Viscosity measurement using Brookfield viscometers and rheometers

This category groups together our different solutions for characterizing the kinematic or dynamic viscosity, the rheological properties, the texture of liquid, pasty or powder products.

• Brookfield Viscometers
• Brookfield special configurations
• Brookfield Accessories
• Specific viscometers
• Brookfield rheometers
• Powder analyzer

What device should I use to measure a product?

A viscometer, a rheometer or a texturometer? Here are a few tips.

As part of the development of your new products, you ask yourself what material to use to describe your product. A viscosity cup, viscometer, rheometer, consistometer, penetrometer or texturometer.

Labomat Essor, a company specializing in the advice, sale, installation, calibration and maintenance of measurement and testing equipment since 1992, is the exclusive Brookfield representative for France, the Maghreb and French-speaking Africa.

This is why Labomat Essor brings you here its advice in the choice of material to use.

Viscosity, Rheology or Texture, how to navigate?

We call a Newtonian product, a product whose viscosity does not depend on agitation. These perfect fluids are generally quite rare, for example, water, standard oils, honey, etc.

However, most of the products are non-Newtonian, so we will find shear-thinning fluids (whose viscosity decreases when stirred), shearthickening fluids (whose viscosity increases when stirred), fluids with a flow threshold ( for which it is necessary to apply a force so that they start to flow), thixotropic products whose viscosity varies over time ... These non-Newtonian products represent the vast majority of products that are encountered in everyday life and for which they are shaken before use without even thinking about it. Non-Newtonian products are products whose viscosity depends on the conditions of agitation.

Brookfield viscometers and Brookfield rheometers simulate the behavior of the product under different agitations. They measure the dynamic viscosity of products. The dynamic viscosity of a product is expressed in pa.s or poises.

We speak of absolute viscosity when the measurement is carried out at a known shear rate (also called a speed gradient or shear rate) or at a known shear stress.

We speak of relative viscosity or apparent viscosity when we define the viscosity in function of the mobile / speed combination used. This is the case when using the standard mobiles supplied with the Brookfield viscometers and rheometers. The measurements being carried out in a beaker, we do not know exactly how far the disturbance caused by the rotation of the mobile will go so we do not know how to calculate at what shear rate we are working but we know under what conditions we have worked, the result is then relative to the mobile / speed combination used.

Thus with Brookfield viscometers it is possible to measure the viscosity of the products under different stirring conditions and to express the relative viscosity (for example 2500 cp measured at 25 ° C with the RV number 2 mobile at a speed of 10 revolutions per minute) or the absolute viscosity (e.g. 2500 cp measured at 25 ° C at a shear rate of 10 s-1

What difference can there be between a viscometer and a rheometer?

It's about the same difference as between a car and a sedan… Both are cars but one and more efficient than the other. Clearly, viscometers in general and Brookfield viscometers among others, are intended to measure the viscosity of a product under a limited number of stirring conditions.

This is why devices generally have a limited number of speeds, a few tens to a few hundred. The purpose of rheometers is to describe the evolution of viscosity under a multitude of stirring conditions.

Brookfield rheometers thus have thousands of speeds in order to be able to draw rheological curves called rheograms describing the evolution of the rheological properties of products as a function of agitation, of the speed of rotation, of the shear rate (these terms being homogeneous between them) or the force applied to them (shear stress). Rheometers also make it possible to perform mathematical calculations by applying mathematical models according to the laws of Newton, Casson, Bingham, Oswald, Herschel Buckley but also to determine the thixotropy index, the flow threshold, the flow index etc ...

While viscometers are used more in quality control, rheometers are mainly used in research and development.

Labomat Essor and Brookfield offer 3 main families of rheometers:

Fixed speed rheometers such as models DV2T and DVNext or CAP 2000+.

These imposed speed rheometers have hundreds or several thousand speeds. The principle is to impose, as on viscometers a speed of rotation or a shear rate and to measure the force of resistance of the product to the rotation of the mobile (or shear stress) then to automatically change speed in order to be able to build flow curves (rheological curves, rheograms)

Brookfield RST models at speed or constraint:

These rheometers impose a shear rate and measure a shear stress, or conversely impose a shear stress (that is to say a torque on the mobile) and measure the speed of rotation of the mobile.

Finally, there are oscillating rheometers which make it possible to determine the viscoelastic properties of a product.

Instead of having a rotating mobile and playing on the speed of rotation, Broofield RSO oscillating rheometers work by making the mobile oscillate. Brookfield RSO oscillating rheometers will then be able to work in amplitude sweep or frequency sweep.

