AMBIENT CONDITIONS NECESSARY
FOR
PROPER
OPERATION
OF
ELECTRONIC BALANCES
This publication aims at description of problem of measuring process with reference to
ambient conditions. It consists of three chapters. Chapter one covers basic information
referring to preparation of a balance to operation. It discusses problems on leveling a balance,
accuracy adjustment, weighing result readout, etc.
Chapter two includes information on workstation of balances and its factors, like temperature,
humidity and electromagnetic compatibility. Special attention was paid to relation: factor –
balance reaction – which is extremely important in case of very precise measurement of mass.
Chapter three discuses influence of physical conditions like temperature, air density,
gravitational acceleration force, etc. onto weighing process.
Janas Sławomir
Testing Laboratory Manager
e-mail: janas@radwag.pl
RADWAG WAGI ELEKTRONICZNE - TESTING LABORATORY
/ TECHNICAL SUPPORT /
26-600 RADOM, Bracka 28 Street
phone.+ 48 (0-48) 38 48 800 ext. 536 phone / fax. +48 (0-48) 385 00 10,
http://www.radwag.pl
INTRODUCTION
Quality management system, which is present in most of organizations give specific
requirements for measuring instruments and ambient conditions in which they should operate
properly. Due to its specification, it is very hard to define influence of ambient conditions onto
measuring instrument in a specific quality management system. Such features should be
provided by a manufacturer, who is to perform a set of tests to confirmation of its product
parameters. It is particularly important in case of precise measurement of mass.
Fig. 1. Influence of ambient conditions of measuring result
On one hand, ambient conditions may have decisive influence on obtained measuring results,
independent on balance settings. In such case, only change of ambient conditions – elimination
of detrimental factors – may give desired effect, in this case stable measurement and small
indication error.
Fig. 2. Influence of balance parameters on weighing time
On the other hand, parameters of a balance should be set optimally, which guarantees
appropriate speed of measuring process. In practice, it is not possible to separate these two
problems. The operator should focus on providing good ambient conditions (monitor them)
and proper parameters of the balance (change of settings, consultation with technical support).
-2-
1. Preparation of a balance to operation
A balance, which is an electronic instrument , requires initial preparation stage for operation. It
derives from construction features of a balance, its accuracy and ambient conditions on
workstation. In most cases, this process is described in balance documentation, but as
observed, it is not always performed because of operator negligence of the problem.
Preparation of a balance to operation should be comprehended as not only plugging it to
mains, but also its initial assessment, which may include change of specific settings.
Section 1. Balance preparation to use
3
1.1. First plugging to mains .........................................................................................................4
1.2. Leveling a balance ................................................................................................................4
1.3. Preparation of a balance to weighing – accuracy adjustment ..............................................5
1.4. Weighing time– readout collection.......................................................................................7
1.5. Balance adjustment to present working conditions ..............................................................8
1.6. Weighing pan ........................................................................................................................9
1.7. Application of weighing vessels ...........................................................................................9
1.8. Additional weighing pan shields...........................................................................................9
Section 2. Balance workstation
10
2.1. Positioning a balance ..........................................................................................................11
2.2. Weighing room cubic capacity ...........................................................................................11
2.3. Weighing room temperature ...............................................................................................11
2.4. Weighing room humidity....................................................................................................12
2.5. Lighting...............................................................................................................................13
2.6. Air movements....................................................................................................................13
2.7. EMC Electromagnetic compatibility ..................................................................................13
Section 3.Physical conditions during weighing process
14
3.1. Temperature during an analysis ..........................................................................................15
3.2. Sample structure, hygroscopicity........................................................................................15
3.3. Electrostatics in weighing process ......................................................................................16
3.4. Magnetizm as weighing process disturbing factor..............................................................17
3.5. Free air thickness and indication of balances .....................................................................17
3.6. Grativational acceleration force – influence on weighing process. ....................................20
-3-
1. 1. First plugging to mains
Plugging to mains should take place only after positioning of a balance on workstation. In
most cases, as the balance is assembled on workstation, it is plugged to mains and assessed for
the very first time. It is a correct procedure, which allows for assessing whether the product is
compatible with purchase order.
However, in some cases, it is necessary to wait a couple of hours before plugging the balance
to mains. Such situation takes place if a balance temperature is much lower than temperature
present on workstation, e.g. balance temperature is 5ºC and room temperature is 22ºC. if
plugged to mains, the balance temperature will raise quickly, which may cause condensation ,
and part of water may drop onto electronic modules of weighing mechanism. Generally, such
situation is undesired for all kinds of electronic devices, and may be a source of defects. Thus,
it is recommended to maintain warm up and acclimatization period for balances. It is a
guarantee for proper operation of a balance and its accurate measurements.
