The procedure for differential depth measurement is as follows (see illustration below):
1. Test step
First, the indenter is pressed with the test pre-force (also referred to as pre-force or preload) to a penetration depth of h0 in the specimen to be tested. h0 defines the reference level (basis) for subsequent measurement of the residual indentation depth (h).
2. Test step
Next, the additional test force is applied for a dwell time defined in accordance with the relevant standard (several seconds), whereby the indenter penetrates into the specimen to a maximum indentation depth of h1. The test pre-force plus the additional test force gives the total test force (also referred to as total force or main load).
3. Test step
After the dwell period, the additional test force is removed, the indenter moves up by the elastic proportion of the penetration depth in the total test force and remains at the level of the residual indentation depth (h). This is also referred to as the depth differential (difference in indentation depth before and after application of the total test force). The hardness testing machine then calculates and outputs the hardness value using the residual indentation depth (h) and a formula defined in the relevant standard.