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GAUGE BLOCK CALIBRATIONBY
MECHANICAL COMPARISON(GAUGE BLOCK COMPARATOR)
Arrangement bySaraswanto Abduljabbar,
Valunteer, specialist and advisor Length Metrology
Measurement
1. BACK GROUND
International standard ISO 3650 cover the range of accuracy
requirements along
the traceability chain and calibration method is selected
according to accuracy require-
ments of the standard and the user. In mechanical comparison
methods, the similar
nominal size gauge blocks are compared to each other by suitable
probing elements.
Since the compared gauge blocks are in the same nominal sizes,
the inductive probes
which have a short measurement range with high accuracy is used
in the mechanical
comparison technique, figure 2.
Factors influencing the measurement are: the length calibration
of the standard, factors
inherent in the comparator equipment used to measure the length
difference such as
scale linearity and reading capability, gauge geometry with
respect to its effect on
probing the length difference, the temperature and other
environmental factor, etc.
2 Basis of measurement, traceability, reference condition1
2.1 Unit of length: metre
The metre is defined as the length of the path travelled by
light in vacuum in 1/299 792
458 of a second (17th General Conference of Weights and
Measures, 1983). The
definition is realized by working wavelength standards
recommended by the Interna-
tional Committee of Weights and Measures (CIPM).
2.2 Traceability of the length of a gauge block
The measured length of a gauge block is traceable to national or
international length
standards, if the measurement result can be related by an
unbroken chain of compari-
son measurements each with stated uncertainties to a gauge block
which has been
calibrated by interferometry/using appropriate wavelength
standards.
1EN ISO 3650:1998, Geometrical Product Specification (GPS) -
Length Standards Gauge Blocks, Second Edition,
1998-12-15.
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2.3 Reference temperature and standard pressure.
The nominal length and the measured lengths of a gauge block
apply at the reference
temperature of 20 C (see ISO 1) and the standard pressure 101
325 Pa = 1,013 25
bar2.
NOTE: The effect on the length of a gauge block caused by
deviations from the standard pressure may
be ignored under normal atmospheric conditions.
2.4 Reference orientation of gauge blocks
The length of a gauge block up to and including 100 mm nominal
length refers to the
vertical orientation with the measuring faces horizontal. The
length of a gauge block
over 100 mm nominal length refers to the horizontal orientation
with the block supported
on one of the narrow side faces, without additional stress, by
suitable supports each at
a distance of 0.211 times the nominal length from the ends. When
such a gauge block
is measured by interferometry in horizontal orientation, the
weight of the auxiliary plate
wrung to one of the measuring faces shall be compensated.
2.5 Dimensional stability
The maximum permissible changes in length per year of gauge
blocks are stated in
Tabel 1. They apply when the gauge blocks are not exposed to
exceptional tempera-
tures, vibrations, shocks, magnetic fields or mechanical
forces.
Tabel 1 Dimensional Stability
Grade Maximum permissible cahnge in length per yearK0
(0.02 + 0.25 10 )
12
(0.05 + 0.5 10 )
NOTE : is expressed in millimeters
Table 2 Flatness3 Tolerances
Nominal Length, mm
Flatness tolerance,
K 0 1 20.5 150 0.05 0.1 0.15 0.25150 500 0.1 0.15 0.18 0.25500
1000 0.15 1.18 0.2 0.25
2ISO 1:1975, Standard reference temperature for industrial
length measurements.
3ISO 1101:, Geometrical Product Specifications (GPS) Geometrical
tolerancing Generalities,
definitions, symbols, indication on drawings
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Table 3 Limit deviation , of the length at any point of the
measuring face from
and tolerance, , for the variation in length.