1.2. Leveling a balance
Leveling a balance is an activity during which a balance will take a position
which was set during factory adjustment procedure. This way, a balance has
the same parameters of checking and operation in various locations. Thus, it
can be claimed, that if properly leveled, balance indications are not influenced by uneven
surface.
Accuracy of standard levels used in electronic balances is approximately 5’ – 20’. It should be
stressed here, that model of a level depends on mechanical construction of a balance. Tests for
surface level influence on balance indications are performed in accordance to norm EN 45501
section 3.9.1. ,,Inclination” during certification tests of a product. This test refers only to
balances that have type approval certificate issued by a Notified Body.
Another solution which assists in process of leveling a balance is use of electronic level. It is a
set of electronic circuits which can set the zero point with very high precision, even with
resolution 0,06’. General evaluation of this aspect, gives a conclusion, that in case of
traditional level, the main problem refers to their hysteresis, i.e. lack of repeatability with equal
inclination of the set. Additionally, there is also human factor, which is subjective assessment
of level accuracy. None of the two aspects refers to electronic level, even though cost of their
adaptation to electronic balances is much higher. Application of an electronic level, gives
possibility of setting alarm signal if level is not maintained.
Summarizing the problem of balance leveling, it can be claimed, that no level is
disadvantageous for balances and may give below form:
• Changes in zero indication of a balance, if the weighing pan is not loaded (operator of a
balance can zero this drift)
• Mass indication error (operator can not correct this error)
-4-
1.3. Preparation of a balance to weighing - accuracy adjustment
Presently it is assumed, that a balance is ready for operation the moment it is
needed. This is correct, but only in case of weighing instrument with low
resolution. In case of balances with reading unit 0,1mg and lower, balance
accuracy should be adjusted. Thus, procedures like GLP and GMP are so
popular. They oblige an operator to obey specific regulations, and demonstrate
by means of certain actions an objective proof, that balance indications are correct within a
series of measurements. Adjustment of balance accuracy is rather a simple activity, especially
in case of modern balances, which are equipped with system of automatic internal calibration.
Generally, it is assumed, that before start of weighing procedure, a balance should be checked
for its proper operation. In practice, such evaluation is performed by means of checking of
balance mass indications for a known reference mass. Reference mass should be
approximately equal to mass of measured objects. As observed in practice, many operators
comprehends control of balance accuracy in their own way. In some cases, of measured
objects are 20-50 grams, operators use 200 g reference mass for checking balance indications.
Such control has not much to do with evaluation of balance accuracy in utilized measuring
range.
Each organization establishes its own criteria for accuracy checking of instruments, but the
aim of tests should always be the same. Reference masses used for checking instrument’s
accuracy MUST be related to their supervising references. This is a guarantee of accuracy of
their mass and reliability of weighing process.
Depending on balance design, accuracy adjustment can be performed with use of:
• External reference weight with specific mass, so called external calibration
• Automatic internal calibration with reference mass built into balance casing
Generally speaking, calibration is referring reference mass (external or internal) to a mass that
is specified in factory menu of a balance. In result of such process, balance accuracy is
corrected. In case of balances equipped with system of automatic internal calibration, one can
differentiate three criteria for calibration:
• Initial calibration (on balance start)
If switched on, a balances automatically performs calibration process in order to correct
possible errors
• Temperature calibration
Activated automatically on change of temperature in balance mechanism. This change
must exceed the threshold set in factory parameters of a balance
• Time calibration
Activated automatically on elapsing time. Time interval is defined in factory parameters
of a balance and it is measured from the last performed calibration.
-5-
Fig. 3. Drift of balance indication with change of temperature, e.g. process of self-heating of a balance
A – Initial calibration (on balance start)
B – Temperature calibration
C – Time calibration
Broken line stands for indication error in case calibration process is not activated. According to
norms of Good Laboratory Practice, adjustment of balance accuracy should be performed in
set time intervals and before start of weighing process. It is also recommended to save the
records of such procedure.
Summarizing the problem of balance accuracy adjustment, it should be stressed, that with
adjustment process (calibration process), balance accuracy is restored to its proper value.
