An arbitrary standard measure of consistency (hardness) based on ASTM D217 (ISO 2137). The softer the consistency, the higher the penetration number.
The ASTM Definitions are:
Penetration – The depth that a standard cone penetrates a sample of lubricating grease under prescribed conditions of cone weight, time and temperature. Penetration is measured in tenths of a millimeter, sometimes abbreviated dmm.
Undisturbed Penetration – The penetration of a sample of lubricating grease that has not been stirred or disturbed measured at 25°C (77°F).
Unworked Penetration – The penetration at 25°C (77°F) of a sample of lubricating grease that has received only minimum disturbance in transferring to a grease worker cup or dimensionally equivalent container.
Worked Penetration – The penetration at 25°C (77°F) of a sample of lubricating grease immediately after it has been subjected to 60 double strokes in a standard grease worker.
Prolonged Worked Penetration – The penetration at 25°C (77°F) of a sample of lubricating grease that has been subjected to more than 60 double strokes in a standard grease worker at a temperature of 15°-30°C (59°-86°F). Typical numbers of double strokes used for prolonged working are 10,000 and 100,000. After the prescribed number of double strokes, the sample is brought to 25°C (77°F), and worked an additional 60 double strokes, and penetration is then measured without delay.
Block Penetration – The penetration at 25°C (77°F) of a sample of lubricating grease that is sufficiently hard to hold its shape, determined on the freshly prepared face of a cube cut from a block of grease.