Nominallength,
Calibration grade K Grade 0 Grade 1 Grade 2limit
deviationof length
at anypointfrom
nominallength
tolerancefor the
variationin length
limitdeviationof length
at anypointfrom
nominallength
tolerancefor the
variationin length
limitdeviationof length
at anypointfrom
nominallength
tolerancefor the
variationin length
limitdeviationof length
at anypointfrom
nominallength
tolerancefor the
variationin length
0.5 1010 2525 5050 75
0.20.30.40.5
0.050.050.060.06
0.120.140.2
0.25
0.10.10.1
0.12
0.20.30.40.5
0.160.160.180.18
0.450.60.81
0.30.30.3
0.35
75 100100 150150 200
0.60.81
0.070.080.09
0.30.40.5
0.120.140.16
0.60.81
0.20.2
0.25
1.21.62
0.350.40.4
200 250250 300300 400
1.21.41.8
0.10.1
0.12
0.60.70.9
0.160.180.2
1.21.41.8
0.250.250.3
2.42.83.6
0.450.50.5
400 500500 600600 700
2.22.63
0.140.160.18
1.11.31.5
0.250.25
3
2.22.63
0.350.4
0.45
4.456
0.60.70.7
700 800800 900
900 1000
3.43.84.2
0.20.2
0.25
1.71.92
0.30.350.4
3.43.84.2
0.50.50.6
6.57.58
0.80.91
3. Measurement by comparison
3.1 Principle of measurement
In order to determine the length of a gauge block by compa-
rison, the difference of its central length from that of a
reference
gauge block is measured and applied algebraically to the
length
of the reference. For the probing, the measuring faces of
each
gauge are touched from opposite directions as shown in
Figure
1, and the length difference is measured by a high
resolution
length indicator.
algebraically difference (see arrow point 1) = = Measured
length
= Nominal length
Fig. 1 Measurement of central length by comparison taking the
perpendicular
distance from the centre of a measuring face to the opposite
one
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3.2 Central length
A measurement by comparison transfers the central length of a
standard gauge block to
a gauge block under test. The reference gauge block may either
directly be measured
by interferometry or related through one or several stages by
comparison to a reference
gauge measured by interferometry.
NOTE:
The effect of one wringing, which is included in the length of
the reference gauge block
measured by interferometry, is transferred by the comparison
measurement.
3.3 Method of determining length by comparison
The relatively small difference in central length between a
reference gauge block of
known central length and another gauge of unknown central length
is measured by a
high resolution length indicator. (see Figure 2).
Figure 2 Measurement of central length by comparison taking the
perpendicular distance from the
centre of a measuring face to the opposite one. Schematic
diagram of a gauge block comparator with
Measured Length Difference
Reading ()
Calibration ()
Drift ()
Temperature
Reading ()
Calibration ()
Drift ()
= 20
PenetrationFlatness
= = +
InductiveProbe 1
InductiveProbe 2
Parallelism
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opposing styli and digital readout. The diagram depicts the
influence factors in the measurement, and the
mathematical symbols which will represent them in the text.
Figure 2 shows the reference gauge block in the reading position
between an upper
contact and one beneath. The anvils are retractable and the
weight of the block is
supported independently. The connection line between the two
anvils is perpendicular
to the measuring faces. A reading of the indicator is taken at
the centre of the reference
gauge block which is then replaced by the gauge block to be
measured and a central
reading is taken on it. The vertical position is used for
comparison of gauge blocks of
nominal lengths up to 100 mm; Gauge blocks of nominal lengths
longer than 100 mm
can also be measured by comparison with a reference gauge block
in the horizontal
position. If a vertical orientation as specified in fig.2 is
used, the supports are adjusted
horizontally and vertically so that one anvil of the comparator
contacts the centre of one
measuring face of the gauge block and the second anvil is moved
over the second face
until the minimum reading is obtained.
4. HOW CHOOSING THE RIGHT GAUGE BLOCK COMPARATOR
For mainly economic reasons, most of customers of accredited
laboratories prefer their
standards being calibrated by mechanical comparison because of
lower costs and
shorter calibration time. In the market there are many products
of the Gauge Block
comparators. All of them of course have advantages and
disadvantages. This white
paper will guide you how to excecute the right Gauge Block
Comparators.
As you know many comparisons, especially those in dimensional
metrology, cannot be
done simultaneously. Using a gage block comparator, the
standard, control (check
standard) and test block are moved one at a time under the
measurement stylus. For
those comparisons each measurement is made at different time.
The term calibration
design can be applied to experiments where only differences
between nominally equal
objects or groups of objects can be measured.
Ordinarily the order in which these measurements are made is of
no consequence. The
usefulness of drift eliminating designs depends on:
1. The stability of the thermal environment, accuracy required
in the calibration. The
environment has to be stable enough that the drift is
linear.