Calibration eliminates errors sourcing from:
- drift of indications as result of temperature change
- Drift of indications as result of mechanism and electronic set wearing off process
- Relocating the balance from one place to another with change of gravitational acceleration
value (change of latitude and altitude above sea level)
Another aspect of balance characteristics is its repeatability, which does not depend on balance
accuracy. Balance operator does not influence its repeatability.
-6-
1.4. Weighing time – readout collection
Weighing speed is very important in cases where weighing is part of production process. It has
economic aspect “time is money”, especially if most of organizations are private entities which
have to give profit. In case of a laboratory, as a testing unit which focuses on accuracy,
measurement repeatability and reliability are put as most important, before weighing speed.
Weighing time, is a time that is counted from the moment of placing a load on balance
weighing pan until the moment of obtaining stable weighing result. Value of time interval is
expressed by most of the manufacturers between a couple to several of seconds, depending on
design and accuracy of a balance.
Weighing time results are obtained in very good laboratory conditions, and they do not reflect
real use of a balance. Thus, operator should have objective approach to weighing time, which
recently became a kind of pursue “on who is the fastest” between manufacturers.
Another aspect of weighing time, is determination of stable weighing – which criteria
determine stability of a weighing result. If stability criterion is modified by means of “result
confirmation” parameter, can either shorten or lengthen weighing time.
Fig. 4. Stability criteria and weighing time
Present designs of electronic balances use various methods for marking stability of weighing
result. They may take a form of a circle, triangle or any other shape, which can not be
interpreted as weighing result.
Real weighing time depends on multiple factors, like breeze, vibrations, temperature drifts, etc.
It should be pointed here, that in most of laboratory balances, an operator has wide possibilities
of optimizing weighing time according to working conditions. Modification of balance settings
results in longer or shorter weighing time in the same working conditions. Thus, it is possible
to discuss a term of “optimization of weighing speed”.
-7-
1.5. Balance adjustment to present working conditions
Each balance that leaves premises of a manufacturer, is set for operation is some specific
working conditions. It is generally assumed, that such working conditions, are compatible with
manufacturer requirements on temperature, humidity and possible disturbances.
As visible in practice, such assumptions are not always correct.
In case of most electronic balances, adjusting their parameters to working conditions is:
Selection of other scale for signal filtering processes /higher of lower filter setting /
Selection of other criterion of weighing result stability.
Fig. 5.Graph of balance stabilization of weighing process
Due to practical purposes, names of filter settings like fast, average, slow or very slow are
used. In case of stability of weighing result criterion, it is expressed as: fast, fast + reliable,
reliable. Generally, it is possible to express a common rule:
The higher the disturbances, more precise are filter settings.
The consequence of such operation is extension of weighing time, but in some circumstances it
is the only possible way to determine accurate weighing result. Above all, the best solution is
to eliminate the source of disturbance.
-8-
Fig.. 6. A graph of balance indication stabilization with disturbances
1.6. Weighing pan
Weighing pan of a balance should be selected with attention paid to the weighed object. The
weighed object should not protrude outside weighing pan edges. Any protruding object may
cause disturbances in weighing process. This is caused by ambient air drift within weighing
chamber of a balance.
Weighed objects should be placed in the centre of weighing pan. If so, operator eliminates
error of balance eccentricity. In order to evaluate eccentricity error of a balance, operator
should place a load which is ⅓ of balance max capacity and place it in ½ weighing pan (which
is according to norm EN 45501 section A.4.7. determination of eccentric weighing).
1.7. Application of weighing vessels
While applying weighing vessels, the operator should focus on below aspects, which may
influence correct outcome of performed analysis:
Vessel size
always use vessels that are corresponding to size of weighed sample.
Vessel material
while applying vessel made of plastics, make sure there is no risk of electrostatic discharges.
Electrostatics may occur in case of low humidity, approximately 25-30%. In such case,
weighing result will bear substantial error, and weighing result indication will not stabilize.
Vessel temperature
should be equal as temperature of sample, that is going to be weighed
in this vessel. Neglecting this condition may result in creation of air
drifts which may influence weighing result. It is particularly
important in case of weighing with resolution 0,1mg and higher. The
same sample if weighed in cold and than in warm vessel will have
different mass.
-9-
1.8. Additional weighing pan shields
They are designed for separation of weighing pan from air movements. One can select
between:
Shields put onto balance casing,
Protective covers on each of 4 sides of a balance, so called anti-draft shields.
