fluctuation of the dial pointer, inconsistency of the readings on the digital display.
(B) Damage to the pivot tip and pivot bracket will affect the accuracy and repeatability of the viscometer. The following oscillation test will allow you to assess the condition of these components:
1. The viscometer must be level, no moving device must be attached, the power switch must be OFF for analog models, the power switch must be ON and the motor switch must be OFF for viscometers. digital.
2. Turn the mobile coupling axis to deflect the measurement pointer or the digital display until a percentage of torsion of 5 to 10 %. Release and let the axis freely return to its initial position.
3. If the pointer or digital display swings freely without jerking, and returns to zero each time the test is performed, the pivot tip and pivot bracket are in good working order. If not, the viscometer may be out of specification and the device may require maintenance.
(C) We have never encountered a spring made of a beryllium / copper alloy that exhibits changes in characteristics due to fatigue or heavy use, even after hundreds of thousands of flexes. For this reason, checking the calibrated spring is not necessary. There is no zero adjustment required on analog models for this reason, so no external adjustment knob. The zero adjustment on some digital models has been made necessary to compensate for drifts due to the rise in temperature of certain electronic components.
(D) The use of a calibration oil is recommended to perform a final performance check. Measure the standard solution the same way you use to monitor any other fluid. Brookfield viscosity standard solutions (calibrated at + - 1%) are ideal for this test. The use of fluids other than standard solutions is not recommended due to the probability of unpredictable rheological behavior.
(E) If the viscometer passes all the tests described above, its performance can be considered satisfactory. If the accuracy or use of the device still seems suspect, refer to paragraph 3.5.
Recalibrate the Brookfield Viscometer
In many cases, it is impractical to use a low-size 600 ml beaker to take a measurement with the Brookfield viscometer. It may be preferable to use a different container if the transfer of the product is found to be messy or if it takes a long time. Sometimes, users prefer to use the viscometer without its yoke in order to avoid additional cleaning of this part. All of these practices involve recalibration of the device if accurate results are to be obtained.
If measurements have been taken under certain well-established conditions and you wish to establish a new reference point for that same product under new conditions, the following procedure may be sufficient:
1. Measure the product in the old container and in the new one and / or with the caliper in place or removed. Make sure you are using the same mobile and the same speed. Check that the temperature of the product is the same.
2. Note the new measurement. This is your new benchmark corresponding to your initial value. This procedure can be used to establish quality control methods and when the operator does not have to worry about the centipoise value of the material.
When your job requires knowing the actual centipoise value of the material, we suggest the following procedure if a different container is to be used or if you no longer wish to use the caliper:
1. Follow the procedure described earlier in this chapter.
Measure the viscosity of a Newtonian fluid using a standard 600 ml low size container. Brookfield standard oils are ideal for this. Take this measurement with care, as the accuracy of the final result depends on it. Multiply the viscometer reading (%) by the appropriate conversion factor to determine the viscosity of the fluid in centipoise.
2. Transfer the standard oil to the container you wish to use and for which the viscometer is to be calibrated. In case you only want to remove the caliper, remove it and keep the same container. Make sure the oil temperature is always the same.
3. Use the same mobile and measure the viscosity of the oil in the new container and / or without the caliper. Note the reading of % torque and speed S1.
4. The new measuring range is determined by the following formula: R1 = 100xviscosity / x where R1 represents the full scale range under the new conditions, viscosity is the measurement of standard oil viscosity measured in step 1 and where x is the measurement of torsion obtained in step 3.
To calculate the resulting new full-scale ranges with the same mobile used at different speeds under the same conditions, use the following formula: R1 / R2 = S1 / S2 where R1 is the range defined in step 4 for speed S1 (in RPM), and S2 the speed at which the new range should be calculated. 6. The multiplier coefficient (f) for the new conditions can be determined by the following formula: f = R1 / 100 where R1 is the range for the rover / speed combination as defined in step 4.
To calculate viscosity, multiply the reading by % obtained on the viscometer by the coefficient f.
Maintenance of the viscometer
Brookfield viscometers are very reliable provided they are handled correctly. Most problems are effectively detected through the calibration check described in paragraph 3.3.9. To avoid potential problems, it is worth remembering some points:
(A) The forces to which the viscometer reacts are extremely small. Optimal performance of the device depends on the elimination of any unnecessary friction that may affect its sensitivity. It means: Cleanliness. Care must be taken to prevent dust, fumes, liquids and all other forms of contamination from entering the interior of the viscometer housing. If it is necessary to use the device in such situations, it is recommended to use mobile extensions and / or purge systems to minimize the ingress of contaminants. Information on these accessories is provided in section 2.1.10.
(B) Never turn the device upside down while an uncleaned mobile is still attached.
(C) Never expose the viscometer to temperatures exceeding 75 ° C. When measuring products at high temperatures, it is recommended to use the mobile extensions or the Thermosel accessory.
(D) Avoid brutalizing the mobile connector by lateral blows or downward pressure which could damage the point and the pivot support. Always lift the mobile connector slightly to position or remove the mobiles. Do not hit the mobile against the walls of the container. Do not pull down on the mobile or the mobile connector.
(E) Do not drop or shake the device. The supplied Brookfield laboratory tripod offers a stable stand suitable for measurement. If the viscometer is to be used as a portable device, it must be returned to its transport case after use. If the viscometer is physically damaged or if it does not pass the oscillation test, it must be returned for repair to Labomat Essor.
The need for periodic preventive maintenance depends on the conditions of use. Under normal conditions, an annual overhaul should be sufficient to maintain the viscometer in its optimum condition for use. More brutal use may require more frequent revisions.
Specific diagnostic procedures are detailed in the user manual supplied with each model of Brookfield viscometer.
This section lists the most common problems you may encounter when using your viscometer, along with their probable causes and suitable solutions.
The mobile is not spinning
- Make sure the viscometer is connected to the mains.
- Check the voltage on your viscometer (115V, 220V); it must match the voltage of your power supply network.
- Make sure the power button is in the ON position.
- Make sure that the speed selector is in the correct position.
The rotation has a runout or the mobile seems twisted
Make sure the rover is securely attached to the viscometer rover connector.
Check that the axes of the other moving parts are all straight. Replace mobiles
Check the cleanliness of the screw thread of the viscometer mobile connector and that of the mobile.
Inspect the threads. If they are damaged, the device must be serviced.
Check whether the moving parts are turning eccentrically or whether they are out of round. A slight runout of 1 mm is tolerated on either side of the axis, ie 2 mm of total runout (horizontal measurement at the tip of the mobile during rotation in the open air).
Check if the viscometer mobile connector is bent. If necessary, the device must be returned to Labomat Essor for repair. If you continue to have problems with your viscometer, we recommend that you follow the diagnostic procedure in the instrument's instruction manual to isolate the potential problem.
Perform an oscillation test
- Remove the mobile and put the motor in the OFF position.
- Gently lift the mobile connector of the viscometer.
- Turn the connector until the red dial needle indicates 15-20 on the analog viscometer dial or when the display reads 15-20% for digital models Carefully release the connector
- Watch the red needle swing freely (or the display value for digital models) and return to zero position. If the pointer freezes, sticks or does not return to zero, the device needs repair.
Perform a calibration check
- Check the choice of model, speed and mobile used.
- Check the test parameters: container, volume, temperature, method.
- Perform a calibration check as described in the procedure described in the user manual of your viscometer.
- Check that the tolerances have been calculated correctly. Verify that the calibration verification procedures were followed exactly.
If the device is out of tolerance, it needs to be serviced.
For more information on the operation and use of a Brookfield viscometer, contact Labomat Essor
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