2. Each measurement must be made in the same amount of time so
that the
measurements are made at fairly regular intervals. Finally, the
measurements of
each block are spread evenly as possible across the design. The
designs are
constructed to:
Be immune to linear drift.
Minimize the standard deviations for test blocks (as much as
possible)
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Spread the measurements on each block throughout the design
Be completed in 5-10 minutes to keep the drift at the 5 nm
level
4.1 Critical contributions to the uncertainty
Critical contributions, which could be dimi-
nished, are temperature deviations, calibra-
tion of the comparator and calibration of
reference gauge blocks. Temperature devi-
ations are influenced by the air temperature
deviations and by radiation of illuminating
bodies and of the operator.
Temperature can not be measured
on the gauge block and therefore table
temperature is assumed to be the tempe-
rature of the gauge block. However, devia-
tions between these two temperatures can differ for up to 0.05C.
Additional problems
appear by longer gauge blocks (near 100 mm) because temperature
is not constant
along the height. Beside that, thermal expansion coefficient of
gauge blocks is usually
not exactly known. It may differ for 1 10. Thermal expansion
correction is
therefore not accurate.
These problem happened because difficult to know what is
excactly the CTE4 of
the material gauge block. According to Ted Doiron and John Beers
5, The uncertainty in
the expansion coefficient of the gauge block is more difficult
to estimate.
Table 4: shows that as the blocks get longer, the thermal
expansion coefficient becomes systematically
smaller. It also shows that the differences between blocks of
the same size can be as large as a few
percent. Because of these variations, it is important to use
long length standards as near to 20 C as
possible to eliminate uncertainties due to the variation in the
expansion coefficient.
4CTE (Coefficient Thermal Expansion).
5Gage Block Hand Book, Dimensional Metrology Group Precision
Engineering Division, National Institute of
Standards and Technology (NIST-USA).
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4.2 UNCERTAINTY BUDGET AND CRITICAL CONTRIBUTIONS
In order to establish proper environmental conditions, special
microclimatic chamber
with own precise air conditioning system was built in the
laboratory. Temperature
deviations in the measuring space of gauge block comparator are
limited to 0,1C.
Temperature is the most important influence factor on measuring
uncertainty in
calibration beside the accuracy of calibration of reference
gauge blocks. The expanded
uncertainty of the calibration of gauge blocks by mechanical
comparison in the machine
is currently (0.03 + L/3000) m, L in mm*. Using coverage factor
k = 2. It was
calculated according to guide ISO GUM 2 edition, 1995 and
comprises all important
components. Some of them were evaluated statistically by
repeated measurements,
some of them were evaluated by experience and some of them on
the basis of different
data from manuals and calibration certificates. All the
measurements including tempera-
ture measurements were performed with traceable equipment and
measuring uncer-
tainties were evaluated for all measurements.
5. STRATEGY FOR DECREASING CURRENT LEVEL OF
UNCERTAINTY 6
5.1 Study of temperature influences
The following temperature parameters have been analysed:
change of gauge block temperature after putting it on the
comparator,
temperature difference between the gauge blocks and the
comparator table,
temperature influence of the illumination and the operator,
and
thermal expansion coefficients of gauge blocks.
All the above parameters were detected as critical and are
therefore subject of optimisation. Research was focused into
experimental simulations of different materials and colours
of
these materials. It was found out that the radiation of
different
materials is quite different and that calibration results are
critically
influenced by radiation. The first resource of radiation are
lights
producing constant radiation that can be eliminated from
calibra-
tion result by calculation. The second source of radiation is
the
operator. This source is not constant and is quite critical
because
the gauge block that is closer to the operator is heated
stronger
that the gauge block standing behind the first one. Shield that
is
put between the operator and the comparator is made of
transparent plastics in order to enable the operator to look
into measuring area.
6B. Acko, A. Sostar and A. Gusel, REDUCTION OF UNCERTAINTY IN
CALIBRATION OF GAUGE BLOCKS
Laboratory for Production Measurement, Faculty of Mechanical
Engineering, University of Maribor, 2000, Maribor,Slovenia.
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However, the isolating properties of this material are not good
enough to stop the
radiation of a human body.
