Balance calibration - Better Work and Standards Programme

Report
Bangladesh BEST Programme
Calibration of Balance
Nihal Gunasekara
Sri Lanka
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Bangladesh BEST Programme
Types of Balances
•
•
•
•
•
Electromagnetic force balance (Top loading)
Single-pan, two-knife-edge balance
Two-pan, three-knife-edge balance
Triple beam balance
Weighing scale
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Bangladesh BEST Programme
Analytical Balance
Balance with an enclosed weighing chamber and a
resolution 0.01 or 0.1 mg
Micro Balance
Balance with a resolution of 1 microgram
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Bangladesh BEST Programme
Micro Balance
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Bangladesh BEST Programme
Semi Micro Balance
Balance with a resolution of 10 microgram
Balance with a resolution of 0.1 microgram
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Bangladesh BEST Programme
Resolution or Discrimination
The smallest change in mass which can be detect by the
balance
Sensitivity
This is a measure of the ability of the balance
to detect changes in the load applied to it
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Bangladesh BEST Programme
Response time
Time taken for the weighing instrument to indicate the value
of a change in load
Stability
A measure of the time for which the reading on the balance
remains unchanged
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Bangladesh BEST Programme
Correction
Correction = True Value - Reading
Error
Error = Reading - True Value
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Bangladesh BEST Programme
Departure From Nominal Value
The amount by which the reading on an instrument depart
from its nominal value
Hysteresis
The indications of a measuring instrument when the same
value of the quantity measured is reached by increasing or
decreasing that quantity
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Bangladesh BEST Programme
Standby
Operating state of an electronic instrument where
one part is switched off for energy saving reasons
Accuracy class. (mass)
Legal metrology term. Classification of weights for calibration
purposes by OIML R111-1 weights of Accuracy class E1, E2,
F1, F2, M1, M2, M3 from 1 mg to 50 kg
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Bangladesh BEST Programme
Different accuracy classes of mass
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Bangladesh BEST Programme
Repeatability
 Closeness of the agreement between the results
of successive measurements of the same
measurement carried out under the same
conditions of measurement.
 Repeatability condition include :
• the same observer
• the same measuring instrument, used under the same
conditions
• the same location
• repetition over a short period of time
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Bangladesh BEST Programme
Reproducibility
 Closeness of the agreement between the results of measurements of
the same measurement carried out under changed conditions of
measurement.
 A valid statement of reproducibility requires specification of the
conditions change
 The changed conditions may include
– principle of measurement
– method of measurement
– observer
– measuring instrument
– reference standard
– location
– conditions of use
– Time
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Bangladesh BEST Programme
Influence Factors
Air movements and heat sources
Support surfaces and vibration
Electrical and electromagnetic interference
Magnetic effects
Buoyancy effects
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Bangladesh BEST Programme
Influence factor
Influence
Parameter
Source
Effect
Test for effect
Precautions
Vibration
Various, Eg. Machinery,
roads, railways, aircraft
operator
Changes forces on the
weighing machine due
to acceleration
Primary
Use of a high-mass
support, set on a solid
surface, for the
weighing machine,
independent of the
adjacent flooring, Eg
fixed to bedrock
factor
Environmental
Vibration causes
damages
Stability of reading
under constant load
Secondary
Accelerometer
measurements
Note : low frequency,
vibration is likely to be
more serious in its
effects than high
(acoustic) frequencies
Rubber pads used to
damp high frequencies
may pass low frequency
vibration, and may
cause tilting of the
weighing machine
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Bangladesh BEST Programme
Influence factor
Influence
Parameter
Source
Effect
Test for effect
Precautions
Temperature: air,
weight, weighing
machine
Sunlight, heating
systems, operator’s
body, greenhouse
heating in the weighing
chamber
Temperature changes
and differentials can
change the weighing
machine response,
cause convection
effects and change
buoyancy
Temperature monitoring
instruments
Avoid direct sunlight
and other connections
to the external
environment; avoid
changes in heat and
lighting input. Leave
balance cases ajar when
not in use. Avoid
excessive handling of
equipment during
weighing. Allow
weighted object to
acclimatize
factor
Environmental
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Bangladesh BEST Programme
Influence factor
Influence
Parameter
Source
Effect
Test for effect
Precautions
Humidity
Air supply ,air
conditioning, changes
in activity levels
Buoyancy, absorbed
moisture, condensation
Rh monitoring
instruments
Avoid liquid water in the
working area unless rh
is too low .