It should be noted that the Brookfield RSO oscillating rheometers have a range of use much greater than those of the Brookfield RST imposed stress rheometers, itself still much greater than that of a standard Brookfield viscometer. The measurement range of a Brookfield RSO rheometer is 2000 times wider than that of a conventional Brookfield viscometer ...

Viscometers and rheometers are used to demonstrate the rheological properties of liquid or pasty products but are limited when it is desired to measure more complex products such as products having pieces.

For example, for a fruit jelly, it is very easy to measure its viscosity using a Brookfield viscometer equipped with the Helipath accessory, or even the rheological properties with an RST rheometers or even its visco-elastic properties at l. using an Oscillating rheometer, but to measure a jam with pieces it will be necessary to use a Brookfield model CT3 or model CTX texturometer.

Brookfield CT3 or Brookfield CTX texturometers are tensile-compression testing devices.

The principle is to apply a controlled strain on the product and measure its response in terms of force as a function of the strain, or to apply a force and measure the strain. There is a wide range of accessories and measuring mobiles to simulate the behavior of products. For example, we will find accessories of the butter cutting wire type to simulate the effort that must be produced to cut the butter. The applications are extremely varied, and the Brookfield CT3 or Brookfield CTX texturometers are widely used to test food products (fruits, vegetables, dairy products, processed products such as cakes, pastries, ready meals), cosmetic or pharmaceutical products, packaging, materials (grease, glues, rubber, etc.). This equipment makes it possible to reproducibly and objectively determine quantities such as: firmness, consistency, hardness, adhesiveness, elasticity, resilience, cohesiveness, apparent modulus, macability, elasticity, breaking point, gelling power, maturity, pour point, force stickiness, spreadability, applicability ...

Viscometers

To choose the type of viscometer to use, the following parameters must be taken into consideration:

- Measuring range

Brookfield viscometers are offered with different types of sensors: Low viscosity LV sensors which, with standard mobiles, allow to measure from 15 cp and up to a few million mpa.s (from a centi-poise with l 'Ultra Low Adapter accessory). RV sensors which measure average viscosities (from 100 cp and up to tens of millions of mpa.s), and HA or HB type sensors and their multiples for measuring very high viscosity products ... ). It is imperative to choose the right type of sensor supplied with the device according to the products you will have to measure because it is generally impossible or very difficult to compare results obtained with two devices equipped with different sensors. If a single sensor cannot cover the viscosity range of your products, then either purchase a second device or consider switching to a Brookfield RST rheometer or a Brookfield RSO oscillating rheometer.

- Sample volume

Brookfield viscometers are delivered with their stand, a transport case, a mobile case including standard mobiles, the measuring bracket, the user manual and the calibration certificate. Brookfield type LV viscometers are supplied with 4 standard mobiles LV1 LV2 LV3 and LV4, the LV5 mobile is optional. Brookfield type RV viscometers are supplied with the 6 standard mobile rovers RV2 RV3 RV4 RV5 RV6 and RV7, the RV1 rover is optional. These mobiles have been designed to allow the products to be measured directly in a beaker (ideally, a low-size 600 ml griffin type beaker should be used). This is called measuring Brookfield viscosity or apparent viscosity. It is therefore necessary to have sufficient quantities of product of the order of a few.

This is why Brookfield has developed a whole series of accessories for working on small quantities of products (SSA low volume adapter, ULA low viscosity adapter, Thermosel, cone-plane geometry). With these different measuring geometries or accessories, the quantities of sample required will range from 0.5 to 16 ml of product. The advantage of these measurement geometries is also that they can be used in research and development to determine the absolute viscosity of the product (that is to say at a known shear rate) and thus make mathematical models.

It is therefore possible with these accessories and with the Rheocalc T control software to transform a Brookfield DV2T viscometer used for quality control into a simple, efficient and economical rheometer, which will then respond to 90% market needs and which will allow you to complete the majority of your R&D projects.

- Temperature

Temperature is an essential parameter when performing viscosity tests. Indeed, even if the temperature does not influence the technical measurement of measurement in itself, it influences the viscosity of the product itself (for example the butter does not have the same viscosity when it is measured by it. out of the fridge or after half an hour of waiting at room temperature.). Brookfield DV1M viscometer models have an optional temperature probe, Brookfield model DV2T and DVNext viscometers and rheometers are standard temperature probes. In addition Brookfield offers a wide range of thermostatic or refrigerated baths allowing to go down to -20 ° C or Thermosel ovens allowing to work up to 300 ° C.

- Type of products

Brookfield viscometers can measure liquid products of course but not only:

Brookfield viscometers also allow you to work on pasty products, gels, products with particles by using the following accessories: the Helipath, the "spiral adapter" or the "vane spindles" which fit on viscometers and rheometers Brookfield standard.

