The need for consistency
Consistency is something we encounter frequently in semi-solid, pasty, creamy or highly viscous products without ever giving it much thought.
• Cosmetics: Lipstick consistency that is too high often breaks on application.
• Personal Care/Medical: High consistency lotions/ointments stick on the skin for wound treatment, low consistency lotions soak in easily
• Food: Spreadable fats and cheese need to have optimum consistency to spread without drip
• Machinery: Hard grease will not properly lubricate a bearing, low consistency grease will leak from the bearing
• Road surfacing: Soft bitumen without sufficient binder stiffness in high temperatures will cause road rutting. Very hard bitumen consistency under cold conditions may result in road fatigue cracks
The application of penetrometry
Penetration is the method used to measure consistency which is the resistance a sample exhibits to deformation by an applied force.
A Penetrometer is a device that provides a rapid empirical method for measuring the denseness, compaction, consistency or penetrability of a wide variety of solid, semisolid, food and non-food products. These typically include soil, agricultural produce, or semisolid petroleum products in the pharmaceutical (such as creams and ointments), civil engineering, agriculture, geology and scientific exploration industries.
It is a simple device that was invented in 1846 and typically measures the resistance of a substance to penetration to a given depth by a weight-driven cone or needle of a specific shape by gravity at a specific temperature. This penetration is indicated in penetration units of 0.1mm or directly in millimetres which allows a rating of the plasticity or consistency of the sample. Over time, many standard methods have developed that use this principle but vary in the Penetrometer cone/needle geometry, time and load applied.
The Plus range of Texture Analysers can accommodate a Penetrometer unit to provide this measurement and extend the range of applications for the instrument.
How the Penetrometer works
Typically, a standard cone or needle is automatically released from the Penetrometer rig and allowed to free-fall into a contained sample for 5 seconds (or a different specified cone release interval) at a constant temperature after which time the cone is automatically retained. The depth of penetration of the cone or needle into the sample is measured and recorded in mm, PU (Penetrometer) units along with the additional measurement of apparent yield stress (σapp). The deeper the cone/needle sinks into the material, the softer the material is.
*Whilst some standard methods call for the use of a sample ‘pot/cup’ preference, operators have the option to use the samples original container. This avoids textural damage that may compromise the direct measurement of properties such as hardness during the transfer of the sample from its original packaging to the measurement cup. In all instances the surface of the specimen must be smooth and flat.
The hollow Penetrometer Probe and shaft for the European Pharmacopoeia 2.9.9 method is 150g (+/- 0.05g) and made from brass with a steel tip. This cone geometry and total weight is also in accordance with cone methods applicable to greases and to petrolatum which are described in ASTM Test Methods D 217 and Test Method D 937, respectively.
This rig and cone also complies with the following standard methods:
Collecting and displaying Penetrometer data
The above graph shows the cone release interval 5 seconds
Typical spreadsheet in Exponent Connect software
The Penetrometer is only compatible with PLUS model Texture Analysers