The first of two is used in balances with 1 mg resolution. The other is applicable in balances
with resolution at least 0,1 mg (analytical balances, microbalances). While operating a
balance with anti-draft shield, operator should avoid putting their hands into it. It causes
increase of temperature and this prevents from obtaining repeatable weighing results.
Remember to open the anti-draft shield only when necessary, due to stable conditions in the
weighing chamber. After a couple of hours of continuous operation of an analytical balance, its
temperature increases by 1-2oC as result of operator’s heat radiated from their organism.
- 10 -
2. Balance workstation
Repeatable weighing result from a series of measurements depend not only on balancer
parameters, but also on ambient conditions in which it is operating. As revealed by tests and
experience, optimization of workstation is the first factor which leads to obtaining reliable and
accurate measurements.
Content
2.1. Positioning a balance ..........................................................................................................11
2.2. Weighing room cubic capacity ...........................................................................................11
2.3. Weighing room temperature ...............................................................................................11
2.4. Weighing room humidity....................................................................................................12
2.5. Lighting...............................................................................................................................13
2.6. Air movements....................................................................................................................13
2.7. EMC Electromagnetic compatibility ..................................................................................13
- 11 -
2.1. Positioning a balance
Balance location should be stable and free from vibrations and
shakes. If such disturbances occur, they should be efficiently
eliminated by balance positioning base, e.g. anti-vibration table.
Balance positioning table and the ground should not bend under
human and samples weight.
It is unacceptable to have magnetic elements within weighing
area, which may disturb weighing process. Weighing area should
be also free from static electricity, as it significantly influences
process of precise measurement of mass. Weighing workstation
should be also free from air breeze. Finally, supply network
should be stable and free from disturbances.
2.2. Weighing room cubic capacity
When deciding on balance location, operator should consider size of a weighing room and
quantity of people who work there at the same time. Such assessment is necessary, as balance
requires thermal stabilization of its workstation.
A room that is too small will heat quickly by present personnel, and this may cause occurrence
of dynamic temperature error. From the other point of view, keeping stable room temperature
requires continuous operation of air conditioning system, which is a source of air breeze.
Economics is another important factor, which refers to continuous operation of devices
sustaining proper microclimate. Thus, the operator should evaluate and monitor temperature
changes during a working day, and decide whether to limit quantity of people working in a
room.
2.3. Temperature in weighing room
It is one of the most important factors during weighing process. Temperature in a weighing
room should be constant and set according to user manual specification. A question refers to
word “constant”, as it is comprehended differently in case of balances with resolution 100 mg
and in case of resolution d=0,01mg. Generally, it is assumed that a temperature is stable if its
changes do not exceed 0,5oC per hour.
Measurement of temperature stability is especially important in case of balances which
accuracy is set with external standard mass – balances with external calibration system. If
ambient conditions change, than such balance needs to be re-calibrated.
For balances equipped with system of internal automatic calibration, the process of accuracy
adjustment takes place automatically with consideration of time and temperature changes.
Such solution is commonly used in balances manufactured by RADWAG.
- 12 -
The effect of dynamic temperature changes in a weighing room is dynamic temperature error.
In practice, if dynamic temperature error occurs, measuring parameters of a balance, like
repeatability and linearity, may get worse.
Fig. 7. Dynamic temperature error
The problem of dynamic temperature error is particularly dangerous for balances equipped
with system of external calibration. Such balances are not equipped with system notifying on
temperature change.
In case of slow temperature changes, a balance will warm together with a weighing room.
Such process is not source of errors due to its slow dynamics, and static errors compensating
factors. The process of determining temperature factors of a balance is realized during
production stage of a balance, and it refers to static temperature stages.
Fig. 8. Static temperature error of a balance
2.4. Humidity in a weighing room
Humidity should be set in between 40% and 60%. A balance should not be utilized below 30
% and over 80 % relative humidity. Too high humidity creates operation discomfort for the
operator and causes moisture absorption by a sample.
Too low humidity content may be a reason for creation of electrostatic changes, which
influence readability and stability of a balance. If such is present, it is described as static
electricity, which is defined as a set of factors which accompany unbalanced electric charge on
substances with low conductivity (like plastics, glass).
- 13 -
2.5. Lighting
Apart from display backlight systems installed in a balance, lighting of a weighing room
should provide proper readout of weighing result. Balances should not be placed nearby
windows (despite good light), and close to sources of light. Both are sources of heat, and may
influence balance weighing parameters.
2.6. Breeze of air
If present, they are visible in a form of unstable weighing results or too
long weighing period. A workstation should not be located close to
door or windows. Workstation location should be also relatively far
from sources of breeze sources, such as air-conditioning, vents or
draughts.
