What factors can cause errors when using a digital Rockwell hardness tester?

Taiwan Nakazawa answers for you: Causes of error include: the surface of heat-treated parts may have salt stains, sand, or other substances. When a load is applied, the part may slip. If grease is present, it will act as a lubricant when the diamond head presses in, reducing friction and increasing the indentation depth. This will cause the measured hardness value to be lower. For the tested area of a part, a thin and loose oxide scale layer will decrease the hardness value, while a thick and dense oxide scale layer will increase the hardness value. For parts whose hardness is to be measured, the oxide scale must be removed, and they must be wiped clean and free of any dirt.