GUM’s Rockwell hardness standard machines after modernization


ACTA IMEKO | www.imeko.org December 2020 | Volume 9 | Number 5 | 240 

ACTA IMEKO 
ISSN: 2221-870X 
December 2020, Volume 9, Number 5, 240 - 246 

 
 

GUM’S ROCKWELL HARDNESS STANDARD MACHINES  

AFTER MODERNISATION 
 

J. D. Fidelus1, M. Kożuchowski2 

 
1 GUM, Central Office of Measures, 00139 Warsaw, Poland, janusz.fidelus@gum.gov.pl   

2 MERICORE, 02654 Warsaw, Poland, m.kozuchowski@mericore.com  

 

Abstract: 

This article describes two twin deadweight-type 

Rockwell hardness standard machines (HSMs – 

HSM-S02 and HSM-NT) of GUM after 

modernisation. A new control system for the station 

with database, a hydraulic pump, a displacement 

measuring system and an application enabling the 

operator to operate the measuring station are 

described. The adjustment process of the HSM-S02 

with entering automatic compensation (correction) 

enabling the improvement of the measured result in 

real time is presented. 

Keywords: Rockwell hardness standard 

machine; deadweight-type machines; adjustment; 

industry 4.0, LabVIEW, database  

1. INTRODUCTION 

The GUM HSMs were designed and 

manufactured in the 1970s by Ernst Leitz GmbH 

Wetzlar (Germany). In order for the systems to 

work in a fully automatic mode and in accordance 

with Industry 4.0 requirements [1], their modernisa-

tion was necessary. The proper and precise 

functioning of the HSMs requires their accurate 

adjustment and calibration. Although there is a pre-

cise procedure for calibrating the reference standard 

blocks, there is no such procedure for the HSMs. 

2. GUM’S ROCKWELL HARDNESS 
STANDARD MACHINES 

In cooperation between GUM (Główny Urząd 

Miar/Central Office of Measures) as the National 

Metrology Institute of Poland for standardisation of 

hardness measurement and MERICORE, a concept 

has been developed for the automation and 

modernisation of two twin HSMs - scale A, B, C, D, 

E, F, G, H, K (HSM-S02) and scale N, T (HSM-NT), 

in accordance with PN-EN ISO 6508-3 [2, 3]. As 

part of the modernisation process, a new control 

system for the station, a hydraulic pump, a 

displacement measuring system and an application 

enabling the operator to operate the measuring 

station were made, see Figure 1. 

 
Figure 1: GUM’s Rockwell hardness standard machine 

composed of the following elements: 

1. Indenter 

2. Specimen support 

3. Preliminary load 

4. Hydraulic cylinder 

5. Weights 

6. Body of the hardness tester 

7. Electric drive of the pump actuator 

8. Monitor 

9. Electric control cabinet 

10. Controller's rack 

11. Hydraulic cylinder of the pump 

The station controller was built in accordance 

with industrial and laboratory standards, combining 

reliability and precision required in laboratory 

equipment. The use of the latest solutions made it 

possible to prepare the station to be networked in 

accordance with the idea of Industry 4.0. 

The use of an industrial computer, a system of 

distributed islands for the acquisition of signals 

from sensors, LAN communication with individual 

executive systems of the system, and a 22-inch, 

touch screen monitor creates an extremely efficient 

and convenient to use facility. 

The hydraulic pump uses an electrically 

controlled actuator to control the piston position 

with single micrometre resolution. This makes it 

possible to control the movement of the overlap of 

individual loads unheard of in commercial 

workstations. Due to the fact that the modernised 

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ACTA IMEKO | www.imeko.org December 2020 | Volume 9 | Number 5 | 241 

stations date back to the 1970s, it was necessary to 

replace the original sensing of the mechanical 

systems. Contact ends and switches were replaced 

by inductive proximity sensors which, thanks to the 

absence of moving parts and contacts, ensure 

failure-free operation even in the case of 

contamination or flooding with oil. 

Depth displacement is measured using the 

Renishaw RESOLUTE system, which consists of an 

optical encoder (head) and a stainless steel belt scale 

with laser-engraved absolute code (Figure 2). The 

system used has a measurement resolution of 1 nm 

and an error of 3.5 µm per metre of scale length. 