factor
Environmental
Air conditioning,
humidity controller
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Bangladesh BEST Programme
Influence factor
Influence
Parameter
Source
Effect
Test for effect
Precautions
Airflow
Air conditioning,
wind, convection,
operator movement
Varies force on
weighing machine
Primary
Avoid temperature
gradients. draught
free working space,
enclose weighing
factor
Environmental
Stability of reading
under constant load
Secondary
machines
Enclose weighing
machine and observe
change of reading
stability
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Bangladesh BEST Programme
Influence factor
Influence
Parameter
Source
Effect
Test for effect
Precautions
Magnetic
Constructional
steelwork,
ferromagnetic weight
(s) electrical induction
earth’s magnetic field,
moving-coil meters
Change of current in
force compensation
coil, direct attraction
to/from surrounding
structure Motiondependent restoring
forces
Primary
Avoid weights with
residual magnetism
(de-gauss) Test with
compass or
gaussmeter Limit
magnetic
permeability of
weights weighed
check for magnetic
fields with
gaussmeter Screen
with mumetal
factor
Environmental
Use a magnet to check
for induced reading
changes
Secondary
Monitoring readings
when weights are
rotated and at varying
heights above the
weight receptor (place
on/under non-magnetic
mount)
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Bangladesh BEST Programme
Source of error
Type of error
Load cell hysteresis
Repeatability
Electrical Interference
Repeatability
Mains fluctuation
Zero
Incorrect horizontal support
Sensitivity, Zero, Repeatability
Mechanical errors
Repeatability
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Bangladesh BEST Programme
ACCURACY CLASS (OIML R111-1 :2004)
Nominal value
Tolerance in mg
Class E1
Class E2
Class F1
Class F2
Class M1
Class M2
Class M3
10 kg
5.0
16
50
160
500
1600
5000
5 kg
2.5
8.0
25
80
250
800
2500
2 kg
1.0
3.0
10
30
100
300
1000
1 kg
0.5
1.6
5.0
16
50
160
500
200 g
0.10
0.3
1. 0
3
10
30
100
100 g
0.05
0.16
0.5
1.6
5
16
50
50 g
0.030
0.10
0.3
1.0
3
10
30
10 g
0.020
0.06
0.20
0.6
2.0
6.0
20
5g
0.016
0.05
0.16
0.5
1.6
5.0
16
2g
0.012
0.04
0.12
0.4
1.2
4.0
12
1g
0.010
0.03
0.10
0.3
1.0
3.0
10
100 mg
0.005
0.016
0.05
0.16
0.5
1.6
50 mg
0.004
0.012
0.04
0.12
0.4
10 mg
0.003
0.008
0.025
0.08
0.25
5 mg
0.003
0.006
0.020
0.06
0.20
0.003
0.006
0.020
0.06
0.20
1 mg
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Bangladesh BEST Programme
Lowest class of weights required for different types of weighing
machine
Resolution
Capacity
100g
10g
1g
10mg
1mg
0.1mg
0.01mg
or less
M1
M1
F2
F1
E2
M1
M1
F2
F1/E2
E2
E2
E2
E2
Up to 200g
200g to 1 kg
100mg
1 kg to 30kg
M2
M2
M1
F2
E2
30kg to 100kg
M2
M1
F2
F1
E2
Above 100 kg
M2
M1/F2
F1
E2
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Bangladesh BEST Programme
Calibration of Top Loading and Analytical Balances
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Bangladesh BEST Pregramme
Before use a Balance…………
•
•
Keep the Balance power on appropriate time to warm up.
Clean the balance as best as no contamination on the balance pan.
•
•
•
•
If it is applicable, level the balance before use.
Don’t move the balance after the calibration.
Lint or any other soft gloves should be worn by the user.