2.7. EMC Electromagnetic compatibility
The notion of electromagnetic compatibility refers to devices that are used at a workstation.
A device is compatible if:
It emits electromagnetic disturbances that do not cause malfunctioning of other
devices, and
It’s operation is not disturbed by disturbances emitted by another device..
Bearing in mind the above, one can also discuss ambient conditions controlled for
electromagnetic disturbances. In practice, an operator can not control compatibility of
neighbouring devices, as they base on conformity declarations attached to the devices.
Fig. 9. Compatibility of electromagnetic devices
In case of verified balances, declaring EMC compatibility only with norm EN-45501 is not
sufficient. This norm covers tests for disturbance resistance but it does not check emission test.
Thus, it is necessary to perform additional tests which should FULLY confirm balance
compatibility with norm EN 61326-1.
- 14 -
3. Physical conditions during weighing process
Physical conditions influence the balance and the weighed object. Total evaluation of such
process should be divided and diagnosed. Which of the two, balance or weighed object, has
bigger influence on proper outcome of an analysis. On which part of the process one should
concentrate:
A balance – external factor, or
a sample – external factor?
CONTENT
3.1. Temperature during an analysis .........................................................................................15
3.2. Sample structure, hygroscopicity.......................................................................................15
3.3. Electrostatics in weighing process .....................................................................................16
3.4. Magnetizm as weighing process disturbing factor.............................................................17
3.5. Free air thickness and indication of balances.....................................................................17
3.6. Grativational acceleration force – influence on weighing process. ...................................20
- 15 -
3.1. Temperature during an analysis
Temperature should be stable before, during and at the end of an analysis.
In some cases, even if ambient conditions are fine, a balance does not stabilize, and operator
can observe drift of indications. It can be caused by lack of temperature stability of a balance
that has been plugged to mains just before the measurement procedure. When assessing
influence of temperature on a sample or a weighing vessel, it needs to be stressed, that if a
temperature of the sample or weighing vessel is significantly different than ambient
temperature, than it may cause air drifts.
It is well known, that air drift is one of disturbing
factors. As result of air drift, the operator obtains
too low or too high weighing result, depending on
drift direction. Above data create practical aspects
referring to weighing processes:
do not weigh samples that are taken out directly
from a drier
acclimatize the samples before analyzing their
mass
operator should not put their hands into
weighing chamber (causes change of weighing
chamber temperature)
pick up the samples with use of tweezers or
other holders (if touched with hands, samples
change their temperature)
Fig. 10. Temperature during an analysis
3.2. Sample structure, hygroscopicity
Weighing result, as obtained during an analysis period is influenced by a series of factors
relating to balance and ambient conditions. However, one should also consider sample
features during weighing process. Samples that are liquids, can undergo process of
evaporation. In such case, balance indication will be influenced by a drift. Weighing result will
continuously decrease. Additionally, it can be concealed by balance filters. In order to prevent
such situations, liquid samples should be weighed in weighing vessels, like bulbs with narrow
necks or vessels with top cover. If operator is determining evaporation level, not the mass,
balance settings should be modified, so that it is possible to perform ordered analysis.
A factor reverse to evaporation is absorption of moisture from ambient air by a sample. it is
very important in case of hygroscopic samples. The effect of moisture absorption is differences
in mass determination, each measurement will have higher mass readout than the previous one.
For the purpose of proper weighing of such substances, weighing vessel should be clean and
dry. The easiest way to eliminate moisture absorption factor is application of hermetic vessels.
- 16 -
3.3. Electrostoics in weighing process
Evaluation of electrostatics in weighing process is complicated, as an operator
has to determine a factor that is not visible. The operator can only observe the
effect of electrostatic presence. Electrostatic discharges may occur on:
compensated ions (positive or negative) are taken over from the air,
by rubbing two non-conducive substances,
touching a sample with hand
low humidity in weighing room
According to Columbus right, interaction force between two point electric charges is directly
proportional to the product of these charges and inversely proportional to the square of
distance between them. In practice, it may cause a reaction between weighed sample and
components of weighing chamber.
q ⋅q
F =k⋅ 1 2
r2
where:
[1]
F – force of the interaction between two point electric charges,
k – proportionality coefficient
q1 q2 – point electric charges
r2 – distance between them
a visible effect of electrostatics presence is::
slow drift of weighing result,
large dispersion of weighing results in a series of measurements, and
no return to zero if a load is taken off the weighing pan.