When measuring displacements of 200 µm, the 

scale error is negligible. The head has its own 

electronics, which controls the optoelectronics of 

the reading but also monitors whether the setting of 

the optical axis to the scale is correct. If any 

deviation is detected, the measuring system sends 

information to the control application that the 

reading surfaces must be adjusted or cleaned. In 

combination with the digital readout transmission, 

this makes the system extremely resistant to 

interference and errors.  

 
Figure 2: GUM’s Rockwell hardness standard machine 

equipped with the Renishaw RESOLUTE system, which 

consists of an optical encoder (head) and a stainless steel 

belt scale with laser-engraved absolute code 

The software for the workplace control has been 

developed from scratch in the LabVIEW 

environment, which is dedicated to creating 

applications in laboratory and industrial 

applications. In particular, it is dedicated to the 

operation of measuring systems due to the 

implemented functions of hardware operation and 

data analysis and archiving capabilities. 

The control application enables the measurement 

to be carried out and also includes an equipment 

diagnostic module. Thanks to it the user can easily 

check the correctness of operation of particular 

systems. In addition, the application will make it 

impossible to switch on the station or perform a 

measurement if it finds that any of the systems are 

not working properly or return values outside the 

accepted control limits.  

Each station has its own database in which all 

data of orders placed as well as the results of 

conducted tests are stored. For the convenience of 

users, the database is accessible through a specially 

created program in LabVIEW using a web browser. 

This makes it possible to enter and modify data from 

any computer in the same network (Figure 3). 

 
Figure 3: A computer with dedicated software (control 

and device diagnostics application written in LabVIEW) 

for data analysis and archiving 

In accordance with the requirements of safety 

standards, the controller has been equipped with a 

certified safety controller and an emergency stop 

button, thanks to which the user can stop the 

hydraulic pump operation at any time, protecting 

himself or his equipment. Resumption of the 

workstation is only possible after the appropriate 

safety system restart procedure. 

3. HSM-S02 – FORCE CALIBRATION AND 
MECHANICAL STUDY 

3.1. Force Calibration 
The force calibration of the machines using the 

force-proving instrument composed of the force 

transducer (type Z3H2 R, Hottinger) and the 

measuring amplifier (type DMP41-T2, HBM). Data 

archiving and analysis was carried out, i.e. using 

HBM (Catman) software. The exact description of 

the force calibration will be the subject of a separate 

work. 

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3.2. Preliminary Hardness Measurements 
In order to verify the indications of the Rockwell 

hardness standard machine (HSM-S02), a number 

of hardness blocks measurements were made for the 

HRC scale, which are shown in Table 1. Each 

sample was measured at five points and then the 

average value was determined, which was 

compared with the valued of the measured standard. 

Based on the measurements carried out, a statistical 

analysis was performed to adjust the characteristics 

of the HSM-S02 to the hardness reference blocks. 

Thanks to this, the automatic compensation 

(correction) entered into the system can correct the 

measurement result in real time. 

Table 1. Measurements of Rockwell hardness C without compensation 

Hardness blocks Measurements (HRC) Difference 

Designation HRC 1 2 3 4 5 Average Uniformity 

MPA NRW 1214601.1006 21.62 22.95 22.80 22.93 22.86 23.00 22.91 0.20 1.29 

MPA NRW 863401.0206 30.20 30.91 31.08 31.10 31.06 30.98 31.03 0.19 0.83 

MPA NRW 6975302.0916 44.71 44.91 44.82 44.70 44.64 44.74 44.76 0.27 0.05 

AS 2271/98 55.90 56.39 56.22 56.29 55.9 56.08 56.18 0.49 0.28 

MPA NRW 900809.0306 61.13 60.87 60.83 60.69 60.88 60.66 60.79 0.22 -0.34 

MPA NRW 863905.0206 64.94 64.51 64.50 64.50 64.51 64.60 64.52 0.10 -0.42 

 

First, a plot was made where measurement 

values were placed on the X axis, while the hardness 

values of the tested blocks were placed on the Y 

axis. Figure 3 presents a graph of this relationship. 

 
Figure 3: The graph presents the relationship between the 

hardness values measured on the HSM-S02  

and the nominal hardness values of the HRC blocks 

Then, a trend line was determined which was 

finally fitted using a 3rd degree polynomial 

(R2 = 0.999), given in equation (1). 

