FAQ Viscometry

Frequently Asked Questions

Viscometry

1.1. In general

• Why should I even measure the viscosity as a processor of polymers?
  During processing, polymers are damaged to a higher or lesser extent, depending on the conditions (temperature, shearing forces). This changes the quality of the final product, e.g. the mechanical characteristics such as strength and toughness. The polymer decrease can be determined by assessing the viscosity of polymer solutions, where e.g. the viscosity number or the intrinsic viscosity are used as a measuring number for the final result. The solution viscosity is therefore an important metric for the quality control of plastics.
• Can I evaluate the measured results by means of a software program?
  For the AVS®370 and the AVS®Pro, a software evaluation is possible. For the work with dilution viscosimeters with AVS®370, a special software for the automatic measurement of serial dilutions is available.
• Which prerequisites should I arrange in the laboratory for the capillary viscosimetry?
  Depending on the planned viscosimetry system, you must ensure sufficient space on a laboratory bench or a similar surface. The device should be protected from dust, as dust particles will impair the measurement in the viscosimeter. A connection to running water close by is only required in case of counter-cooling of the thermostat bath, but recommendable in any case. Compressed air is not required for SI Analytics systems. Depending on the application, a suction/exhaust system is recommendable or even required, if you are working with chemicals that are hazardous to your health (e.g. if you are measuring PET solutions). For manual measurements and cleaning, a vacuum pump (the simplest: water jet pump) should be available for drying. The chemicals needed for the sample preparation, must be disposed of properly following the measuring process. Please contact SI Analytics or our sales partners for concrete questions: www.si-analytics.com/kontakt.html
• Can the bath temperature be regulated/controlled by means of software?
  With the AVS®Pro III, the temperature is regulated via the software, with the AVS®370 (WinVisco) and AVS®470, a temperature regulation is not possible.
• Can the AVS measuring systems be connected to the LIMS?
  The WinVisco software for the AVS®370 and the software for the AVS®Pro III can be connected to LIMS.
• Which PC requirements does the WinVisco software have?
  PC with operating system Win 7 or Win XP as well as an available USB or COM interface
• What does suction via N1 mean?
  N1 is the designation of the top measuring level, therefore corresponds to the top ring marker on viscosimeters for manual measurement.

1.2. Application

• Which devices are suitable for sample preparation?
  For polymer analytics, the plastic samples must be dissolved in suitable solvents. Here, you must observe the applicable standard: For example, polyamide is often measured as per the standard ISO 307, PET as per ISO 1628-5. In all cases, a certain concentration is started, e.g. 0.5 g in 100 ml of solution. In order to precisely start this concentration, you will need a scale for analytic purposes (accuracy ± 0.1 mg) and a volumetric flask (e.g. 50 ml). The starting of the samples can be simplified if the solvent is added (metered) via a flask burette (e.g. Titronic® 500). This allows for variable sample weights, while a certain plastic quantity must be weighed for a process with volumetric flasks - you will only need normal sample bottles for this. In many cases, 100 ml screw bottles have been used here, sometimes, smaller sample bottles of 50 or 40 ml have been sufficient. Magnetic stirrers or shakers are required for the dissolution process. Some samples (e.g. PET) must be dissolved at higher temperatures (100°C - 125°C), so that at least one heatable magnetic stirrer (a heating stirrer block would be better) is required..
  