A pair of forceps or a pair tweezers with thermal insulating ends should
be used to lift and place weights.
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Bangladesh BEST Programme
Before use a Balance
• Make sure with the soft placement of weights on the balance pan as
level best of possible.
• Readings should be taken after the balance is stabilized well.
• The place should be free from vibration.
• Any other relevant precaution practiced by the user.
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Bangladesh BEST Programme
Balance Calibration
The calibration of the electromagnetic force
compensation balance consists of the following tests:
•
•
•
•
•
Test 1 - Scale Value
Test 2 - Repeatability of Reading
Test 3 - Departure from Nominal Value
Test 4 - Off centre Loading
Test 5 – Hysteresis
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Bangladesh BEST Programme
Scale Value
Internal Scale Adjustment –Internal Mass
External Scale Adjustment – External Mass
Procedure – Balance Operating
Manual
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Bangladesh BEST Programme
Repeatability of Reading
Objective of Measurement :
The repeatability test is carried out to determine the ability of the
balance to give under defined conditions of use closely similar
responses for repeated measurements
The balance Standard Deviation is the “repeatability of
measurement”
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Bangladesh BEST Programme
Repeatability of Reading
:
• M 1 -Set the reading to zero without any mass on the
pan
• M 2 - Record the zero reading of the balance (z)
• M 3 - Place the mass on the pan and record the reading
(r)
• M 4 - Remove the mass from the pan
• M 5 – Repeat M2 to M4 ten times
• during the process the balance should not be zeroed
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Bangladesh BEST Programme
Repeatability of Reading:
Performed at three points :
 Near zero
 Near half load
 Near full load
Bangladesh BEST Programme
DATA RECORDING
Mass 1 :
No
Zero
reading
(z)
Mass
reading
(r)
Mass 2:
Corr.
reading
(r-z)
Zero
reading
(z)
Mass
reading
(r)
Mass 3:
Corr.
reading
(r-z)
Zero
reading
(z)
Mass
reading
(r )
Corr.
reading
(r-z)
1
2
3
4
5
6
7
8
9
10
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Bangladesh BEST Programme
Repeatability of Reading
Mass 1:
Mean value
Std. Dev. ( σ )
Max. Diff.
Uncer. Type A
Mass 2:
Mass 3 :
:
:
:
:
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Bangladesh BEST Programme
Repeatability of Reading
Repeat three different values of M ; near zero, half maximum
and maximum load.
Load on Pan
Standard
Deviation of
Reading (mg)
Maximum Difference
Between Successive
Readings (mg)
0.05
0.067
0.2
100
0.052
0.1
200
0.052
0.1
(g)
Bangladesh BEST Programme
Departure from Nominal Value:
Objective of Measurement :
“The departure from nominal value measurements
are carried out to determine the deviation of the
balance reading from the expected value”
This is the Correction to Balance Reading
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Bangladesh BEST Programme
Departure from Nominal Value:
1 Divide the range into at least 10 equally spaced steps
2 The masses corresponding to M, 2M, 3M, 4M,
5M, 6M etc. until the capacity of balance is reached
3 In addition choose four more test points
corresponding to 20 %, 40 %,60%and 80 % of the first
step for single range.
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Bangladesh BEST Programme
Departure from Nominal Value:
1 - Read the zero z
2 - Place the weight on the pan &record reading r
3 – Remove the weight momentarily and place it back on
1
1
the pan
and note the new reading r2
4 - Remove the weight and read the zero z
5 – Repeat 1 to 4 for weights of masses 2M,3M,4M,5M etc
2
until the capacity of balance is reached
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Bangladesh BEST Programme
Departure from Nominal Value:
•
Average zero reading z = (z + z )/ 2
•
Average mass reading r = (r + r )/ 2
•
Difference d = r – z
i
1
i
i
i
1
2
2
i
• Correction c = M – d
i
i
i
• M is the conventional value of the mass
i
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Bangladesh BEST Programme
Correction to Balance Reading
Pan Load
Conv. Value
Mi
Reading
M- 100
100.02
0.00
100.00
0.00
100.02
2M
3M
4M
5M
6M
7M
8M
9M
10M
Mean
0.00
100.01
Difference
ri - zi
100.01
Correction
Mi - di
0.01
Bangladesh BEST Programme
Departure from Nominal Value:
Reading
Correction(mg)
Expanded Uncertainty k=2
(g)
Before the
Calibration
After the
Calibration
+(mg)
4
--
0.0
0.096
8
--
0.0
0.097
12
--
-0.1
0.097
16
--
0.1
0.098
20
0.2
-0.0
0.099
40
--
0.0
0.098
60
--
0.0
0.098
80
0.4
-0.1
0.099
100
--
-0.1
0.099
120
0.6
-0.1
0.099
140
--
-0.1
0.10
160
0.4
-0.2
0.10
180
--
-0.3
0.10
200
0.5
-0.3
0.11
220
--
-0.3
0.11
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Bangladesh Best Programme
Off Centre Loading:
Objective of Measurement :
“The off center loading is carried out to find the
variation in the displayed weighted as the object
is moved to various positions on the weighing pan”
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Bangladesh BEST Programme
Off Centre Loading
This test to find the variation of the reading for several place of the
balance pan. Test should be done for rectangular type balance pans
and as well as for circular type balance pans.
A single weight should be used
3
4
1
5
2
3
4
1
2
5
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Bangladesh BEST Programme
Off Centre Loading:
Back (g)
100.0000-100.0000=0.0000
Center (g)
Left (g)
100.0000-100.0000=0.0000
100.0001-100.0000=0.0001
Max Difference
=0.0002 g
Place a mass on
the centre of the pan and
move it successively to the
front rear, left and right
positions ( position being 2/3
of the distance between the
centre and the lip of the
pan) on the pan recording
the reading each time
Right (g)
99.9999-100.0000= -0.0001
Front (g)
100.0001-100.0000=0.0001
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Bangladesh BEST Programme
Off Centre Loading:
Mass of 100g was moved to various positions on the pan. The balance
readings obtained relative to the centre reading
Centre(g)
Front(g)
Back(g)
Left(g)
Right(g)
Maximum
Difference (g)
0.0000
0.0001
0.0000
0.0001
- 0.0001
0.0002
Place a mass on the centre of the pan and move it successively to the front
rear, left and right positions ( position being 2/3 of the distance between the
centre and the lip of the pan) on the pan recording the reading each time
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Bangladesh BEST Programme
Hysteresis:
Objective of Measurement :
“The hysteresis measurements are performed to determine the
difference in the indication at the same load , depending upon the
direction of approach to that load ie increasing or decreasing the
load”
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Bangladesh BEST Programme
Hysteresis
Perform the test at one point about mid range
•
•
•
•
Zero the balance and record the reading z
Place a mass M on the plan and record the reading r
Add extra mass M to bring the balance close to full range
Remove the extra mass M and record the reading with the
mass M still on the pan
• Remove the mass M and record the reading r
• Repeat above steps three times
• Take the average of three readings
1
1
1
2
2
1
1
2
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Bangladesh BEST Programme
Hysteresis
Mass M =
g
Pan Load
Reading 1
Reading 2
Reading 3
Zero ( z1 )
M ( m1 )
M + M1
M ( m2 )
Zero ( z2 )
m1 - m2
z1 - z2
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Bangladesh BEST Programme
Hysteresis:
Pan Load (g)
Hysteresis Error(mg)
0
0.0
100
0.1
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Bangladesh BEST Programme
Hysteresis:
Mass Value VS Display Value
100
Hysteresis
error
80
90
Display Value
70
60
Increasing
50
Decreasing
40
30
20
10
0
0
20
40
60
80
100
Mass Value
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Bangladesh BEST Programme