𝐻𝑅𝐶 = 0.00006 ∙ 𝑋3 − 0.0077 ∙ 𝑋2

+ 1.3469 ∙ 𝑋 − 5.8 
(1) 

where the X value is the raw result of the hardness 

measurement at the HSM-S02. 

After taking into account equation (1) in the 

calculations of the HRC result value from the 

measurements according to Table 1, the relation 

between the deviations of the values calculated from 

the measurements and the nominal values was 

obtained (Figure 4). 

As shown in Figure 4, the distribution of 

deviation values is quite random and their values do 

not exceed ± 0.5 HRC. Therefore, it was decided to 

compensate the remaining deviation values by 

creating a table of correction values in specific 

intervals. The results are shown in Table 2. 

Table 2: Summary of final corrections 

Interval 

number 

HRC hardness 

range 

Correction 

accepted 

1 0-30 -0.15 

2 30-35 -0.1 

3 35-40 +0.1 

4 40-47.5 +0.2 

5 47.5-50 +0.1 

6 50-53.5 -0.1 

7 53.5-58 -0.25 

8 58-60 -0.1 

9 60-62.5 +0.0 

10 62.5-68 -0.3 

 

 

Figure 4: Deviations of the measurement HRC values 

from the nominal values after applying the compensation 

equation 

20

25

30

35

40

45

50

55

60

65

70

20 30 40 50 60 70

N
o

m
in

a
l 

va
lu

e
 (

H
R

C
)

Measured value (HRC)

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

0,4

20 40 60 80

H
R

C
 v

a
lu

e
 d

if
fe

re
n

ce

Measurement points

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In order to verify the correct operation of the 

HSM-S02 with the compensation equation and table 

of corrections, after 3 months from the first 

measurements summarised in Table 1, a series of 

measurements were made on the same hardness 

blocks. Measurement were made at 23 °C ± 0.5 °C 

using the same indenter. The results of the 

verification measurements are summarised in 

Table 3. 

Table 3. Hardness measurements of HRC blocks with active compensation 

Hardness blocks Measurements (HRC) Difference 

Designation HRC 1 2 3 4 5 Average Uniformity  

MPA NRW 1214601.1006 21.62 21.50 21.51 21.36 21.53 21.55 21.49 0.19 -0.13 

MPA NRW 863401.0206 30.20 30.25 30.21 30.19 30.07 30.28 30.20 0.21 0.00 

MPA NRW 6975302.0916 44.71 44.99 44.75 44.84 44.80 44.77 44.83 0.24 0.12 

AS 2271/98 55.90 55.89 56.06 56.05 55.70 55.68 55.88 0.38 -0.02 

MPA NRW 900809.0306 61.13 60.99 61.25 61.04 61.15 61.18 61.12 0.26 -0.01 

MPA NRW 863905.0206 64.94 65.01 64.88 65.01 65.00 65.12 65.00 0.24 0.06 

 

The results of the introduced compensation are 

shown in Figure 5 and Figure 6. Figure 5 shows the 

deviations of the calculated values of the five 

measurements with and without active 

compensation, while Figure 6 shows a comparison 

of the uniformity values of both measurement 

series. 

 
Figure 5: Difference of measured hardness value and 

HRC nominal hardness blocks value.  Comparison of 

average values from the measurement series without 

compensation (in blue) and with active compensation (in 

orange) 

 

Figure 6: Uniformity for individual measurement series 

(measurements without compensation – in blue;  

measurements with compensation – in orange) 

As shown in Figure 5, measurement results with 

active compensation differ much less from the 

reference blocks than in a series of measurements 

without compensation. By reading the difference in 

Table 3, the average hardness measurements of each 

block do not differ by more than 0.15 HRC from its 

nominal value.  

Figure 6 summarises the values of measurement 

spread for measurement series made before and 

after the introduction of deviation compensation. In 

both cases, we can see that the scatter values for 

individual blocks for both measurement series are 

very similar and oscillate around 0.2-0.25 HRC. The 

exception are the uniformities when measuring the 

blocks with a value of 55.9 HRC, which shows 

higher uniformity (older generation block with 

higher uniformity of surface hardness). 

