Getting the right mix: measuring viscosity wirelessly

Mixing is a fundamental process in the manufacture of many products; in many cases, it is an exact science, as over- or under-mixing can leave various component materials unevenly distributed, or change the state of the end product. Viscosity measurement is believed to be the most accurate way to carry out mixing; however, rotating mixers can lead to tangled wiring. Here, Mark Ingham of Sensor Technology addresses wireless solutions as a viable alternative.

When mixing sauces, such as tomato sauce and mayonnaise, any notable change in texture or viscosity could dissuade consumers from purchasing products. Mixing is thus a crucial process for the food industry.

While there are a number of technologies available for measuring viscosity, a popular option is the rotational viscometer, which measures viscosity by monitoring the torque required to rotate a spindle at a constant speed within the fluid. The torque, generally measured by determining the reaction torque on the motor, is proportional to the viscous drag of the fluid. The rotational viscometer can run throughout the mixing process, logging data to provide a log of the change in viscosity over time. This profile can be compared with historic data from earlier mixes to provide information that would not be available from final target viscosity readings.

Torque can also be difficult to measure; due to the rotating spindle, wires attached to a torque sensor on the shaft would wind up and quickly snap. Approaches using slip rings are available, but unsuitable due to set-up time and wear and tear. A wireless alternative has been developed using TorqSense rotary torque transducers from Sensor Technology. These do not require a physical connection to the rotating shaft, as they use a radiofrequency (RF) link to send power to the sensing element on the spindle, and to receive torque reading signals from it.

The sensors use two surface acoustic wave devices (SAWs) made of ceramic piezoelectric material containing frequency resonating combs. The SAWs are glued into the drive shaft at 90° to one another. As the torque increases, one comb expands and the other contracts proportionally to the torque, acting as strain gauges, measuring changes in resonant frequency.

An RF transmitter/receiver mounted near the spindle emits radio waves towards the SAWs and collects them when they are reflected back. The change in frequency of the reflected waves identifies the current torque.

In the rotational viscometer, the sensor is mounted between the motor and the paddle. A double bearing eliminates any side loads, while a torque provides protection, if the paddle mechanism seizes. With the motor operating at constant speed, the transducer provides an output of torque that changes according to the viscosity during mixing, allowing the operator to measure the viscosity of the mix.

The system can also be pre-calibrated using specific paddles immersed in fluids with a known viscosity. By calibrating the viscometer to a known sample, the absolute torque figure can then be derived in the application from the relative torque figure and the required viscosity then measured.

TorqSense rotary torque transducers are designed to simplify the design of rotational viscometers while increasing accuracy, providing a good solution when mixing requires accurate viscosity data.

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