Components of a Knoop hardness value:
1. The numeric hardness value;
2. The two letters "HK", standing for "Hardness according to Knoop";
3. The applied test load in kgf;
4. According to ISO 4545: The dwell time of the test load, but only if this is not between 10 and 15 seconds (uncommon in practice)
Examples:
870 HK 1
- 870 …hardness value
- HK …as per Knoop
- 1 …with test load of 1 kgf (or test force of 9.807 N )
870 HK 1/30
- 870 …hardness value
- HK …as per Knoop
- 1 …with test load of 1 kgf
- 30 …dwell time to test force outside recommended duration, in this case: 30 seconds