5.2 Surfaces for thermal stabilisation of the gauge blocks
Surfaces for thermal stabilisation of gauge blocks have a great
influence on
thermal behaviour of gauge blocks during the calibration. If
these surfaces get other
temperature than the air and the comparator table, the gauge
block temperature
changes rapidly. Our analyses have shown that an aluminium plate
with bright shining
or light grey surface gives the best results. Long term
temperature measurements are
shown in Figure 3 as an example. The white plastic surfaces used
before these studies
are always warmer than the air and cause additional heating of
gauge blocks. The
same properties have steel surfaces. The stone that has been
used in the past has the
worst characteristics and is not recommended at all.
In this research four materials (aluminium, steel, stone and
plastic) in four different
colours (black, white, grey and shining) were tested.
Temperature was measured in the
air, on the tested surface and on the gauge blocks, that were
put on these surfaces.
Long term and short term deviations were measured with the light
turned on and with
the light turned off in the measurement room. When the light was
turned off, the
differences were not so critical because the radiation was very
low. In contrary, when
the lights were turned on, big differences in temperature
changes were observed. The
worst behaviour was detected at dark steel and stone
surfaces.
Figure 3. Temperature comparison between the aluminium base
plate and the gauge
block.
5.3 Material of the comparator table
Some simulations with different materials have shown the
importance of the
material and the colour of the table. The existing tables
(tested on Mahr and Cary
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comparators) do not have ideal properties because of dark
colours. The temperature is
constantly above the air temperature. When gauge block is put on
the table, the
temperature starts to grow and the length of the gauge block is
changing.
Typical temperature behaviour after putting gauge block on the
table is shown in figure
3. In this case gauge block was put on the comparator table at
10:41 and at 10:50
calibration of this gauge block was finished. From this case we
can learn, that the
gauge block should not be measured immediately after putting it
on the table. It should
be stabilised for approximately three minutes. Probe 1 in Figure
3 was measuring the
reference gauge block temperature and probe 2 was measuring the
temperature of
measured gauge block. We can see that the temperatures of both
gauge blocks after
stabilisation are not equal.
5.4 Thermal expansion coefficient of the gauge blocks
Thermal expansion coefficient must be determined with standard
uncertainty
0.3 10. For this we shall determine thermal expansion
coefficient within the
limits of 0.5 10. Usually this is not a problem, but sometimes
we get gauge
blocks without any data about . In such cases we can not be sure
that was
determined with proper accuracy. Temperature deviations must
therefore be dimini-
shed to minimum possible values.
Another problem is the difference between thermal expansion
coefficients of the
reference and measured gauge blocks. Uncertainty contribution of
his difference in mm
is expressed by the equation:
= ()
Where:
- uncertainty contribution in mm() - uncertainty of the
difference of thermal expansion coefficients ( ( - nominal length
of gauge blocks - maximum possible temperature deviation
If uncertainty of the difference between thermal expansion
coefficients is 0.6 10
and maximum possible temperature deviation is 0.1C, than the
uncertainty contribution
in calibration of 100 mm gauge block is 0.006 m what is the
maximum possible limit for
our demands. If temperature deviation is 0.3C, we get the value
of 0.018 m, what is
far too much.
From the above example we can conclude that temperature
deviations and the
limits of determination of thermal expansion coefficient must be
within very low values.
These two components are in strong correlation and influence the
length dependent
part of the combined uncertainty.
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Figure 4. Thermal behaviour of gauge blocks after putting them
on the comparator
table
5.5 Indentation of different gauge blocks by the applied
measuring force
Research in this field is made in order to be able to calibrate
gauge blocks of
different materials. The indentation is calculated by the
following (Roark's) equation:
= 1.4
=1
+1
Where:
- indentation - probing force - probing ball diameter - Poisson
number of the probing ball material - Poisson number of the gauge
block material - elasticity module of the probing ball material -
elasticity module of the gauge block material
If probing force is 0,1 N, probing ball of d = 2 mm is made of
hart metal and gauge block
is made of steel, than the indentation is 0.116 m. For the gauge
block made of hart
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metal this value would be 0.081 m. The difference of the both
indentations is 0.035
m. The greatest problem in this calculations is to get right
values for and . Every
deviation of these values causes uncertainty of indentation
calculation. The second
problem is evaluation of probing force, which shall be measured
very precisely. There-
fore, some experimental work has already been performed in order
to evaluate uncer-
tainty of indentation. It was found out that the standard
uncertainty can be held in the
limits of 0.008 m.