As it is not possible to remove the source of electrostatics, it is common to use factors that
eliminate or compensate the influence of undesired electrostatic charges.
One of the methods that partially eliminates the problem is providing proper air humidity in a
weighing room. It is recommended to set relative humidity of a weighing room in between 40
% and 60 %. There are, however, cases where setting such humidity is not desired or
impossible. If such is the case, than operator should install an ionizer – ionizing frame.
Ionizing frame is a device that generates ions, so called
aero-ions, which charge is opposite to charges that
operator wants to eliminate. The means of operation of a
ionizing frame is eliminating charges that are on
operator’s hand and inside weighing chamber on its
opening. If balance conditions, ionizing rate is equal to
recombination rate, which maintains constant ionization
rate. Additionally, an operator can apply specially
designed mats, antistatic foils and selected uniforms.
Fig. 11. Ionizing frame for an analytical balance
(optional equipment)
- 17 -
3.4. Magnetizm as weighing process disturbing factor
Most of weighing mechanisms of high resolution balances are constructed on basis of
electromagnetic sets which include a force-motor and magnet. In case magnetic loads are
measured, there is a risk, that electromagnetic field of a balance is disturbed or weighed
sample is influenced by magnet installed in a balance. The effect is incorrect mass reading of a
weighed sample.
A solution for this problem is removal of a weighed sample from electromagnetic field of a
balance, i.e. increasing a distance between a sample and balance mechanism. It is possible
through so called under-hook weighing with application of special racks or hooks made of
aluminum.
3.5. Free air thickness and indication of balances
In case of very precise measurements, the operator should consider free air thickness. As has
been proofed by laboratory tests, determination of mass in air and in vacuum gives different
results. If, for instance, a 100 g steel load and its liquid counterpart are placed on a lever scale,
mass in air is equal. If the same reference masses are placed on a lever scale in vacuum, it
occurs that, liquid mass is heavier than steel mass.
Fig. 12. Free air thickness
The above is a confirmation to Archimedes right, which quotes:
,,A body immersed in a liquid or gas is influenced by vertical upward force of buoyancy.
Force value is equal to weight of liquid displaced.’’
Thus, in case of very accurate analysis, it is recommended to control free air thickness, which
depends on humidity, temperature and air pressure. It is assumed, that on the sea level, and
temperature 20°C dry air thickness equals approximately 1,2 kg/m3
Free air thickness and humidity
Humid air thickness is lower than dry air. It is explained by a right by Italian scientist Amadeo
Avogadro. The right says, that at the same temperature and pressure in the same volume (1m3),
there is always the same quantity of gas particles. If in the mentioned 1m3 gas particles are
changing into lighter, than thickness in this volume will decrease.
Such is the instance if water vapour will mix with dry air. The air consists mainly from
nitrogen (two atoms of N2 with atomic mass 14 each, i.e. 28) and oxygen (two atoms O2 with
atomic mass 16 each, i.e. 32). Atomic mass of dry air is approximately 28+32=60. If atomic
mass of water particle is 18 (two hydrogen atoms with atomic mass 1 each and one atom of
oxygen 16).
Humidity does not have significant influence of air thickness compared with temperature and
pressure.
- 18 -
Air thickness and temperature
Nitrogen, Oxygen and other gasses particles (air components) arte in continuous movement,
and they hit each other and area around them. The higher their temperature, the faster they
move. Thus, if air is heated, the molecules speed up, and in result their hitting force is bigger.
If warmed air is surrounded only by air (with other temperature), it will repel the surrounding
air. Similar process takes place in atmosphere – air thickness decreases if its temperature raises
(as it is heated).
Air thickness and pressure
Pressure influences air conversely to temperature. As the pressure raises, the thickness also
raises. Pressure systems also influence air thickness, but not as much as altitude above sea
level.
It can be noticed, that air thickness is lower at height on a hot day, when air pressure is low.
The air thickness is the highest at low altitude, high pressure and low temperature (sunny but
freezing day).
The size errors sourcing from air thickness changes very much depends on density of
measured sample. For samples with density 500 – 4000 kg/m3 the error sourcing from air
thickness changes is big and visible. Its size also depends on sample size.
