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The National Lubricating Grease Institute (NLGI) determines grease grades based on the consistency of the grease, which is measured by its penetration depth at a standard temperature. The procedure involves using a penetrometer, which consists of a standardized cone that is allowed to penetrate the grease sample under specific conditions.

The steps to determine NLGI grades are as follows:

  1. Sample Preparation: The grease sample is brought to a standard temperature of 25°C (77°F) and then worked 60 strokes in a standard grease worker to ensure uniformity.

  2. Penetration Test: A penetrometer cone is released to penetrate the grease for 5 seconds. The depth to which the cone penetrates into the grease is measured in tenths of a millimeter.

  3. Consistency Measurement: The penetration depth measurement is used to assign an NLGI grade. The NLGI consistency number ranges from 000 (very fluid) to 6 (very firm), as shown in the table below:



NLGI Grade Penetration Range  Consistency Food Analogy
000 445-475 Semi-fluid Ketchup
00 400-430 Semi-fluid Applesauce
0 355-385 Very soft Brown Mustard
1 310-340 Soft Tomato Paste
2 265-295 Moderate (normal) Peanut Butter
3 220-250 Firm Vegetable Shortening
4 175-205 Very firm Frozen Yogurt
5 130-160 Hard Smooth Pâté
6 85-115 Very hard Cheddar Cheese Spread





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