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The process of choosing a spindle and rotational speed for use with your Brookfield viscometer is ultimately one of trial and error. However, there are several means to narrowing the possibilities before any testing is performed. If a sample has historically been tested using a particular speed and spindle combination, the user should use that same combination. This will help to ensure test results are accurate and repeatable. The objective is to choose a spindle that will produce a dial or digital display reading between 10% and 100% torque.
The maximum viscosity range of a spindle running at a given speed is equal to the spindle factor multiplied by 100. This is the full scale range for that spindle/speed combination. The minimum viscosity that can be measured is equal to 10 times the factor, or one tenth of the full scale range. Knowing the full scale range, you can then determine if your fluid fits within the capabilities of that spindle/speed combination.
Additionally, digital viscometers equipped with the AUTO RANGE key allow the full scale range to be obtained automatically. Enter a spindle code and an RPM setting and press the AUTO RANGE key. The viscometer will calculate and display the full scale range viscosity for that spindle and speed combination.
If the reading is below 10% or above 100%, the user should choose a different speed to obtain a reading in the recommended range. If changing speeds doesn't provide readings between 10% and 100%, the user should try another spindle. Generally, if the reading is above 100% at the lowest speed, the next smallest spindle should be used. If the reading is below 10% at the highest speed, the next largest spindle should be used.
To test a fluid at multiple speeds, choose a spindle that will produce readings between 10% and 100% for at least three speed settings.
Viscosity standards are used to check the accuracy of your Brookfield viscometer. Brookfield Silicone and Mineral Oil Standards provide you with a fluid viscosity value that is constant at 25°C, making calibration verification easy to establish. Standards are a traceable part of calibration verification - an important component of many certifying quality systems like ISO, ASHTO, etc.
Generally one or two is sufficient. Select a fluid with a viscosity value that fits the measuring range of the spindle and speed that you use most often.
This choice is dependent on your product. Silicone is used most often because it is less temperature sensitive. Some manufacturers, such as the paint industry, cannot have silicone in their process, so mineral oil becomes the choice.
Brookfield has specific recommendations:
Brookfield has a wide variety of silicone and mineral oil viscosity standards.
The viscosity of your sample fluid is typically of little importance when checking the accuracy of your Brookfield Viscometer, because:
Your Brookfield Viscometer/Rheometer is guaranteed to be accurate to +/- 1.0% of full scale range in use. You can check the accuracy of your Viscometer by performing a calibration check.
You must use:
Instrument Accuracy is 1% of Full Scale range in use (FSR) and is added to Standard Fluid Accuracy which is 1% fluid value in cP.
FSR is the factor for the spindle and speed in use multiplied by 100; FSR can also be found on digital models, by selecting a spindle, speed and then pressing the AUTO RANGE button; the screen will then display a cP value @ 100% (of FSR); 1% of that displayed FSR is instrument accuracy.
Fluid value is on the label of the jar, above the word "value", and is stated in cP(mPa·s). Calculate 1% of this value.
Test your Brookfield Viscometer/Rheometer with a Brookfield Viscosity Standard Fluid at 3 different rotational speeds to verify how the instrument responds when sensing low, medium, and high % torques.
RV Viscometer with RV #3 spindle at 20, 10, and 5 rpm
Brookfield Viscosity Standard Fluid: Nominal value = 5000 cP; Actual value = 4850 cP
|rpm||% trq||FSR (1%)||Fluid (1%)||+/-|
Viscosity results obtained from Brookfield instruments are reproducible regardless of whether an instrument is a dial reading or digital. Assuming that all other conditions are identical - spindle, speed, temperature, container size, spring torque, etc., a reading produced on a dial reading viscometer is as valid, accurate, and reproducible as a reading produced on a digital viscometer.
Generally speaking, viscosity has an inverse relationship with temperature. As temperature increases, viscosity decreases. For this reason, it is critical to control the temperature of a sample during any viscosity measurement.
Brookfield temperature accuracy is +/- 1 °C up to 150 °C, and +/- 2 °C above 150 °C up to 300 °C. This accuracy applies for RTDs in DV-I+ w/Temperature Probe, DV-II+, Programmable DV-II+, DV-II+ PRO, DV-III, DV-III+, Thermosel w/Model 74,75, 85 and 106 Controllers, CP/CPE-44PY cups, SC4-**RPY Chambers.
