Calibration of Balance
Sri Lanka
1
Nihal
Gunasekara
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
2
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Programme
Analytical Balance
Balance with an enclosed weighing chamber and a
resolution 0.01 or 0.1 mg
3
Micro Balance
Balance
with a resolution of 1 microgram
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Programme
Micro Balance
4
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Programme
Semi Micro Balance
Balance with a resolution of 10 microgram
5
Balance with a resolution of 0.1 microgram
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Programme
Resolution or Discrimination
The smallest change in mass which can be detect by the
balance
6
Sensitivity
This
is a measure of the ability of the balance
to detect changes in the load applied to it
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Programme
Response time
Time taken for the weighing instrument to indicate the value
of a change in load
7
Stability
A
measure of the time for which the reading on the balance
remains unchanged
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Programme
Correction
Correction = True Value
-
Reading
8
Error
Error = Reading
-
True Value
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Programme
Departure From Nominal Value
The amount by which the reading on an instrument depart
from its nominal value
9
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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Programme
Standby
Operating state of an electronic instrument where
one part is switched off for energy saving reasons
10
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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Programme
11
Different accuracy classes of mass
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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
12
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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
13
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Programme
Influence Factors
14
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Programme
Air movements and heat sources
Support surfaces and vibration
Electrical and electromagnetic interference
Magnetic effects
Buoyancy effects
Influence factor
15
Use of a high
-
mass
support, set on a solid
surface, for the
weighing machine,
independent of the
adjacent flooring,
Eg
fixed to bedrock
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
Primary
Stability of reading
under constant load
Secondary
Accelerometer
measurements
Changes forces on the
weighing machine due
to acceleration
Various,
Eg
. Machinery,
roads, railways, aircraft
operator
Vibration causes
damages
Vibration
Environmental
Precautions
Test for effect
Effect
Source
Parameter
Influence
factor
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Programme
Influence factor
16
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
Temperature monitoring
instruments
Temperature changes
and differentials can
change the weighing
machine response,
cause convection
effects and change
buoyancy
Sunlight, heating
systems, operator’s
body, greenhouse
heating in the weighing
chamber
Temperature: air,
weight, weighing
machine
Environmental
Precautions
Test for effect
Effect
Source
Parameter
Influence
factor
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Programme
Influence factor
17
Avoid liquid water in the
working area unless
rh
is too low .
Air conditioning,
humidity
controller
Rh
monitoring
instruments
Buoyancy, absorbed
moisture, condensation
Air supply ,air
conditioning, changes
in activity levels
Humidity
Environmental
Precautions
Test for effect
Effect
Source
Parameter
Influence
factor
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Programme
Influence factor
18
Precautions
Test for effect
Effect
Source
Parameter
Influence
factor
Avoid temperature
gradients. draught
free working space,
enclose weighing
machines
Primary
Stability of reading
under constant load
Secondary
Enclose weighing