The final stage of verification of the correctness 

of the HSM-S02 stand and the compensation used 

was the performance of the third series of 

measurements on other blocks than those used in the 

first two tests with a maximum uncertainty of 

0.4 HRC. 

Table 4 summarises the results from the 

measurements of the third series. These 

measurements were carried out on the same day as 

series 2 to eliminate the effect of time on possible 

position instability. 

Figure 7 shows the difference between the HRC 

value of the block and the average value of its five 

measurements. The measurements were performed 

with active compensation taking into account the 

table with corrections to maintain consistency with 

the second measurement series. 

Analysis of the results showed that the values of 

uniformities and average of individual 

measurements are acceptable. The block 22.73 HRC 

is an exception. 

 

-1

-0,5

0

0,5

1

1,5

21,62 30,2 44,71 55,9 61,13 64,94

H
R

C
 d

if
fe

re
n

ce

HRC nominal value

0

0,1

0,2

0,3

0,4

0,5

0,6

21,62 30,2 44,71 55,9 61,13 64,94

H
R

C
 u

n
if

o
rm

it
y

HRC nominal value 

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Table 4. Results of measurements of 3rd series made on additional blocks with a maximum measurement uncertainty of 

0.4 HRC 

Hardness blocks Measurements (HRC) Difference 

Designation HRC 1 2 3 4 5 Average Uniformity 

19776 D-K 22.73 21.42 21.52 21.25 21.33 21.47 21.40 0.27 -1.33 

PRESS-9794 35.80 36.07 35.96 35.90 35.99 35.91 35.97 0.16 0.17 

19777 D-K 45.91 45.56 45.64 45.57 45.59 45.66 45.61 0.10 -0.30 

AS4322/85 50.50 50.76 51.00 50.81 51.01 50.95 50.91 0.25 0.41 

AS 60/98 61.00 60.73 60.64 60.54 60.92 60.75 60.72 0.38 -0.28 

19778 D-K 65.86 65.52 65.49 65.49 65.65 65.57 65.54 0.16 -0.32 

 

 
Figure 7: The values of the difference between the mean 

value of the measurements and the value of the hardness 

block in the 3rd series. Measurements with active 

compensation 

The average values of measurements for all 

blocks, except the block with the lowest hardness, 

are within 0.4 HRC according to the documentation 

of blocks. The average value of the measurements 

for the 22.73 HRC block measured on the HSM-S02 

stand is 1.33 HRC too small in relation to its 

nominal value. 

Compared with the measurements from the first 

series of the 21.62 HRC block, it can be seen that 

they were 1.29 HRC too high in relation to its 

nominal value. Therefore (to check the uniformity 

of the soft 21.62 HRC block), a comparative study 

was conducted with the HSM-S01 stand, which uses 

the same type of indenters and is very similar in 

construction. The results for both blocks and stands 

(HSM-S01, HSM-S02) are presented in Table 5. 

First, the values of uniformities were analysed. 

According to Figure 8, the dispersion values are 

comparable and according to the data from Table 5 

for individual standards, they do not differ by more 

than 0.01 HRC, which can be considered a 

negligible value. 

In addition, the scatter values for both blocks are 

small, which suggests that both blocks have similar 

uniformity and the positions are repeatable and 

stable. 

 

 

 

Figure 8: Uniformities for comparable test results at 

HSM-S01 (in grey) and HSM-S02 (in black) stands 

 

The following conclusions can be drawn from 

the conducted tests: 

1. It should be assumed that the block with a value 
of 21.62 HRC has a different actual value than 

the declared nominal value. 

2. Assuming no fatal error in the measurements at 
the HSM-S01 and HSM-S02 stands and 

theirstability on the basis of the uniformities 

shown in Table 5 and Figure 8, it is suggested 

to take the actual value of the hardness of the 

21.62 HRC block as the average of the 

measurements on the HSM-S01 stand and 

measurements from the first series on the 

HSM-S02 stand without compensation, giving 

a calculated average difference of 1.062 HRC.  

3. If it is assumed that the average value of the 
21.62 HRC block measurements taken on the 

HSM-S02 stand in the first series without 

compensation should be corrected by the 

calculated average value from comparative 

measurements according to point 2, then the 

average value from the results of the block with 

the 22.73 HRC value is -0.269 HRC. This value 

is close to the value measured at the HSM-S01 

stand.  

