Introduction to Behavioral Instrumental Techniques

The processing industries materials are full of measuring instruments and standards intended to quantify macroscopic properties related to the flow, texture or mechanical strength of ingredients, finished or semi-finished products.

In this article, we briefly present these techniques and their operating principles, thus offering an unprecedented analysis grid of their possible uses and their limits.

Empirical instrumental techniques by gravity flow

The most rudimentary of the rheological measurement techniques aim to quantify the flow of matter under the effect of its own weight, by gravity.

Depending on the geometry and the principle of the device, a flow time, a distance travelled, a characteristic orifice size, an angle, will be measured, which will give an indication of a tendency for the product to flow. We often talk about viscosity index for fluids, flowability for powders, consistency for pasty products.

On this principle, there are also efflux viscometers, devices of the viscosity cup type equipped with temperature control means (see our article A brief history of viscometry and rheometry).

Progress point: empirical techniques measuring purely operational quantities

In these various techniques, often described as empirical, the quantities measured are generally not defined scientifically, but essentially operational : the quantity measured is defined exclusively by what the device quantifies through its own mode of operation, without any particular relationship to a property established on the theoretical level.

For example, the consistency measured with the Bostwick consistometer combines at least contributions linked to the density of the product (conditioning the effect of gravity), to its rheology (more or less flow threshold) and its physics at the interfaces (more or less sticky or slippery products will flow very differently). Thus, very different products will be likely to have equivalent consistencies. However, for products with similar formulations, the differences in measured consistency will constitute an indicator of their difference.

Another example, the flow time of a product measured during its passage through a viscosity cup presents an arithmetic relationship with the viscosity of the fluid only in the particular case of Newtonian fluids (see our article Viscosity, rheology and confusion). In the case of fluids exhibiting more complex rheologies (non-Newtonian, threshold, etc.), the flow time, although capable of operationally reflecting a tendency relating to flowability, cannot constitute a measurement of viscosity.

Instrumental techniques with mechanical constraints

In contrast to these techniques based on a quantification of behavior under the effect of gravity, there are instrumental techniques based on theapplication controlled mechanical constraints: instrumental techniques with mechanical constraints.

Instrumental techniques with mechanical constraints are likely to exploit the different modes of stresses and deformations of matter known at the macroscopic scale: compression, traction, torsion, bending, shear.

The durometer quantifies the force necessary for a defined indentation, in different hardness scales (Shore, Rockwell, Vickers, Brinell) suitable for testing different types of materials (elastomers, plastics, metals, leather, wood, etc.).

There are also durometers and associated standards for brittle or thin materials (Knoop test).

In practice, particularly in the case of Brookfield viscometers, the most common, the measurement is carried out with a spindle whose speed of rotation is fixed (in rpm, round-per-minute, i.e. in revolutions-per-minute). -minutes), coupled with a temperature measurement.

Viscometers are dedicated to rather fluid products, with certain limitations depending on the specific properties of the product (see our article Viscosity, rheology and confusion).

Compression gelometers are variants of the Bloom tester, developed in the 1920s for gels and gelatins, and similar to the operating principle of the durometer. This involves determining the penetration limit mass of a standardized plunger causing a certain deformation of the gel without rupture; this weight is translated into a gel strength/consistency in Bloom units. Contemporary versions rather determine a limit force before rupture.

Shear gelometers operate on the principle of a viscometer with a mobile hook whose rotation is progressively prevented by the gelation or setting of the product. Kinetic monitoring makes it possible to determine a freezing time.

More exotic approaches use different mobiles and test benches adaptable to the texturometer to impose stresses according to more specific configurations, from slight deformation to the penetration of the mobile, the slicing of the product, its bending, its rupture (for example in the case of crunchy products).

The texturometer translates well this notion of behavioral instrumental technique, in which it is a question of reproducing macroscopic constraints by the methods of implementation of an instrumentation with the principles of robust mechanical operation on the scientific level. The protocol configuration options, which vary according to the models, open up many ways of behavioral quantification.

Mainly used in the field of polymers, DMA is of interest in principle for all types of flexible or phase-change materials.

Like texturometers, tribometers are often equipped with modules allowing constraints to be applied according to movements (and within ranges of effort) characteristic of the application context to be simulated: four-balls, ball-on-disc , ball-on-three-plates, ring-on-ring, discs, …

conclusions

At the end of this non-exhaustive overview of common instrumental techniques in the processing industries, it is possible to draw up an initial assessment:

1. Far from being reduced to measuring a quantity (as one would do with a ruler or a tape measure to measure a distance), behavioral instrumental techniques impose mechanical constraints on matter in order to determine reactions (responses) of matter in return.
2. These stresses are imposed at the macroscopic scale by gravity flow, compression, shear, extension, bending or torsion. Environmental constraints (temperature in particular) can also be controlled.
3. These constraints are more or less configurable according to the instrumentations, limiting the determination of response to single values ​​or sets of points (function) according to the imposed constraints. Thus, the variety of possible stress modes multiplies the types of responses, physical, mechanical, rheological, tribological, texture, etc.
4. The nature of the states of matter depends on the type of device and the possible parameterization conditions of the stresses to be applied.

These considerations on instrumental techniques should not make us forget that any measurement is underpinned by a context of questioning. Often, the views focused on measurement disregard the questions that motivate it, as if the quantity to be measured to answer such and such a question was self-evident. This is not the case, in particular for industrial issues, and the variety of instruments mentioned constitutes concrete proof of this.

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