machine and observe
change of reading
stability
Varies force on
weighing machine
Air conditioning,
wind, convection,
operator movement
Airflow
Environmental
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Programme
Influence factor
19
Precautions
Test for effect
Effect
Source
Parameter
Influence
factor
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
Primary
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)
Change of current in
force compensation
coil, direct attraction
to/from surrounding
structure Motion
-
dependent restoring
forces
Constructional
steelwork,
ferromagnetic weight
(s) electrical induction
earth’s magnetic field,
moving
-
coil meters
Magnetic
Environmental
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20
Repeatability
Load cell
hysteresis
Repeatability
Electrical Interference
Zero
Mains fluctuation
Sensitivity, Zero, Repeatability
Incorrect horizontal support
Repeatability
Mechanical
errors
Type of error
Source of error
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21
ACCURACY
CLASS (OIML R111
-
1 :2004)
0.20
0.06
0.020
0.006
1 mg
0.20
0.06
0.020
0.006
0.003
5 mg
0.25
0.08
0.025
0.008
0.003
10 mg
0.4
0.12
0.04
0.012
0.004
50
mg
1.6
0.5
0.16
0.05
0.016
0.005
100 mg
10
3.0
1.0
0.3
0.10
0.03
0.010
1 g
12
4.0
1.2
0.4
0.12
0.04
0.012
2 g
16
5.0
1.6
0.5
0.16
0.05
0.016
5 g
20
6.0
2.0
0.6
0.20
0.06
0.020
10 g
30
10
3
1.0
0.3
0.10
0.030
50 g
50
16
5
1.6
0.5
0.16
0.05
100 g
100
30
10
3
1
. 0
0.3
0.10
200 g
500
160
50
16
5.0
1.6
0.5
1 kg
1000
300
100
30
10
3.0
1.0
2 kg
2500
800
250
80
25
8.0
2.5
5 kg
5000
1600
500
160
50
16
5.0
10 kg
Class M
3
Class M
2
Class M
1
Class F
2
Class F
1
Class E
2
Class E
1
Tolerance
in mg
Nominal value
0.003
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Lowest class of weights required for different types of weighing
machine
22
E2
E2
E2
F2
M1
M2
M2
1 kg to 30kg
E2
E2
F1/E2
F2
M1
M1
200g to 1 kg
E2
F1
F2
M1
M2
30kg to 100kg
E2
F1
F2
M1
M1
Up to 200g
E2
F1
M1/F2
M2
Above 100 kg
0.01mg
or less
0.1mg
1mg
10mg
100mg
1g
10g
100g
Resolution
Capacity
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Programme
Calibration of Top Loading and Analytical Balances
23
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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.
24
Bangladesh BEST
Pregramme
•
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.
25
Before use a Balance
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
26
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Programme
Scale Value
Internal
Scale
Adjustment
–
Internal
Mass
External
Scale
Adjustment
–
External
Mass
Procedure
–
Balance Operating
Manual
27
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Repeatability of Reading
Objective
of
Measurement
:
T
he
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”
28
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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
29
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Repeatability of Reading
:
Performed at three points :
Near zero
Near half load
Near full load
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Programme
DATA RECORDING
Mass
1
:
Mass
2
:
Mass
3
:
31
No
Zero
reading
(z)
Mass
reading
(r)
Corr.
reading
(r
-
z)
Zero
reading
(z)
Mass
reading
(r)
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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Programme
Repeatability of Reading
Mass
1
:
Mass
2
:
Mass
3
:
Mean value :
Std. Dev. (
σ
) :
Max. Diff. :
Uncer
. Type A :
32
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Programme
Repeatability of Reading
0.2
0.1
0.1
0.067
0.052
0.052
0.05
100
200
Maximum Difference
Between Successive
Readings (mg
)
Standard
Deviation of
Reading (mg)
Load on Pan
(g)
Repeat three different values of M ; near zero, half maximum
and
maximum load.
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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
34
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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.