6. DRIFT ELIMINATING DESIGN
The sources of variation in measurements are numerous.
Measurements on gauge
blocks are subject to drift from heat built-up in the
comparator. This effect cannot be
minimized by additional measurement because it is not generally
pseudorandom, but a
nearly monotonic shift in the readings. In dimensional work the
most important cause of
drift is thermal changes in the equipment during the test.
The purpose of drift eliminating design is remove the effects of
linear instrumental drift,
but also alowing the measurement of the linear drift itself.
This measured drift can be
used as a process control parameter. For small drift rates an
assumption of linear drift
will certainly be adequate. But, for high drift rates or long
measurements time the
assumption of linear drift may not be true.
Measurements on gauge blocks are subject to drift from heat
build-up in the compa-
rator. This drift must be accounted for in the calibration
experiment or the lengths
assigned to the blocks will be contaminated by the drift term.
The designs are
constructed so that the solutions are immune to linear drift if
the measurements are
equally spaced over time. The size of the drift is the average
of the difference
measurements. Keeping track of drift from design to design is
useful because a marked
change from its usual range of values may indicate a problem
with the measurement
system.
7. OVERVIEW OF THE GAUGE BLOCK COMPARATOR PRECIMAR 826 VS
TESA UPG.
PRECIMAR 826 TESA UPD TESA UPCDESIGN: rigid cast-iron stand with
vertical
guiderigid cast-iron stand with verticalguide
Measuring range 0.5 mm to 170 mm 0.5 mm up to 100 mmWeight 37 Kg
(comparator only) 23 kg (comparator only)NOTE:Heavier Unit more
steady (immune from vibration)Repeatability (with no influenceof
external temperature)
0.01 m 0.04 m
Measuring uncertainty (.+ )/ , = (. + . ( For instance Gauge
Block 100 For instance Gauge Block 100
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mm, = (0.03 + 100/3000) = 0.03 + 0.033
= 0.063
mm, = (0.07 + 0.5 0.1) = 0.07 + 0.05
= .NOTE:Following the hierarchy of calibration, calibrator must
be 10% or 20% smaller then tolerance of theworking standard. For
example, Gauge Block tolerance 0.12 m (Grade 0, length 100 mm),
must becalibrated by Gauge Block Comparator with Uncertainty
minimum 0.12 x 0.2 = 0.024 m. Following thisreason, methode direct
measurement is not acceptable. In the comparison method the
stability of theinstrument is 0.02 + 0.25 10 ( Precimar
826).Repeatability (with no influenceof external temperature)
0.01 m 0.025 m
Measuring uncertainty (.+ )/ , = (. + . ( For instance Gauge
Block 100mm, = (0.03 + 100/3000)
= 0.03 + 0.033= 0.063
For instance Gauge Block 100mm, = (0.05 + 0.5 0.1)
= 0.05 + 0.05= .
Repeatability (with no influenceof external temperature)
0.01 m 0.015 m
Measuring uncertainty (.+ )/ , = (. + . ( For instance Gauge
Block 100mm, = (0.03 + 100/3000)
= 0.03 + 0.033= .
For instance Gauge Block 100mm, = (0.02 + 0.2 0.1)
= 0.02 + 0.02= .
NOTE:Following the Gauge Block tolerance in the Table 3, UPC and
UPD can calibrate Gauge Block Grade 1and 2 only. Precimar 826 can
calibrate Gauge Block Grade 0, 1 and 2. Only TESA with= (0.02 + 0.2
( can be used for Grade 0, but this instrument still struggle to
get the uncertaintybecause it will depends on the temperature
correction. You know that in the paragraph 4.1 explainedThermal
expansion correction is therefore not accurate. So that is why the
Comparator need the deviationof the temperatur is 0.03C.Material
Housing Diecast Iron Diecast AluminumSensors for length
dimensions:
Double Probes Yes Yes Yes
NOTE:The measuring configuration consisting of two probes
aligned opposite one another combined with theconcept and quality
of the measuring system is the guarantee for an extra low
measurement uncertainty.