• How many different solvents can I use with one device?
  For the AVS®370 and AVS®Pro, you can rinse with a max. of 2 different solvents. In general, the first solvent must be a good solvent for the sample previously measured in the viscosimeter. The second solvent is used for drying and must therefore be volatile (e.g. acetone or dichloromethane): It will rinse out the residue left behind by the first solvent and will be removed during the subsequent drying process by an air stream or a vacuum by evaporation. The second solvent does not need to be a solvent for the measured sample, if the sample residue was already be removed by the first solvent.
• How can I avoid the generation of bubbles in Ubbelohde or micro Ubbelohde viscosimeters?
  The main cause for bubbles with Ubbelohde viscosimeters: At the end of a measurement, a small volume of the sample remains in the capillaries. When the sample is repumped into the measuring sphere, air bubbles may form from this sample residue and the air above the level vessel. With the software-controlled devices AVS®370 and AVS®Pro III, the capillary can be blown out prior to a measurement by a brief puff of air - this will prevent these bubbles from forming.
• When does the measurement take place with suction or pressure, respectively?
  The classic standard measuring mode is the pressure operation. It is still used often today for non-critical samples (non-toxic, not aggressive). In case of a malfunction, the liquid is pressed out of the capillary pipe and then usually flows into the thermostat bath. Therefore, the suctioning operation is recommended for aggressive samples - such as polymer solutions. With this operating mode, the capillary pipe is connected to the ViscoPump via a safety bottle. In case of a malfunction, the sample will enter the safety bottle and will be detected there by a safety sensor. In addition to this safety advantage, only the suctioning operation allows the use of a disposal/cleaning system. Therefore, the work is done in a suctioning mode for all AVS®370 devices with a disposal system and for all AVS®Pro systems.

1.3. The right choice

• Which device is the suitable one for me (selection scheme)?
  We can not offer a general selection scheme at this time, as the demands and prerequisites of our customers are too varied. Please contact SI Analytics or our sales partners by telephone or by e-mail for a non-committal consultation: www.si-analytics.com/kontakt.html
• Starting at which number of samples does an automation pay off?
  A ViscoClock is already profitable for a small number of samples based on its low acquirement cost, e.g. for the occasional measurement of oils. An AVS®370 or AVS®470 is profitable if you are regularly measuring about 5 samples per day. In addition to the advantages of less work, there is also the high reproducibility and safety that play an important role - the latter especially with typical polymer applications, where you work with aggressive samples that are hazardous to your health.
• For which applications should an opto-electronic probing be used for automatic viscosity measurements and for which a thermo-electric probing?
  The opto-electronic probing is the standard method for transparent liquids. It is also suitable for colored oils or strongly dyed solutions, such as the deep blue CuEn, a solvent for cellulose. The thermo-electric TC viscosimeter is used for opaque liquids, especially for used oils. As the thermistor sensors are surrounded by glass and melted tightly into the viscosimeters, you can also measure conductive and very aggressive liquids. As the thermistor sensors become hot during use (up to 150°C), TC viscosimeters should not be used for samples where solvents may evaporate: Impairment by the formation of incrustations on the sensors.
• When is the AVS®Pro profitable?
  An AVS®Pro system is usually used once a sample quantity of about 20 samples/day/shift is processed.
• Which viscosimeter is suitable for my measuring process?
  SI Analytics generally recommends Ubbelohde or micro Ubbelohde viscosimeters: With this type of viscosimeter, the run time does not depend on the filled volume, so that the measuring accuracy is higher - with simpler ease of use, as you do not have to watch for accurate filling. Sometimes, the use of other viscosimeters is recommended: I. E. for measurements in the petrochemical area, traditionally, Cannon-Fenske viscometers are often used, and for foaming samples (e.g. beer, blood), micro Ostwald viscometers have proven to be effective. All viscosimeters are available in different capillary sizes, which must match the respective sample viscosity, so that the run time is between 100 and 500 seconds.
• Are there also micro viscosimeters and how big does the minimum sample volume need to be?
  Micro Ubbelohde viscosimeters and micro Ostwald viscosimeters require only 3 to 4 ml or 2 ml of sample volume.