 

 

-1,5

-1,0

-0,5

0,0

0,5

22,73 35,80 45,91 50,50 61,00 65,86

H
R

C
 d

if
fe

re
n

ce

HRC nominal value

0

0,05

0,1

0,15

0,2

0,25

0,3

21,62 22,73

H
R

C
 d

if
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HRC nominal value



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After introducing a change in compensation and 

correction value, the current graph in Figure 7 is 

shown in Figure 9. 

Final calibration and corrections on HSM-S02 

(and HSM-S01) stands will be introduced after 

conducting bilateral comparative tests with the 

National Institute of Metrological Research (INRiM, 

Italy) [4]. 
 

Table 5. Comparison of measurement results for the 21.62 HRC and 22.73 HRC blocks at HSM-S01 and HSM-S02 

Hardness blocks Measurements (HRC) Difference 

Designation HRC 1 2 3 4 5 Average Uniformity 

HSM-S01 

MPA NRW 1214601.1006 21.62 22.36 22.36 22.56 22.56 22.44 22.46 0.20 0.84 

19776 D-K 22.73 22.64 22.56 22.60 22.48 22.76 22.61 0.28 -0.12 

HSM-S02 

MPA NRW 1214601.1006 21.62 21.50 21.51 21.36 21.53 21.55 21.49 0.19 -0.13 

19776 D-K 22.73 21.42 21.52 21.25 21.33 21.47 21.40 0.27 -1.33 

 

 
Figure 9: Measurement results of the 3rd series after 

updating the correction value for the 0-30 HRC range. 

Measurements with active compensation 

4. SUMMARY 

GUM’s deadweight-type Rockwell hardness 

standard machines were modernised in accordance 

with Industry 4.0 requirements.  

At the stage of preparing the article, the stand 

was not yet ready to conduct the final research and 

determine the uncertainty budget. The stand was 

manufactured in 1975 and has been used 

sporadically in recent years, which raised concerns 

about its technical condition. After the 

modernisation, its stability and repeatability had to 

be determined and the developed algorithm to 

compensate for measurement deviations had to be 

tested. For this purpose, hardness blocks were used, 

which were measured under reproducible 

conditions with an interval of three months. The 

obtained measurements, which were used in the 

article, confirmed that the station’s mechanisms 

work properly, and the compensating algorithm 

allows correction of any deviations to the level of 

0.05 HRC for the entire scale range. 

 

The presented way of the adjustment HSM-S02 

with the introduction of automatic compensation 

(correction) allows to improve the measured result 

(hardness) in real time. 

On this basis, an uncertainty budget will be 

developed for the stand without using plate hardness 

standards. The compensation algorithm can be used 

to adjust the characteristics of the indenter or the 

system for measuring the displacement of the 

indenter if comparative measurements made with 

the laser interferometer showed its non-linearity or 

offset. 

It is planned to use the HSMs (the hysteresis / 

Rockwell hardness investigations) in the EMPIR 

project with the acronym ComTraForce. 

5. ACKNOWLEDGEMENTS 

Modernisation works of the HSMs were 

financed from funds for investment tasks for the 

GUM in 2019.  

As a part this work had been carried out in the 

framework of the project with the title 

“Comprehensive traceability for force metrology 

services”. The 18SIB08 ComTraForce project has 

received funding from the EMPIR programme co-

financed by the Participating States and from the 

European Union’s Horizon 2020 research and 

innovation programme. 

 
 

 

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

0,4

0,5

22,73 35,80 45,91 50,50 61,00 65,86

H
R

C
 d

if
fe

re
n

ce

HRC nominal value

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6. REFERENCES 

[1] Klaus Schwab, The Fourth Industrial Revolution, 
2016 

https://luminariaz.files.wordpress.com/2017/11/th

e-fourth-industrial-revolution-2016-21.pdf    

[2] EN ISO 6508-1/2/3: Metallic Materials- Rockwell 
hardness. 

[3] J. D. Fidelus, Participation of the Central Office of 
Measures in the European industrial project EMPIR 

in the field of force, Metrology and Hallmarking - 

the Bulletin of the Central Office of Measures,  

1 (24) / 2020, 7-14. BazTech. 

[4] A. Germak, J. D. Fidelus, C. Origlia, Bilateral 
comparison in Rockwell C hardness scale between 

INRiM and GUM, ongoing study. 

 

 

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