Air thickness calculation:
ρa =
0,348444 ⋅ p − h ⋅ (0,00252 ⋅ t − 0,020582)
273,15 + t
where:
[2]
where:
[3]
ρa – air thickness [kg/m3]
p – air pressure [hPa]
h – air humidity [%]
t – air temperature [oC]
calculation of proper mass [m] corrected by air buoyancy
ρa
ρc
m=
⋅w
ρa
1−
ρ
1−
m – sample mass
ρa – air thickness [kg/m3]
ρc – sample density which was used for balance calibration [8000kg/m3]
ρ – weighed sample density [kg/m3]
w – weighing result
- 19 -
Instance:
Air pressure 996hPa, humidity 45%, temperature 25oC, sample: cork with density 240kg/m3
and mass 5g
Calculation of air thickness:
ρa =
0,348444 ⋅ 996 − 45 ⋅ (0,00252 ⋅ 25 − 0,020582)
= 1,1576kg / m3
273,15 + 25
Calculation of real sample mass:
1,1576kg/m3
1−
8000kg/m3
m=
⋅ 5 g = 5,023506
1,1576kg/m3
1−
240kg/m3
In practice, all measurements made by laboratory personnel on everyday basis do not include
above specified relationship. In order to correctly specify mass of tested sample it is necessary
to know:
sample density and
air thickness
and calculate corrective factor for weighing process. Determination of sample density with
proper accuracy is the first important problem. Thus, determination of real masses is applicable
in laboratory tests. Some models of RADWAG balances offer assistance to operator in a
function of air buoyancy correction.
Activation of this function allows for calculation of air thickness. The procedure is very
simple: first, the operator is to weigh 100 g steel standard mass, and next weigh 100 g
aluminum standard mass. as density of steel standard mass is 8000kg/m3 and aluminum
2400kg/m3 buoyancy force influencing both standard masses is different. It allows to
determine value of air thickness, which is needed for determining real mass of a sample.
- 20 -
3.6. Gravitational acceleration force – influence on weighing process.
Apart from evident advantages of electronic balances, like ease of operation, weighing speed
ergonomics, there is also one more, negative factor, which is underestimated or often omitted.
It is terrestrial gravitational force, which may cause change of electronic balances accuracy.
This problem does not refer to all kinds of weighing instruments, and it relates to instrument
design and its accuracy. Detailed analysis of this problem is described in WELMEC 2
document – Gravity zones 3.3. which stresses importance of the problem.
As practice shows, not all users and manufacturers are aware of the errors that may occur as
result gravitational acceleration force change. Value of gravitational acceleration force
depends on:
latitude and
altitude above sea level, which additionally makes the problem more complex for the
operators and manufacturers.
Scaling of balances in units of mass requires taking into consideration value of gravitational
acceleration force “g’’ which is present on operation location. If a balance is relocated from
calibration to place to operation place, than it requires consideration of accuracy changes
caused by change of gravitational acceleration force.
Such correction can be performed by various means, depending on functionality of a balance.
In case this procedure is neglected, it may be a source of errors in weighing process. In order
to make operators aware of the problem, it is recommended to do practical tests and
calculations on changes in gravitational acceleration force with consideration of latitude and
maximal altitude above sea level. Then refer this result to the errors that occur in the balance.
Detailed procedure for this problem is described by Directive 90/384/EEC.
A formula for calculation of gravitational acceleration force value with consideration of
latitude and altitude above sea level is:
g= 9,780318 {1+5,3024 · 10-3 sin2ϕ - 5,8 · 10-6 sin2(2ϕ)} - 3,085 · 10-6 · a [m s-2]
[4]
Where:
g
- value of gravitational acceleration force
ϕ
- latitude [o]
a
- altitude above sea level [m]
According to Directive 90/384/EEC the errors which may occur as an effect of latitude
changes must not exceed ⅓ of Maximal Permissible Errors (MPE), which is expressed by
below formula.
n(∆gϕ + ∆ga)
gR
≤
MPE
[5]
3e
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where:
n
- quantity of verified divisions of a balance
∆gϕ - deviation resulting from latitude change
∆ga - deviation resulting from altitude above sea level change
gR - nominal value of gravitational acceleration force for selected zone
MPE -maximal permissible error
Considering above condition, any area can be divided into gravitational zones in such a way,
that:
differences present between any random operation locations of a balance in a selected
zone, and
nominal value of gravitational acceleration force [gr] for selected zone
do not cause occurrence of an error exceeding 1/3 of MPE.
Quantity of gravitational acceleration zones depends on resolution of a balance – the higher the
resolution of a balance, the higher quantity of gravitational zones.