1.4. Precision

• How do I check the measuring accuracy of my AVS system? Are there calibration liquids for this?
  The calibration constant of the capillary viscosimeters can be checked by means of calibration liquid, which can be obtained from the Deutschen Kalibrierdienst (DKD, ZMK Wolfen), the German calibration service. Another possibility is the direct comparison with master viscosimeters, which were calibrated i.e. by the PTB or the DKD (ZMK Wolfen). Furthermore, the viscosimeters can be recalibrated at SI Analytics GmbH. And, the temperature of the thermostat bath must be checked with an accurate thermometer: The deviation from the nominal temperature must not exceed 0.02°C; this is especially important for absolute measurements of viscosity. Note the correct sample preparation when measuring polymer solutions: Correct concentration, avoiding polymer decrease by excessive heating or dissolution times, observing contained additives when weighing and more.
• Can measurements be accurate with run time of less than 200 seconds?
  The standards to measure the absolute viscosity with capillary viscosimeters - e. g. DIN 51 562, ISO 3105, ASTM D 446 - require run times of at least 200 seconds. In practice, however, many users work with run times of less than 200 seconds. With viscosity meters with a large capillary diameter (i. e. Ubbelohde type II or larger, all micro Ubbelohde types), the increase of measuring uncertainty at a run time of 100 seconds is only very minor compared to 200 seconds and mostly acceptable in practice. With viscosimeters for low-viscosity liquids (e.g. Ubbelohde types 0, 0c, I, Ic), special flow effects occur with decreasing capillary diameter and decreasing run time, so that the measured run times must be corrected as per Hagenbach-Couette. This correction can only be approximated by means of a formula, therefore, the measuring uncertainty increases with a decreasing run time. Therefore, you must adhere to run times of 200 seconds or longer by any means with viscosimeter types with tight capillaries for precision measurements. With low expectations of measuring uncertainty, you can use shorter measuring times < 200 seconds in practice as well for these tight capillary diameters. In this case, the run times must be corrected as per Hagenbach for sure. These corrections can be found in the instructions for use of the SI viscosimeters (also see download area of the SI home page). For micro Ubbelohde viscosimeters, the Hagenbach correction is generally less than for DIN Ubbelohde viscosimeters, so that micro Ubbelohde viscosimeters can also be used with run times between 50 and 100 seconds. For measurements of relative viscosity of polymer solutions, the standard ISO 1628-1 permits minimum run times of 150 to 50 seconds for the blind value (pure solvent) depending on the capillary size.
• What is the Hagenbach-Couette or HC correction?
  The Hagenbach-Couette correction takes into account the effects during the entry and exit of the sample into or from the capillary as well as the kinetic energy of the sample; therefore, this correction is called the "kinetic energy correction". The correction is comprised of correction times that must be subtracted from the run time. The correction time increases with a shorter run time and the smaller the capillary diameter. The values of the Hagenbach correction can be found in the instructions for use for the viscosimeters (also see download area).

1.5. Mainenance

• Can the viscosimeters be dried after cleaning in the drying cabinet? If yes, what is the maximum temperature?
  The viscosimeter can be dried in the drying cabinet - preferably in a vacuum. We recommend a maximum temperature of 120°C.
• I would like to measure totally different oils. How do I rinse the viscosimeters between measurements?
  Pour the samples out of the viscosimeters or suction them out and then rinse repeatedly with a suitable solvent (such as petroleum spirit). Finally rinse again with a volatile solvent (such as acetone or light petroleum) and dry the viscosimeter using a dust-free air flow, preferably generated by underpressure (for example, water jet pump).

1.6. Tempering

• Why is temperature so important for viscosity measurements? Do I definitely need a thermostat?
  The viscosity is very dependent on the temperature: A temperature change of 1°C leads to a viscosity change of approx. 5%, depending on the sample. This is why an exact tempering is required for measuring. Standards require an accuracy of tempering of ± 0,02 °C.
• Why is a separate control thermometer necessary if the thermostat shows the temperature at 0.01°C?
  The resolution of the thermostat of 0.01°C does not mean that the absolute temperature of the bath liquid is accurately set to ± 0.01°C. In order to adjust the bath temperature to an accuracy of ± 0.01...0.02°C, control thermometers with a low measuring uncertainty are required. The control thermometers by SI Analytics have a resolution of 1/100°C and can be certified (calibrated).
• Why does there have to be counter-cooling when working only slightly above room temperature?
  An exact regulation of the temperature is only possible, if the thermostat bath is heated and cooled simultaneously. Only if the measured temperate is about 20°C higher than the room temperature, the normal heat loss of the thermostat bath will be sufficient as a counter cooling.
• How exactly does the temperature constant need to be in the bath?
  As per DIN 51562, ISO 3104 and ASTM D 445, the temperature uncertainty must not exceed ±0.02°C.

Here you can download the whole handbook from SI Analytics as a PDF: