Calibration of nonautomatic weighing instruments

Calibration of nonautomatic weighing instruments
XVIII IMEKO WORLD CONGRESS
Metrology for a Sustainable Development
September, 17 – 22, 2006, Rio de Janeiro, Brazil
Calibration of nonautomatic weighing instruments
Author 1: Adriana Vâlcu
1
National Institute of Metrology, Bucharest, Romania, adriana.valcu@inm.ro
Abstract: This paper describes the metrological
requirements and the measurements methods for calibrating
nonautomatic weighing instruments. It also may be a useful
guideline for operators working in calibration laboratories
accredited in various fields. This paper is also intended to
enable metrology laboratories to prove a given expanded
uncertainty using suitable procedures. Two examples of
calibration methods are given: one for electronic weighing
instruments and another for the mechanical instruments. The
described methods include information regarding the
measurements standards used for calibration, environmental
conditions, calibration procedures and estimation of the
measurement uncertainty.
Keywords: electronic weighing instruments, mechanical
weighing instruments, multiple ranges and multi-interval
instruments.
Introduction
Calibration methods and the evaluation of the
uncertainty described in the paper are in compliance with
the OIML Recommendation OIML R 76-1 and ISO-GUM
[3] (Guide to the expression of uncertainty in
measurements-1995). The combined standard uncertainty
results from both the type A and type B evaluations of the
measurements uncertainty.
The environmental working conditions shall be suitable
for the instrument to be calibrated. The room where the
balances are installed must be temperature and humidity
controlled. It is not allowed to place balances near
equipment that generates vibration or in a room where dust
may affect them. Also, heat transmission by solar radiation
through the windows shall be prevented.
A. Calibration of electronic balances
Before calibration, an electronic weighing
instrument should be checked on site to make sure it
functions adequately for the intended application. The
preliminary operations are:
- identification of the weighing instrument (type, model,
serial number, etc.);
- checking the leveling;
- ensuring that electrically powered instruments have
been switched on for a period of a least one hour
(preferably overnight) and have reached room
temperature;
- ensuring that the pan of the weighing instrument is
clean and in good condition;
- pre-loading for several times the weighing instruments
to near maximum capacity;
- adjusting the error close to Max, using internal or
adequate external weights, to allow compensation for
changing environmental factors such as temperature
and, implicitly, air density.
Estimating the expanded uncertainty is based on:
- repeatability
- resolution
- eccentricity
- the influence of temperature variations at the site of use
- accuracy measurements
- the standards weights used in the accuracy
measurements
- hysteresis
1. Repeatability
At least ten repeated measurements must be
performed. This test should be done at or near the nominal
maximum capacity of the weighing instrument or using the
largest load generally weighed in applications. In the case
of a zero deviation between the weighings, the instrument
shall be reset to zero, without determining the error of the
zero indication [1]. The uncertainty due to repeatability of
the weighing process, uw, is given by standard deviation s of
several weighing results obtained for the same load under
the same conditions. For multiple range instruments, this
test shall be carried out for each range used, thus for n
measurements:
n

∑  I
uw = s = i =1

i
− I 

2
(1)
n −1
where Ii is the indication of the weighing instrument and n
is the number of repeated weighings:
1 n
I = ∑ Ii
(2)
n i =1
2. Resolution
For balances having the resolution d (equal to the
scale interval), the uncertainty of the rounding error, ur, for
each reading I is [2]:
ur =
d /2
3
=
d
(3)
12
The uncertainty of the rounding effect for I ≠ 0 is given
by [6]
ur =
d I2= 0 + d I2= L
12
(4)
Equation (4) is used for single and multiple range
instruments. For multi-interval instruments, for the different
scale intervals di, the uncertainty due to the rounding effect
is:
ur =
where: d1
di
d12 + d i2
(5)
12
is the smallest scale interval
is the scale interval of the appropriate partial
range.
3. Eccentricity
It is preferable to use large weights instead of
several small weights. The load L shall be applied on the
pan in the positions indicated in Fig. 1 in a sequence of
center, front, left, back, right, or equivalent.
After the first measurement, tare setting may be done when
the instrument is loaded. For instrument having no more
than four points (n ≤ 4) of support, the test load is 1/3 Max.
[1].
3
4
3
5
2
2
5
The load is first placed in position 1 and is
subsequently placed in the other 4 positions in an arbitrary
order. Acceptable solution for uncertainty due to
eccentricity, uex is estimated as follows [2]:
∆
uex =
(6)
2 3
here, ∆ is the largest difference between off-center and
central loading indications;
The eccentric test is not carried out in the case of weighing
instruments with a suspended load receptor.
4. The effect of temperature variations, during the
calibration, uT is calculated from:
uT =
1
12
(∆ t ⋅ TK ⋅ 10 ) ⋅
−6
L
(7)
where: TK
is the effect of temperature on the mean
gradient of the characteristic in ppm/K (estimated or taken
from data information sheet).
∆t = tmax – tmin is temperature variation during the
calibration, for load L.
5. Mass standards
The weights used as measurement standards shall
comply with the specifications in the OIML R111. The
traceability of the standards to the SI unit shall be ensured.
The standards shall be adequately acclimatized before the
calibration (to minimize the effect of convection). A
thermometer kept inside the box with standard weights may
be helpful to check the temperature difference.
• When the indication of the instrument is not corrected
for the errors of the weights (the calibration weights are
introduced as nominal values) the uncertainty of the
reference weights, uref, is estimated as follows:
u ref =
δi
3
or, when two or more weights are used,
u ref =
∑δ
3
i
•
is the maximum permissible error of the “i”
applied weights
When the indications of the instruments are corrected
for the errors of the weights (the calibration weights are
introduced as conventional values) the standard
uncertainty from the calibration certificate (ucert) should
be combined with the uncertainty due to the instability
of the mass of the reference weight (ustab) as following
[2]:
2
U 
2
2
2
=   + u stab
(9)
ucert
+ u stab
k
 
When two or more weights are used for L, the equation
becomes:
uref =
 ∑U i 
2


(9’)
 k  + u stab


Ui (k=2) is the uncertainty of the applied weights from the
calibration certificate.
2
uref =
4
1
1
Fig. 1
where: δi
(8)
(8’)
The calculation of the uncertainty associated with
the stability of the standard (ustab) has to take into account a
change in value between calibrations, assumed that a
rectangular distribution. This component would be
equivalent to the change between calibrations divided by
3:
ustab =
where
Dmax
3
(10)
Dmax represents the drift determined from the
previous calibrations.
If previous calibration values are not available, the
uncertainty from the calibrated certificate is considered to be
an uncertainty associated to the drift.
6. Accuracy measurements
Weighing instruments should be calibrated
throughout their range. When a weighing instrument is used
only over a part of its capacity, the calibration may be
restricted to this part of the measuring range. In this case,
the part range that has been calibrated has to be explicitly
mentioned in the calibration certificate and also a label with
this information should be fixed to the weighing instrument.
Measurements are made at about five equal steps across
the range of the balance (zero, 0.25Max, 0.50Max, 0.75Max
and Max). If the balance is typically used for a particular
load, the accuracy of the scale around this load should be
measured.
• When :
- the weighing instrument was adjusted before
measurement,
- the density of weights is close to 8000 kg/m3 and
- the air density is close to 1,2 kg/m3,
the indication error, EI , is obtained from the difference
between the instrument reading I - upon application of a
load L - and the value of this load (conventional mass value
or nominal value).
EI = I - L
(11)
The weights are applied in increasing and
decreasing loads. The estimation of uncertainty associated
with the indication error taking into account the influences
of repeatability (uw), resolution (ur), reference weights (uref),
temperature (uT) and hysteresis (uH).
Hysteresis occurs when a balance displays a
different reading for the same load, when the load is applied
increasing the weight and decreasing the weight.
If the difference is δx, the standard uncertainty due to
hysteresis is given by [3]:
δx
= 0,29 δx
12
The expression of uncertainty
determining the indication error is:
uH =
u(EI)=
(12)
associated
with
(u w ) 2 + (u r ) 2 + (u ref ) + (uT ) 2 ⋅ L2 + (u H ) 2
2
(13)
and the largest uncertainty u(EI) rel max is taken into account
for further calculations. The indication error is not the same
when the weighing instruments are calibrated using standard
weights under different conditions. In this case, the error of
indication is:

ρ a − ρ a 

1
1
I − L − − L ⋅ (ρ a − ρ 0 ) ⋅ (
)+
−
 =
ρ
ρ
ρc

 
w
c

adj

1

ρw
−
1
ρc
)+
ρa − ρa 
adj
ρc


(15)
Where BC, the buoyancy correction is equal to [8]:

1

ρw
BC = − L ⋅ (ρ a − ρ 0 )⋅ (
where:
−
1
ρc
)+
ρa − ρa 
adj
ρc


(16)
ρw
ρa
ρ0
ρc
= density of the weight
= density of the air during the calibration
= 1,2 kg/m3 is the reference density of the air
= reference (conventional) density of the
adjustment weight equal to 8000 kg/m3
ρa adj = air density at the time of adjustment.
Provided
- the instrument has been adjusted immediately
before the calibration , ρa adj = ρa , the buoyancy
correction may be calculated as:
BC = − L ⋅ (ρ a − ρ 0 )⋅ (
-
1
ρw
−
1
ρc
)
(16’)
the instrument has been adjusted independent of
calibration (ρa adj is unknown) the buoyancy
correction may be calculated as:
BC = − L ⋅
ρa − ρ0
ρw
with the next assumptions: ρa adj= ρ0 and ρw= ρc
2
u BC
=
(16’’)
u ρ2a
ρ
2
w
+ (ρ a − ρ 0 ) 2 ⋅
u ρ2 w
(17)
(18)
ρ w4
Uncertainty of air density u ρa is determined
according to [2]. When the air density is not measured and
the average air density for the site is used instead, the
uncertainty associated to the air density is estimated
(according to chapter C.6.3.4 in [2]) as:
u (ρ a ) =
0,12
[kg/m3]
(19)
3
The expression of uncertainty associated with
indication error (when the buoyancy correction is applied)
is:
u(EI)=
(u w ) 2 + (u r ) 2 + (u ref )
2
+ (u T ) 2 ⋅ L2 + (u H ) 2 + (u BC ) 2 ⋅ L2
(20)
The uncertainty u(EI) should be calculated for each value of
the load used.
From eq. (20), the relative standard uncertainty can be
calculated as:
u(EI) rel= u(EI) / L
(21)
and the largest uncertainty u (EI) rel max is taken into account.
Uncertainty
instrument
E= I – (L+BC)= I – L – BC=
= I − L + L ⋅ (ρ a − ρ 0 )⋅ (
u ρ2
u ρ2
1
1 2
2
u BC
= u ρ2 ⋅ (
−
) + ( ρ a − ρ 0 ) 2 ⋅ w + u 2ρ ⋅ w
4
a
a
ρ w ρc
ρ w4
ρw
the
The uncertainty uEI should be calculated for each
value of the load used.
From eq. (13), the relative standard uncertainty can
be calculated as:
u(EI) rel= u(EI) / L
(14)
=
Starting from eq. (16’) and (16’’), the relative standard
uncertainty associated to the buoyancy correction (uBC) may
be calculated as:
of
measurement
for
the
weighing
The influences of the repeatability and of the
rounding error are assumed to be independent from the load
applied, while all the other components are proportional to
the weight values. The standard uncertainties corresponding
to the components that are proportional to the weight values
are expressed as relative uncertainties. The combined
standard uncertainty uc is based on the parameters described
above (which can be grouped to obtain a simplified
expression that would better reflect the fact that some of the
terms are independent from the applied load, while others
are proportional to the weight value) [5]:
uc = α + β ⋅ L

(22)
When corrections are applied to the error of indication
of the weighing instrument, the expression for
combined standard uncertainty uc is:
2
2
uc = u w 2 + u r 2 + L2 ⋅ (u exrel
+ uTrel
+ u (2EI ) rel max )
2
2
+ uTrel
+ u (2EI ) rel max
= u w 2 + u r 2 + L ⋅ u exrel
(23)
uTrel from eq. (23) is calculated by replacing the temperature
variation during calibration (from eq.7) with the actual
temperature variation recorded during the use of the balance.
 When no corrections are applied to the error of
indication of the weighing instrument, the largest
relative indication error across the range that is
measured EI rel (Max) should be added to uc, in addition to
u(EI)rel(max), as follows:
2
2 + (u
2
uc= u w 2 + u r 2 + L2 ⋅ [u exrel
+ uTrel
( EI ) rel max + E I rel ( Max ) ) ]
=
2
2
u w 2 + u r 2 + L ⋅ u exrel
+ uTrel
+ (u ( EI )
+ E I rel ( Max) ) 2
rel ( Max)
(24)
The expanded uncertainty for k=2 is
U = k·uc
(25)
B. Calibration of mechanical balances
B1. Two pan balances: the balances with two pans
and three knife edges are also known as equal-arm balances
because the knife edges supporting the pans are nominally
equidistant from the central knife edge. The three knifeedges are parallel and lie in the same horizontal plane.
Two-pan balances are generally undamped with a
“rest point” being calculated from a series of “turning
points”. Some balances incorporate a damping mechanism
(usually mechanical or magnetic) to allow the direct reading
of a “rest point”. In all cases, the reading in terms of scale
units needs to be converted into a measured mass difference.
A two-pan balance undamped is used
less
where:
frequently mainly due to the amount of time needed to make
a weighing compared with electronic balances.
There are two methods to calculate the ”rest point” (the
equilibrium positions) “P” for balances (the accuracy of the
second is better than that of the first one):
P = (e1+ 2e2+ e3)/4 or
(26)
P = (e1+ 3e2+3e3+e4)/8
(27)
where e1…e4 are consecutive readings at the extremity of
the swing of the pointer, i.e. where it changes its direction of
motion.
A calibration procedure, [7], is shown in the table 1.
L1and L2 are weights with nominal masses equal to the
minimum capacity of the balance.
L3 and L4 are weights with nominal masses equal to the
maximum capacity of the balance.
The following tests and calculations should be carried out on
a regular basis and are essential to the routine operation of
the balance [7]:
1. Determining scale interval while the balance is loaded
with minimum capacity.
dmin=m sw1/ |P4-P3|
(28)
2. Determining scale interval while the balance is loaded
with maximum capacity:
dmax=m sw2 / |P8-P7|
(29)
3. Determining repeatability while the balance is no
loaded and loaded with maximum capacity (by
determining the experimental standard deviations):
n
∑
R0 =
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Loads applied
on receivers
left
0
L1
L2
L2
0
L3
L4
L4+sw2
0
L3
0
L3
0
L3
0
L3
0
L3
0
L3
right
0
L2
L1
L1+sw1
0
L4
L3
L3
0
L4
0
L4
0
L4
0
L4
0
L4
0
L4
Readings
e1 e2 e3
Equilibrium
position
div
div
∑ (∆
i =1
i =1
; Rmax =
n −1
imed − ∆ i )
2
(30)
n −1
P0i are the equilibrium positions while the balance is
not loaded
P0i med are the mean of equilibrium positions while the
balance is not loaded
∆i are the differences between equilibrium positions
of the balance when it is not loaded and when it is
loaded with maximum capacity
∆i med is the mean of ∆i differences
4. Determining errors due to the fact that the two arms of
the balance are not equal in length (this test is not
applicable to balances with a single pan, case B2,
section to be discussed in).
Table 1. Calibration procedure for two pan balances
No
n
( Poimed − Poi ) 2
Difference
∆i
div
Jmin =
P2 + P3 P1 + P5
−
2
2
(31)
Jmax =
P6 + P7 P5 + P9
−
2
2
(32)
P1
P2
P3
Uncertainty
instrument
P4
P5
P6
of
measurement
for
the
weighing
The standard uncertainty is based on the described above
parameters as following:
P7
P8
P9
P10
∆1= P10-P9
P12
∆2= P12-P11
P14
∆3= P14-P13
P16
∆4= P16-P15
P18
∆5= P18-P17
P20
∆6= P20-P19
•
2
us min = d min ⋅  u sw1  +  u( P 4 − P3 ) 
m   P −P 
3 
 sw1   4
P11
P13
P15
P17
P19
where:
sw1 and sw2 are additional small weights (sensitivity
weights) with mass msw, used to determine the scale interval
of the balance. The sensitivity weights should be calibrated
against suitable mass standards.
uncertainty due to the sensitivity of the balance:
2
(33)
and


2
u
)
us max = d max ⋅  u sw2  +  ( P8 − P7 
m   P −P 
7 
 sw2   8
2
(34)
where: usw is the uncertainty of the additional small
weights sw (sensitivity weights);
(P4 –P3) or (P8 –P7) is the change in the indication of the
balance (due to the sensitivity weights) with the
uncertainties u(P4 –P3) or u(P8 –P7), respectively;
d is the scale interval.
From eq. (33) and (34), the relative standard uncertainty can
be calculated as:
us rel = u s (min, max) / L
(35)
and the largest uncertainty us rel max is taken into account
•
the variance of repeatability :
uw 2 = s2 = R2
•
(36)
uncertainty due to the inequality of the two arms lengths
u P2 6 u P2 7 u P2 5 u P2 9
(in scale divisions)
(37)
u =
+
+
+
+u2
J
4
uJ =
u r2
4
4
 0,2 ⋅ d
+ (u w ⋅ d ) = 
 2 3
2
= 0.0033 ⋅ d + (u w ⋅ d )
2
where:
u w2
w
4
EI = ∆i – mswi – ρa·Vswi
2

 + (u w ⋅ d ) 2 =


(in mg) (37’)
2
is the variance of the repeatability
2
r
u is the variance of limited resolution.
The resolution is equal to 1/10 or 2/10 of the scale interval
d, the standard uncertainty being calculated as:
0,2d
0,2d
ur = 2 =
3
2 3
(38)
Then, the combined standard uncertainty uc can be
calculated as follows:
uc =
(u w ⋅ d ) 2 + u J2 + u s2
rel max
where: sw1... sw4 are sensitivity weights having nominal
mass equal to 1/4...4/4 from the maximum capacity of the
screen;
mswi is the mass of the sensitivity weight applied;
Poi med are the average equilibrium positions (rest points)
while the balance is not loaded.
The indication error of the screen will be calculated as
follows:
⋅ L2 = (u w ⋅ d ) 2 + u J2 + (u s
rel max
⋅ L)
(39)
The expanded uncertainty is reporting by multiplying uc
with the coverage factor k=2
U = k ·uc
(40)
B2. Single pan balances
Displays on these balances tend to be of the optical
variety, the sensitivity of the balance being usually adjusted
by a skilled person.
In the case of single pan, direct reading analytical balances,
the following tests and calculations should be carried out:
repeatability, calibration of the screen and calibration of
built-in weights.
1. Repeatability: this test should be done at or near the
nominal maximum capacity of the weighing instrument or
using the largest load generally weighed in applications.
Repeatability is determined in the same way as was
described in section B1, eq. (30).
2. Calibration of the screen: on can determine the accuracy
measurements for the entire screen by the application
standard weights at various points in the range of the screen
(1/4, 1/2, 3/4 and 4/4) according to the table 2:
(41)
where: ρa is the density of the air and Vswi is the volume of
the sensitivity weight (with uVsw uncertainty).
3. Calibration of built-in weights: the built-in weights are
used in combination during the operation of balance. First of
all, it is necessary to identify the built-in weights as nominal
values. Ideally the weights built into the balance should be
removed and calibrated externally. If this is not possible
they can be left in the balance and calibrated by dialing them
upon combinations. A standard weight S of mass ms and
volume Vs is chosen for calibration, depending on the
accuracy of the balance.
The steps for calibration of built-in weights are [9]:
- record screen reading at no load indication I1;
- record screen reading I2 when loading with
standard weight S;
- record screen reading I3 when loading with
standard S (with volume Vs) and a sensitivity
weight of mass msw (with volume Vsw);
- record screen reading I4 when the standard S is
removed and on pan remains only the sensitivity
weight.
To calculate the mass of the built in weight, the next
formula can be applied:
BW = mS − ρ a ⋅ VS + ρ a ⋅ VBW + K ⋅ (
I1 + I 4 I 2 + I 3
−
)
2
2
(42)
where K is a factor used to convert the reading in terms of
scale units into a measured mass difference.
m − ρ a ⋅ Vsw
K= sw
(43)
I3 − I 2
The formula (42) can be reduced to a simpler one if the
accuracy of the weighing allows it and it is known that K is
constant from the previous measurements:
BW = mS − ρ a ⋅ VS + ρ a ⋅ VBW + K ⋅ ( I1 − I 2 )
(44)
The values for ms, Vs, Vsw, msw , are given in the calibration
certificate.
Table 2. Calibration of the screen
N
o
Load
applied
mg
1
2
3
4
5
6
7
8
9
0
sw1 =1/4
0
sw2 =1/2
0
sw3 = 3/4
0
sw4= 4/4
0
Equilibrium
positions
P1 P2 P3 Pmed
mg mg mg mg
P1
P2
P3
P4
P5
P6
P7
P8
P9
P0imed
mg
Diff
∆i
mg
Mass
of
sensitivity
weight
mg
∆1= P2-P01
msw1
∆2= P4-P02
msw2
∆3= P6-P03
msw3
∆4= P8-P04
msw4
P01
P02
P03
P04
Measurement uncertainty for the weighing instrument
To estimate measurement uncertainty for the
weighing instrument the following parameters need to be
considered:
1. Uncertainty due to the repeatability of the weighing
instrument, uw, - given by standard deviation s of several
weighing results obtained for the same load under the same
conditions, calculated as in eq. (36).
2. Uncertainty associated with the indication error of the
screen - calculated as follows:
u(EI)=
2
2
u sw 2 + u ρ2 a ⋅ Vsw
+ ρ a2 ⋅ uVsw
+ u w2 + u r2
(45)
The above expression includes the parameters described: uw
(repeatability), ur (resolution), u ρa (uncertainty of the air
density), usw (uncertainty of the sensitivity weight), Vswi
(volume of the sensitivity weight with uVsw uncertainty).
3. Uncertainty of the built-in weights - calculated starting
from eq. (42) or (44):
2
2
2 I1 + I 4 I 2 + I 3 2 1 2 2
u BW = u A
+ us2 + u ρ2 (VBW − VS ) 2 + ρa2 ⋅ (uVBW
+ uV2 ) + uK
⋅(
−
) + K ⋅ u [( I 2 + I 3 ) − ( I1 + I 4 )]
a
S
2
2
2
2
uBW= u2A +us2 +uρ2 (VBW−VS)2 + ρa2 ⋅ (uV2 +uV2 ) +uK
⋅ (I1 −I2)2 + K2 ⋅u2(I1−I2)
a
BW
S
(46)
(47)
Since an experimental standard deviation cannot be
calculated for a single measurement to estimate uA, data
obtained from previous repeatability evaluations can be
used, thus resulting a pooled standard deviation.
Standard uncertainty of the reference weight, us is calculated
according to eq. (9) from above.
The combined standard uncertainty uc can be calculated as
follows:
 when corrections are applied to the error of indication,
the expression for combined standard uncertainty uc is:
2
uc = uw 2 + ur 2 + u BW
+ (u( EI ) ) 2