35
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Programme
Departure from Nominal Value:
1
-
Read the zero z
1
2
-
Place the weight on the pan &record reading r
1
3
–
Remove the weight momentarily and place it back on
the pan and note the new reading r
2
4
-
Remove the weight and read the zero z
2
5
–
Repeat 1 to 4 for weights of masses 2M,3M,4M,5M etc
until the capacity of balance is reached
36
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Programme
Departure from Nominal Value:
•
Average zero reading
z
i
= (z
1
+ z
2
)/ 2
•
Average mass reading
r
i
= (r
1
+ r
2
)/ 2
•
Difference
d
i
=
r
i
–
z
i
•
Correction
c
i
= M
i
–
d
i
•
M
i
is the conventional value of the mass
37
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Correction to Balance Reading
Pan Load
Conv. Value
M
i
Reading
Mean
Difference
r
i
-
z
i
Correction
M
i
-
d
i
M
-
100
100.02
0.00
100.00
0.00
100.02
0.00
100.01
100.01
0.01
2M
3M
4M
5M
6M
7M
8M
9M
10M
Bangladesh BEST
Programme
Departure from Nominal Value:
39
0.0
0.0
-
0.1
0.1
-
0.0
0.0
0.0
-
0.1
-
0.1
-
0.1
-
0.1
-
0.2
-
0.3
-
0.3
-
0.3
After the
Calibration
Before the
Calibration
0.096
0.097
0.097
0.098
0.099
0.098
0.098
0.099
0.099
0.099
0.10
0.10
0.10
0.11
0.11
--
--
--
--
0.2
--
--
0.4
--
0.6
--
0.4
--
0.5
--
4
8
12
16
20
40
60
80
100
120
140
160
180
200
220
Expanded Uncertainty k=2
+
(mg)
Correction(mg)
Reading
(g)
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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”
40
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Off Centre Loading
41
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
4
3
1
5
2
3
2
4
5
1
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Programme
Off Centre Loading:
42
Center (g)
100.0000
-
100.0000=0.0000
Left (g)
100.0001
-
100.0000=0.0001
Back (g)
100.0000
-
100.0000=0.0000
Right (g)
99.9999
-
100.0000=
-
0.0001
Front (g)
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
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Programme
Off Centre Loading
:
43
0.0002
-
0.0001
0.0001
0.0000
0.0001
0.0000
Maximum
Difference (g)
Right(g)
Left(g)
Back(g)
Front(g)
Centre(g)
Mass of 100g was moved to various positions on the pan. The balance
readings obtained relative to the centre reading
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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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”
44
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Programme
Hysteresis
Perform
the
test
at
one
point
about
mid
range
•
Zero the balance and record the reading z
1
•
Place a mass M
1
on the plan and record the reading r
1
•
Add extra mass M
2
to bring the balance close to full range
•
Remove the extra mass M
2
and record the reading with the
mass M
1
still on the pan
•
Remove the mass M
1
and record the reading r
2
•
Repeat above steps three times
•
Take the average of three readings
45
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Hysteresis
Mass M = g
46
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Pan Load
Reading 1
Reading 2
Reading 3
Zero ( z
1
)
M ( m
1
)
M + M
1
M ( m
2
)
Zero ( z
2
)
m
1
-
m
2
z
1
-
z
2
Hysteresis:
47
0.1
100
0.0
0
Hysteresis Error(mg)
Pan Load (g)
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Hysteresis:
48
Mass Value VS Display Value
0
10
20
30
40
50
60
70
80
90
100
0
20
40
60
80
100
Mass Value
Display Value
Increasing
Decreasing
Hysteresis
error
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Calibration of Single Pan two Knife Edge Balances
49
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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
50
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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
51
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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 z
1
•
Set the "1" on the dial to be tested, place a calibrated
mass, release the pan and record the reading m
1a
•
Remove the mass , replace it and record the reading
m
1b
•
Return the dial to zero, remove the mass from the pan
and record the zero z
2
•
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
determine the correction
52
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Calibration of Triple Beam Balances
53
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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
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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
55
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Triple Beam Balance
Repeatability of readings:
•
Set the reading to zero without any mass on the pan
•
Record the zero reading
z
i
of the balance without any mass
on the pan
•
Place the mass M on the pan and record the mass reading
m
i
•
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
56
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Triple Beam Balance
•
S
elect 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 z
1
•
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 m
i
•
Remove the mass(
es
), place it back on the pan and record the mass reading m
i2
•
Remove the mass(
es
) and record the zero reading z
i+1
•
Repeat the above steps for the remaining points ( 2M , 3M ,….)
57
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Weighing Scale (digital or analogue)
58
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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
Calibration Results of a Set of Weights
–
Class F
1
59
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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
1 g
1 g
–
0.028 mg
0.03 mg
±
0.1 mg
2 g
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)
Thank you
60
Bangladesh BEST
Programme
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