How to Measure Viscosity Using Rotary Torque Equipment

The widely known principle of rotational viscometers is utilized for calculating the viscosity in most laboratory viscometers, by sensing the torque required for rotating a spindle at a constant speed upon immersing it in the sample fluid.

This is because the torque, which is typically measured with the reaction torque on the motor, correlates to the viscous drag on the immersed spindle and so the viscosity of the fluid.

Using Rotary Torque Equipment for Consistency Measurement

AEA Technology proposed to carry out the consistency measurement of cement mixes for pressure injection into containers filled with radioactive waste. This is because the injection of the mixture at the correct consistency is vital to ensure that it fills all the air spaces and also sets correctly.

AEA found that this could only be achieved through the exact measurement of the drive torque that was required to rotate the mixer paddle at the time of mixing and by employing Rotary Torque Equipment from Sensor Technology.

Several other applications involving the mixing of fluids with solids in suspension were measured similarly after this assessment, for example, plaster mixes, coal slurry, and magnetic particles in a fluid, as it is particularly challenging to measure the viscosity of these mixtures via typical methods.

In most instances, online monitoring is needed. In some other applications, monitoring the viscosity of the fluid during mixing is vital, for example, while manufacturing shampoos and pharmaceutical solutions.

Furthermore, in such applications, it is crucial to determine the relative viscosity during mixing to establish if the consistency or viscosity is at optimum when the process is completed.


The torque will vary in viscosity at the time of mixing as the motor speed is constant, and so the operator can establish the relative viscosity of the mixture. The type and size of the paddle control the relationship between the absolute viscosity and the torque, where the paddle will be designed to ensure mixing.

Traditional viscosity measuring systems use a cylinder instead of a paddle, but a cylinder is not suitable for mixing, so, in general, absolute viscosity cannot be measured at the time of the mixing process.

By measuring a known relative sample via a laboratory viscometer to gather its absolute reading, it is possible to relate the relative reading to the absolute viscosity, enabling the optimum relative torque figure to be established, identifying the viscosity that is required.

Image Credit: Sensor Technology Ltd

The Rotary Torque Transducer is set up between the motor and the paddle. Since the rotary transducers can be sensitive to side loads, care must be taken not to connect the paddle directly with the transducer. This can be ensured by utilizing double bearings to avoid any side loads.

Several mixers are operated by motors, which are several times more powerful than what is needed, and so, if the paddle mechanism is halted, it can result in severe overstrain or breakage of the inline torque transducer. Care must be taken to protect the transducer by utilizing torque-limiting couplings.

This information has been sourced, reviewed and adapted from materials provided by Sensor Technology Ltd.

For more information on this source, please visit Sensor Technology Ltd.


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  1. sami khan sami khan Islamic Republic of Pakistan says:

    for different liquids and pastes different type of paddles and shafts are used like different types of viscometers. My question is that why we use twin paddles in viscometers although different types of paddles are present.

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