With analog and digitalindication system,
Yes Digital Only Analog Only
Measuring force activation Measuring bolt retraction
YesBy electrical vacuum pum lifted
YesBy Vacuum
YesBy electro-motorised
Measurement Method Comparative Measurement DirectMeasurement
ComparativeMeasurement
Advantage:Enhances the measuring condi-tions, thus permitting
all measure-ments to be taken with loweruncertainty. NO linear
error.Disadvantages:Allows gauge blocks of samenominal lengths to
be measuredby comparison. Need 122 GaugeBlock set as a standard
compa-rator.
Advantage:Permits over 90 % of a 122-pieceset to be checked
using the samereference gauge block All nominallengths of the full
gauge set beingcontained within 0,5 and 25 mm.Disadvantages:In fact
the measuring span istherefore exceeded, caused ofLinear Error of
the probes.
NOTE:For the Direct measurement method actually data will found
linear error that is not an advantages for themachine due to the
machine must immune to linear drift. So the choice of the
Comparative methode is thebest solution.
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PRECIMAR 826 TESA UPD TESA UPDComputer-aided calibration
Measurement of single gage blocks Measurement of complete gage
block sets Simultaneous calibration of several
equal gage block sets.
YesYes
Yes
YesYes
Yes
Features of the Hardware: Selection and determination of
measuring sequences.Yes Yes
Management of testpiece andstandard gauge block sets.
Yes Yes
Management of individual gaugeblocks
Yes Yes
Measuring program to perform gaugeblock tests
Yes Yes
Control of all operations and inputs Yes Yes Automatically
assigning the sequenceof nominal dimensions for set tests
Yes Yes
Organization of the measurementprocess for testing multiple
sets
Yes Yes
Printer program for test records andfor the printout of standard
gage blocksets
Yes Yes
Data management Yes Yes
ADVANTAGES: Rigid cast-iron stand, making the
unit temperature stableYes Yes
Easily adjustable vertical slide withupper probe
Yes Yes
Fine adjustment via rigidlyConnected Parallelogram springs
Yes No
NOTE:Fine adjustment is carried out by adjusting a rigidly
connected spring parallelogram system which isintegrated into the
support arm. Electro pneumatic lifting Yes Yes Gentle, precise and
extremely smoothmanipulator operation due to slideways,which are
impervious to dirt
Yes Yes
NOTE:The foot of the base holds the manipulator for correctly
positioning the gage blocks. Smooth movement isensured by high
precision ball bush guides. Ergonomical operation due to the
optimally arranged gauge blockmanipulator
Yes Yes
NOTE:Whenever the gage blocks are moved, the inductive probes
are lifted by means of an electrical vacuumpump. Measurement not
influenced by
manual forceYes Yes
Easy movement of the gage blocksdue to support consisting
ofhardened circular guide bars
Yes Yes
No zero setting required, since theset value is automatically
related tothe nominal value of the respectivereference gage
block
Yes Yes
Very effective protection from heat Yes Yes
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due to an acrylic glass screen alongthe front and both sides of
the unit
Penetration correction Yes NONOTE:See paragraph 5.5, the
correction of the geometric factor of the probe tip penetration.
Correction of differing thermal
expansion coefficientsYes Yes
Computation of mean values Yes Yes Simultaneous calibration of
several
sets possibleYes Yes
Inherent stabilization of temperatureand vibration
Yes Yes
CONCLUTION:
All the concept of the Comparator must be supported for the its
accuracy, with the
requirement of the uncertainty measurement factors, such as:
1. The design, must be immune from the errors, such as measuring
base by refelcted
material to reduce temperature influence.
2. Comparation method, to avoid linear error of the probes.
3. Quick measurement to avoid linear drift cause of time of
measurement.
4. Avoid themperature correction due to difference of the CTE,
unless the themperature
difference over 0.4C.
5. Correction from the elasticity of the material effect from
the contact point.
References:
1. ISO 3650:1999, Geometrical Product Specifications (GPS)
Length standards
Gauge blocks.
2. ISO. GUM: 1995, Guide Uncertainty of Measurement.
3. Ted Doiron and John Beers, Gage Block HandBook, Dimensional
Metrology Group
Precision Engineering Division National Institute of Standards
and Technology.
4. B. Acko, A. Sostar and A. Gusel, reduction of uncertainty
in
calibration of gauge blocks, Laboratory for Production
Measurement Faculty of
Mechanical Engineering University of Maribor, 2000 Maribor,
Slovenia.