Rotational Viscometer Advantages vs. Bostwick Consistometer

VS.

The Bostwick Consistometer was, for quite a long time, the industry standard for food laboratory viscosity testing. It was an extremely user friendly and dependable instrument, however, it had several key limitations which have since been addressed with the introduction of the rotational viscometer.

Made from a stainless steel, level, two compartment trough, the Bostwick Consistometer is a rather simple piece of equipment. The substance which is to be tested is inserted into the first compartment. A gate separates the two compartments and is released during a measurement to allow the liquid substance to flow into the second compartment. The second compartment consists of a graduated parallel lines at 0.5 cm which are 24 cm long. The sample is allowed to flow freely under the force of gravity down the slight slope for 30 seconds and the distance it travels is measured to determine the viscosity.

While a simple process, this instrument is constrained in a number of ways. The first limitation is that the instrument's measurements can't be used to determine rheological information due to the wetting power, surface tension and other factors outside of the liquid's viscosity influencing the reading. The second limitation is the lack of temperature control when performing the test. Room temperature is used by default and is subject to whatever atmospheric temperature changes may occur. Lastly, it is limited in the type of samples which can be tested. Higher viscosity substances are not likely to flow far enough over the 30 second time to allow for substantial variance between samples.

With these constraints in mind, the rotational viscometer was developed. The accuracy and precision of viscosity measurements are greatly improved. The temperature can be controlled using a constant temperature bath. Also, only a very small amount of sample liquid (appoximately 11 mL) is needed when using a rotational viscometer.

When using a rotational viscometer, the resistance of the liquid is measured and the sample viscosity is thus obtained. A rotating spindle is submerged in the sample and the rotational viscometer takes measurements of the additional torque needed to maintain a constant rotational rate while overcoming viscous resistance. This measured data is then converted to a viscosity value and output to the user.

With these factors in mind, it is safe to say that a rotational viscometer is essential for all serious food laboratory research.