Calibration of Single Pan two Knife Edge Balances
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Bangladesh BEST Programme
Balance Calibration
The calibration of the single pan two knife
edge balance consists of the following tests:
•
•
•
•
•
Test 1 - Repeatability of Reading
Test 2 - Departure from Nominal Value
Test 3 – Built-in-Mass
Test 4 - Off centre Loading
Test 5 – Hysterisis
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Bangladesh BEST Programme
Balance Calibration
The calibration of the single pan two knife edge
balance consists of the following tests:
• Test 1 - Repeatability of Reading- same as electronic balance
• Test 2 - Departure from Nominal Value- same as
electronic balance
• Test 3 – Built-in-Mass
• Test 4 - Off centre Loading- same as electronic balance
• Test 5 – Hysteresis- same as electronic balance
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Bangladesh BEST Programme
Balance Calibration
•
•
•
•
•
•
The procedure to determine the Built-in- Mass
With all dials set to the zero and no load on the pan,
release the balance and record the reading z1
Set the "1" on the dial to be tested, place a calibrated
mass, release the pan and record the reading m1a
Remove the mass , replace it and record the reading
m1b
Return the dial to zero, remove the mass from the pan
and record the zero z2
Repeat the above steps for all dial settings of the
balance (i.e. 2 , 3, 10 , 20 ,…………..90 , 100 , 200 )
Follow the same procedure used for linearity test to 52
Bangladesh BEST Programme
Calibration of Triple Beam Balances
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Bangladesh BEST Programme
Balance Calibration
The calibration of triple beam balances
consists of the following tests:
• Test 1 - Repeatability of Reading - same as
electronic balance
• Test 2 - Departure from Nominal Value
(Linearity Test) same as electronic balance
54
Bangladesh Best Programme
Triple Beam Balance
Before the calibration
• Clean the weighing pan and other parts of the balance
taking due care
• Level the balance
• Exercise the balance by making several weighing
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Bangladesh BEST Programme
Triple Beam Balance
Repeatability of readings:
• Set the reading to zero without any mass on the pan
• Record the zero reading zi of the balance without any mass
on the pan
• Place the mass M on the pan and record the mass reading
mi
• Remove the mass M from the pan
• Repeat the above steps ten times (The balance should not
be zeroed during the series of readings)
• Repeat for this for near zero, half load and near maximum
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Bangladesh Best Programme
Triple Beam Balance
• Select a minimum of 10 equally spaced points ( M , 2M ,…..10M ) on the
measuring range in such a way they cover the entire range or the part of the
measuring range to be calibrated. In addition four test points corresponding to
20%,40%,60% and 80 % of the first step
• Record the zero reading z1
•
Set the slider scale to the first step on the measuring range (M) , place the
calibrating mass(es) on the pan until the balance comes to equilibrium and
record the mass reading mi
•
Remove the mass(es), place it back on the pan and record the mass reading mi2
• Remove the mass(es) and record the zero reading zi+1
•
Repeat the above steps for the remaining points ( 2M , 3M ,….)
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Bangladesh Best Programme
Weighing Scale (digital or analogue)
The calibration of weighing scales consists of the following
tests:
Test 1 - Repeatability of Reading - same as electronic
balance
Test 2 - Departure from Nominal Value (Linearity
Test) same as electronic balance
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Bangladesh BEST Programme
Calibration Results of a Set of Weights –Class F1
Nominal Value Conventional Value
Uncertainty k=2
Max. Perm. Error
1 mg
1 mg – 0.001 mg
0.006 mg
±0.02 mg
2 mg
2 mg + 0.011 mg
0.006 mg
±0.02 mg
5 mg
5 mg+0.002 mg
0.006 mg
±0.02 mg
20 mg
20 mg – 0.005 mg
0.01 mg
±0.03 mg
50 mg
50 mg + 0.009 mg
0.012 mg
±0.04 mg
100 mg
100 mg + 0.011 mg
0.015 mg
±0.05 mg
500 mg
500 mg + 0.033 mg
0.025 mg
±0.08 mg
1g
1 g – 0.028 mg
0.03 mg
±0.1 mg
2g
2 g + 0.022 mg
0.04 mg
±0.12 mg
20 g
20 g + 0.009 mg
0.08 mg
±0.25 mg
50 g
50 g – 0.12 mg
0.1 mg
±0.3 mg
100 g
100 g – 0.22 mg
0.15 mg
±0.5 mg
200 g
200 g + 0.04 mg
0.3 mg
±1 mg
Issued by an accredited laboratory (DKD)
59
Bangladesh BEST Programme
Thank you
60

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