(48)
when no corrections are applied to the error of
indication, the indication error across the partial screen
range that is measured EI should be added to uc, in
addition to u(EI), as follows:
2
uc = uw2 + ur 2 + u BW
+ (u( EI ) + EI ) 2
The expanded uncertainty for k=2 will be:
U = k·uc
(49)
(50)
Conclusions
This paper established metrological requirements
for the calibration of nonautomatic weighing instruments
and provided useful information for operators working in
accredited calibration laboratories in various fields, in order
to determine the mass of products.
- A laboratory should not attempt to make measurements
with an uncertainty of “x” using an instrument that has the
readability “x”. If the user wishes to apply no corrections, to
obtain an uncertainty of “x”, he should have a balance with a
readability of “0.1x”, to be sure that are no gross errors
present.
- The balance indications are closer to the conventional mass
than to the true mass, on many occasions, the indication
being directly used as the conventional mass. This is
normally not valid for mass (true mass). In current use, it is
necessary to convert the weighing result from the
conventional mass to the true mass (in section B2 described
above, the weighing result is transformed directly in true
mass).
- When a calibrated instrument is used, the calibration
uncertainty stated in the calibration certificate of that
instrument has to be taken into account when reporting the
measurement uncertainty associated with any measurement
results, but it should be remembered that the calibration
uncertainty represents only one part of the measurement
uncertainty stated in current applications of the laboratory.
Other contributions to the measurement uncertainty that
have to be taken into account are the influence of the
buoyancy correction, the influence of the properties of the
product that is weighed (evaporation, hygroscopic behavior,
electrostatic charging, etc).
- Mechanical balances have generally been replaced by
electronic balances, which often offer better resolution and
are easier to use. The recalibration period for all of them
(mechanical and electronic) can be different for each type,
being influenced by such factors as the usage of the
receives, operator skill and the environment in which the
balance is located. As a general guideline, balance should be
recalibrated yearly, until the stability of operation is
established.
References
[1] International Recommendation OIML R76 -1 “Nonautomatic weighing instruments”, Annex A. pp 68-73
[2] International Recommendation No111 “Weights of
classes E1, E2, F1, F2, M1, M2,M3” ch. 6, pp. 70-74.
[3] “Guide to the expression of uncertainty in
measurement” (ISO, Geneva, 1995), Annex F pp 54 .
[4] International Document OIML D 28 “Conventional
value of the result of weighing in air “pp 7-9, 2004
[5] Document 2089, Edition 00 – October 2000 “Specific
requirements relating to the calibration of nonautomatic weighing instruments”, ch.8.5 - 8.6,
pp 19-24.
[6] DKD –R-7-1, “Calibration of non-automatic weighing
instruments”, issue 98, ch.5
[7] NTM 3-05-77 “Verificarea balanŃelor cl. 3, 4 şi 5” ch.5,
pp 25-31
[8] EA -10 /18 EA “Guidelines on the calibration of nonautomatic weighing instruments” , (draft) July 2004,
ch.4, pp 11 and 2.
[9] Randall M.Schoonover: “Air Buoyancy Correction in
High-Accuracy Weighing on Analytical Balances”.
Anal. Chem. 1981, 53 , 900-902
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