Calibration of Balance

Calibration of Balance
















Sri Lanka

1







Nihal

Gunasekara


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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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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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Micro Balance

4


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Semi Micro Balance


Balance with a resolution of 10 microgram



5

Balance with a resolution of 0.1 microgram


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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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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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Correction


Correction = True Value
-

Reading

8

Error

Error = Reading
-

True Value


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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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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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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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Influence Factors

14


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
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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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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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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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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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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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

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•
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


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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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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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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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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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Repeatability of Reading


Mass

1
:

Mass

2
:

Mass

3

:


Mean value :

Std. Dev. (
σ

) :

Max. Diff. :

Uncer
. Type A :



32


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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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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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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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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


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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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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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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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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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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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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 ,….)







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Weighing Scale (digital or analogue)




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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

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