- Speed or shear range

Like the measurement range in terms of viscosity, the shear range is an important parameter to be taken into account because it makes it possible to evaluate in which agitation range we want to measure the product. Thus, Brookfield offers the models of analog viscometers, DVE, DV1M, DV2T and DVNext which are low gradient viscometers (speed gradient between approximately 0.01 and 300 s-1), high gradient viscometers (cone viscometers plan ICI CAP1000 + or CAP2000 +) which allow to work up to 13300 s-1. Thus, for an architectural or decorative paint used in the building, the first devices make it possible to measure the viscosity of the product in the pot (at rest or in weak agitation), the second will make it possible to reproduce an application of the paint by roller ( typically 10,000 s-1).

Rheometers

Although sometimes used to take quality control measurements, these devices are most intended for Research and Development.

Fixed speed rheometers

The imposed speed rheometers are high-end viscometers which have a very large number of speeds (a few thousand) and a very wide speed range. They are generally used with control software (Rheocalc T for the DV2T and DVNext rheometers and Capcalc for the CAP 2000+) which also allows you to simply control the device, draw the curves and perform mathematical modeling. Brookfield thus offers 3 models of imposed speed rheometers: DV2T, DV3T or CAP2000

RST Brookfield imposed stress rheometers,

The RST Brookfield imposed stress rheometers are mainly used in Research and Development for very sensitive products to shear, to be able to test products over a larger shear range, to determine flow thresholds with great precision, to do on a single piece of equipment what could be done using several conventional Brookfield viscometers or finally for products with more or less large particles.

These Brookfield RST rheometers exist in 3 types of measurement geometry:

- Coaxial cylinders: The sample is placed in a cylindrical test chamber into which is introduced a rotating cylinder which is also cylindrical. This geometry is suitable for measuring liquid products.

- Cone / Plane or Plane / Plane: The sample is placed on a measurement plane, the measurement is carried out using a conical conical mobile (one then speaks of cone / plane geometry) or having the shape of a disc (we then speak of plane / plane geometry). The cone / plane geometry gives better precision because the speed gradient (shear rate) is homogeneous over the entire cone. The plane / plane geometry makes it possible to adjust the plane / plane distance and thus to measure samples having particles up to a few hundred microns.

- Vane Spindle: These are finned spindles made up of 2 perpendicular blades. This makes it possible to work on heterogeneous products with particles or pieces of up to a few mm.

Brookfield RSO Oscillating Rheometers

Finally, there are oscillating rheometers, reserved for the most advanced applications which make it possible to determine the viscoelastic properties of products, complex viscosities, complex complacency, shear moduli, phase angles or damping factors. They can be used in amplitude sweep or frequency sweep.

Texturometers

It is clear that Brookfield viscometers and rheometers are not able to characterize products as frequently encountered as bread, lasagna, jams with pieces, to know if the cheese is enough, if a lipstick is applied as it is necessary… In this case one speaks about measurement of texture.

Widely used in the food industry in particular, because the texture measurement contributes to the organoleptic properties of products, Brookfield model CT3 and model CTX texturometers are also widely used in the cosmetics, pharmaceuticals, packaging, the chemical industry ...

Can be used alone in quality control or to support you in your research by being controlled by computer via the Texture Pro software, the Brookfield CT3 texturometers exist in different measurement ranges. They are available with a whole range of sensors to choose from: from 0-100 g for the most sensitive and from 2 to 50,000 g for the firmest products. Brookfield CTX model texturometers are equipped with interchangeable sensors.

So with the only Brookfield CTX texturometer and several measuring cells we can measure a wide range of products that would have required several Brookfield CT3 texturometer. Texturometers should be supplemented with a sample table and / or test cells as well as a penetrating body. Brookfield therefore offers a very wide range of penetrating bodies: cylindrical, spherical, conical, wire, blade, plane, roller, needle, extrusion system.

Brookfield CT3 or CTX texturometers can thus be used for an extraordinarily diverse range of applications.

Other equipment for characterizing rheological properties: Rheometers for powder

Powders, like fluids, also have flow properties that must be able to be characterized. Unlike liquids which are incompressible, powders are compressible products. Thus the flow properties of the powders are a function of the compression rate to which the product is subjected. Thus, specific equipment such as the Brookfield PFT powder rheometer (also called the Brookfield Powder Flow Tester for powder) can be used to plot the flow curves of powders at different compression levels.