Thermal equilibrium is the state at which every element of a viscosity measurement is at a unified temperature. These elements include, but are not limited to, the spindle, guard leg, sample container, sample, and temperature probe. Because temperature is critical to accurate readings, you should allow sufficient time to achieve equilibrium. In general, one hour is the minimum time to wait. The more viscous the material being measured, the longer you should wait.
Accuracy is calculated as 1% of the full scale range (FSR) of the viscometer. FSR is defined as the highest achievable viscosity reading with a given spindle and speed. The easiest way to determine FSR is by pressing the AUTO RANGE button on your digital viscometer. Pressing this displays FSR for the spindle and speed entered. Taking 1% of this value gives you instrument accuracy. For dial viscometer users, multiplying your factor by 100 gives you FSR. Consequently, you can see that your factor is equal to your instrument accuracy.
Accuracy of the Brookfield CAP viscometer is based on two separate factors. The first factor is the accuracy of the instrument and the second is the accuracy of the calibration fluid. The total accuracy is the instrument accuracy plus the fluid accuracy.
Instrument accuracy is determined by consulting Table 3.1 in the CAP viscometer manual. The operator must determine whether the instrument is a high or low temperature CAP, and whether it is a 1000 or 2000 model. Cross referencing that information with the cone-spindle number in use, they will determine a number on the chart. This number is the percent of the full scale range that must be considered as the instrument accuracy.
For example: A CAP 1000H with cone spindle 3 at 900 rpm.
The operator must first determine the full scale range of the CAP for these conditions. By pressing the RPM button, the display will show 100% = X, X being the value in Poise of the full scale range. Under these conditions, FSR is 8.333 Poise. From Table 3.1, the percent of FSR calculated for accuracy is 2 or 4%, depending on whether the calibration fluid is greater or less than 50% of FSR.
If our calibration fluid is 4 Poise, we would calculate accuracy as 2% of FSR, or 2% of 8.333. Instrument accuracy would thus be .1666, rounded to .17 Poise. The fluid accuracy is 1% of 4 Poise, or .04 Poise. Thus total accuracy is .04 + .17, or .21 Poise.
Most fluids demonstrate a "non-Newtonian" behavior that prevents them from ever displaying a constant centipoise value or dial reading. For more information on Newtonian and Non-Newtonian fluids, consult our free publication More Solutions to Sticky Problems or see our Viscosity Support section.
The spindle may be bent; check the straightness against the dimensions in Appendix A of our publication of More Solutions To Sticky Problems. The jewel bearing, pivot point and shaft may be worn or bent; perform the calibration check to determine if the viscometer is still performing within calibration. The viscometer is in need of service and recalibration.
Various math models were empirically developed by different researchers, in order to fit trends they saw in their own data sets. The Math Model is a "best fit" line that can be used to characterize data from a specific test and has the advantage for easy data storage and comparison with similar tests by looking at slope and y-intercept.
There are some simple models that provide a reasonable fit to many data sets, and have parameters that have some meaning to various practitioners - whether those people are researchers, QA/QC people, or process engineers. The modified Casson model, for example, works well for testing chocolates, among other things. The Herschel-Bulkley model is useful for materials that have a yield point and then "shear-thin" after yielding. This may be good for "gel-like" materials, for example.
Brookfield's "Analysis" module in our applications software (Rheocalc and Wingather) lists the curve-fit parameter results, along with a "Coefficient of Fit". One could try a few of our models, and select the one with the best "CoF", for example - the closer to "1.00", the better the fit. It may be tough to accurately predict product behavior. Conservatively used, rheology models may be good for interpolating apparent viscosities at various shear rates, for example. Nonetheless, some models may be used to extrapolate yield stresses, for example, at shear rate values of zero. The more data points taken, the more reliable the fit.
Please also bear in mind that a certain model may have been used just because it's "simple" and "good enough for a reasonable estimate". That does not mean that it would be the best model to use. One example is in the Petroleum industry: "drilling mud" rheology has been examined with the Bingham model for years, despite the fact that these materials are highly non-Newtonian! The Bingham model assumes Newtonian behavior after yield. Nonetheless, field personnel felt it is was "good enough" to give them an idea of how the material was handling. More practitioners have started using the "H-B" model during the past few years.