In case of weighing instruments class III, it is enough to divide area of Poland into three
gravitational zones. For balances and scales class II, it should be necessary to introduce much
more gravitational zones, but in fact it is pointless, as in the area of single zone, the operator
needs to consider latitude and also altitude above sea level.
Besides this, inaccurate indication of operational location of a balance, i.e. its latitude and
altitude above sea level, may be a source of data for incorrect calculation of corrective factor.
In result, customer will receive a balance with recorded error of indication. It should be
stresses here, that MPE for balances class II is quite narrow, and it is easy to make a mistake
during calculation of corrective factor. Thus, insertion of corrective factor at production stage
is not an applicable solution to this problem.
Fig. 13. Division of Poland into gravitational zones
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The size of errors which may be a result of change in “g” value depend mostly on resolution of
a specific balance, in other words quantity of verified units (n) of a balance. The size of
balance relocation also has its importance with relation to:
change of latitude
change of altitude above sea level
For the purpose of calculation, it has been assumed, that a balance has been adjusted
(calibrated) in Cracow (southern Poland), and transported to Gdansk (northern Poland), where
it has not been re-adjusted (recalibrated).
1. Cracow location: 50o latitude, 250m above sea level
2. Gdansk location: 54 o latitude, 10m above sea level
Technical parameters of a tested balance:
maximal capacity 220g
reading unit d=0,1mg
verified unit e=1
Max
d
e
Error caused by
MPE
Permissible error change of “g”
according
Sum of
from “g” changes Latitude
Altitude errors
to
above
EN 45501
sea level
[MPe]
220 g
0,1mg 1mg 1,5mg
[MPe] / 3
[E]
[E]
[E]
0,5mg
+79 mg
+16 mg
95mg
As observed above, balance errors significantly exceed maximal permissible errors. It is a
subject to conclusion, that if a balance is relocated, it has to be readjusted on new location.
Similar relations apply to balances and scales in accuracy classes II and III.
Records of errors sourcing from balance relocation
One of effective means of “g” sourced error elimination is adjustment of a balance at its
present location. In case of balance class I, and in most balances class II, the problem of is
solved by installation of automatic internal calibration system with a build in weight.
Additionally, this system eliminates possible errors caused by temperature changes, due to
continuous monitoring of working temperature of a balance and detecting changes which may
influence its operation, it is a so called temperature calibration.
There are, however, balances class II, which are verified according to two-staged verification
process. According to this procedure, a balance is adjusted with external standard mass at its
operational location. This way errors sourcing from ”g” change are eliminated.
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Such procedure can also be applied in case of balances and scales class III, if it is recognized
as valid. Another acceptable means of eliminating gravitational acceleration force errors is
introduction of calculated corrective factor during balance/scale production stage. The
corrective factor is a formula which defines difference in “g” between manufacturer’s location
and operation location of a weighing instrument. It is a good solution from the point of
economics, but it requires very precise determination of operational location of a weighing
instrument in relation to its initial calibration location – manufacturer’s site. Such solution can
only be applied in case of weighing instruments with maximal resolution of 5000e.
Literature
[1] Non-automatic weighing instruments metrological aspects – norm PN-EN 45501
[2] WELMEC 2. – Gravity zones 3.3
[3] Electric equipment for measurements, control and use in laboratories. Requirements on
electromagnetic compatibility (EMC) – PN-EN 61326
List of figures
Fig. 1. Influence of ambient conditions of measuring result, page 2
Fig. 2. Influence of balance parameters on weighing time, page 2
Fig. 3. Drift of balance indication with change of temperature, e.g. process of self-heating of a
balance, page 6
Fig. 4. Stability criteria and weighing time, page 7
Fig. 5. Graph of balance stabilization of weighing process, page 8
Fig. 6. A graph of balance indication stabilization with disturbances, page 8
Fig. 7. Dynamic temperature error, page 12
Fig. 8. Static temperature error of a balance, page 12
Fig. 9. Compatibility of electromagnetic devices, page 13
Fig. 10. Temperature during an analysis, page 15
Fig. 11. Ionizing frame for an analytical balance, page 16
Fig. 12. Free air thickness, page 17
Fig. 13. Division of Poland into gravitational zones, page 21
List of referring formulas
[1] Columbus right, page 16
[2] Air thickness, page 18
[3] Calculation of real mass, page 18
[4] Gravitational acceleration force –according to Directive 90/384/EEC, page 20
[5] Conditions for Maximal Permissible Error sourcing from changes in gravitational
acceleration force – according to Directive 90/384/EEC/ page 20
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