This makes it possible to determine very quickly and easily all the rheological properties of powders, but also internal or wall friction angles, normalized flow function, cohesive force, arch dimension, rat hole diameter, bulk density… All these quantities can be useful either in the development of the rheological properties of a powder but also to size: hoppers, silos, transfer pipes, etc.

Kinematic viscometers and falling ball viscometers

In industry, other types of viscometers are also very frequently encountered. These viscometers make it possible to measure the kinematic viscosity of products: for example capillary viscometers or viscosity cups. Viscosity cups, also called consistometric cups, exist in different models: AFNOR, ISO, DIN, FORD, LORY, ZAHN, FRIKMAR, etc.), the principle of which is to time flow time.

These simple measurements are mainly of interest for Newtonian products for which the viscosity does not depend on agitation, but also on non-Newtonian products in order to be able to carry out rapid and economical comparative tests in production.

Mention will also be made of the Brookfield model KF 40 falling ball viscometers, the principle of which is to measure the time of the fall of a ball in a thermostatic cylinder oriented at a defined angle and filled with the product to be measured. These tests can be carried out at an angle defined in standard DIN 53015 or at a variable angle, at different temperatures, with calibrated balls of certified density. The Brookfield KF 40 falling ball viscometers allow measurement according to the Hoppler method with great precision of Newtonian products with a viscosity between 0.5 and 70,000 mPa.s

How does a Brookfield viscometer work?

The operation of a viscometer is as follows:

A stepper motor drives the rotation of the mobile in the product.

Between the motor and the mobile, there is a calibrated spiral spring. The product, depending on whether it is more or less viscous, will apply a reaction force to the rotation of the mobile and will therefore more or less compress the calibrated spiral spring. The torque transducer transforms the torsion of the spring into electric current in order to display the viscosity on the screen of your viscometer.

So as you can see the measurement itself does not depend on the temperature. On the other hand, the product to be measured will be more or less sensitive to temperature. For example, if we take a yogurt, its viscosity will be generally the same at 5 ° C and at 20 ° C, which will not be the case at all for butter ... This is the reason why it is ideal to be able to check (using a thermostatic bath for example) the temperature of the samples or at least measure this temperature. The more temperature sensitive the product, the more it is necessary to tighten the tolerance in temperature regulation.

Measuring range and accuracy of a Brookfield viscometer

The repeatability of a device is of the order of 0.2 % The precision is 1% of full scale. Imagine the old analogue RVT viscometers for which a needle was connected to the calibrated spiral spring and for which the result was displayed on a scale graduated from 0 to 100

The precision was therefore one graduation out of 100, of which 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 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)

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? 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 low speed.

If you want to measure the product in agitation: favor a small mobile and a high speed.

How to check the calibration of your Brookfield viscometer

The calibration procedure is detailed in the appendices of each user manual.In a quick way, you have to use a low-size 600 ml beaker, place the caliper around the mobile, heat the oil on the caliper and the mobile to 25 ° C + -0.1% (in the case of Brookfield silicone standard oils which are certified at 25 ° C)

Take the example of an oil with a commercial value of 1000 cp. The value to be taken into account is that indicated on the certificate: for example 990 cp at 25 ° C The precision on the oil is then 1% of this value is 9.9 cp Measurement range and precision on the measurement

If you take the RV3 rover speed 100, then you will be able to measure between 100 and 1000 cp with an accuracy of + -10CP

Ditto for all the mobile & speed combinations indicated in the following table

So if you check the calibration with the RV1 @ 10RPM or RV2 @ 40RPM or RV3 @ 100 RPM or RV4 @ 200RPM rover, you will have a tolerance linked to the measurement of + -10 cp, i.e. a total tolerance of 19.9 cp ( 9.9 cp for oil and 10 cp for measurement) The device is compliant if the measurement is 990 cp + -19.9 cp i.e. between 970.1 and 1009.9 cp

What parameters should be specified when taking a viscosity measurement?

7 parameters must be specified in the method:

1. Device type (LV, RV, HA or HB)

2. Mobile

3. Speed

4. Sample temperature

5. Measurement time (the viscosity of the products often depends on the stirring time and therefore measurement)

6. 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. beaker)

7. 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…)

Why are my viscosity measurements not reproducible?

The first step is to check that the device settings allow the measurement to be taken with the expected precision. If you are 10% of twist don't be surprised to have random results around the expected value at + -10%, change your method to have a better % twist. For example at 80% of torsion, you will have measurements that will oscillate at + -1.2% around the target value ...

If the settings have been suitably chosen, if the calibration check shows that the device is compliant but that you do not have reproducible values then one of the 7 parameters exposed above has been modified. 'one measure to another ...

Labomat Essor is the official Brookfield distributor for France, the Maghreb and other French-speaking African countries