Influence of seismic on measuring
accuracy of electronic weighing instruments
The design of modern electronic weighing instruments ensures their
stability of indications and measuring reliability by the internal
mechanisms operating in an automatic manner. It is commonly
acknowledged that correct weighing process requires maintaining
specifically determined and stable ambient conditions. However, this
ideology fails if the external phenomena are of macro characteristics,
such as seismic activity. The influence of such activities on the weighing
instruments was demonstrated by the earthquakes that occurred
on 11th April 2012 on the Indian Ocean.
Elaborated by
Sławomir Janas
RADWAG BALANCES AND SCALES
26-600 Radom, Bracka 28 Street, POLAND
e-mail: [email protected]; http://www.radwag.com
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1. Introduction
On 11th April 2012, at 08:38 UTC time a heavy earthquake occurred
in the area of Indian Ocean, approximately 600 km west from the
northern part of Sumatra. The earthquake was recorded by all stations of
Polish seismic network, but only the records of broadband stations were
usable. This resulted from the fact that the earthquake was large but of
relatively shallow centre and its mechanism caused relatively weak waves
elongated in the direction of the remote seismic stations. The tremor
magnitude reached 8,7, and there were 2 secondary tremors of
magnitude 6,0 and 8,2. The records from the seismographs are
presented in below diagrams.
Records from seismic stations made available by Institute of Geophysics, Polish Academy
of Sciences, record in UTC time
Legend:
BEL – seismograph in Belsk
GKP – seismograph in Klasztorna Górka
KSP – seismograph in Książ
UTC time – Universal Coordinated Time is a standard solar time over
the zero meridian. It is a reference point for calculating time in other time
zones, for instance: time CET = UTC + 1, time CEST = UTC + 2.
The moment the Sun is located above the zero meridian, the clocks
displaying the UTC time should exactly show 12:00:00 noon.
The standard time for Poland is:
 Central European (CET) or
 Central European Summer Time (CEST) in the period since its
introduction to the appeal
The Central European time is increased by one hour in relation to
the universal coordinated time UTC(PL). The Central European Summer
Time is increased by two hours in relation to the universal coordinated
time UTC(PL).
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Magnitude – a parameter used for measuring the size of earthquakes,
introduced in 1935 by Charles Richter. The magnitude is calculated using
formulas based on data obtained from seismic records, such as ground
motion amplitude, wave period, epicentral distance and earthquake centre
depth. The magnitude is determined from the longitudinal wave (P),
transverse wave (S) – Mb, surface waves (L) - MS and the seismic
moment - Mw.
A – amplitude; T – period, Δ – distance; H – depth
2. Structure of electronic weighing instruments
The consequence of the earthquakes in the Indian Ocean were
ground vibration, not detectable by a human, but relatively important for
correct operation of electronic weighing instruments of high resolution.
Balance reaction to the earthquake was instability of indications that could
exhibit a defect of mechanical or electronic origin. Balance reaction
resulted from their structure that is generally presented in the below
diagram.
The general principle of operation of electromagnetic weighing
instruments says that: a force generating from a mass placed on the
weighing pan [1] is compensated by the force sourcing from the coil [2]
with current that is located in the magnetic field. The size of current
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flowing in the coil is controlled by a feedback system supported by data
from the positioning system [3].
Consequently, when the weighing pan of a balance is loaded with
mass, the system generates a compensating signal from the coil.
The signal is processed into a stable measurement result. The weighing
pan of a balance tends to reach equilibrium status. In case any vibrations
occur, they are transferred onto the mechanical system of a balance, and
result in impossibility to reach the equilibrium status. Depending on the
character of vibrations, it is possible to obtain intermediate stable results,
but then the repeatability of indications is worse than declared by
the manufacturer.
3. Operating principle of seismographs
A basic components of a seismograph is a seismometer which basic
component is inertial mass that tends to reach balance if affected by any
external forces. The mass is suspended in a way that it forms a physical
pendulum (horizontal or vertical). The vibration period of the pendulum
should be high if compared to the ground vibrations, as the pendulum’s
center is conceived a permanent reference point determining the size and
direction of ground vibrations.
Ground vibrations are processed into electric impulses, amplified and
recorded using a galvanometer on a light-sensitive tape (in traditional
systems, that become out of date) or in memory of a computer. A
simplified diagram of a device is presented in below figure.
1 – housing, 2 – tread, 3 – weight, 4 – scriber, 5 – seismogram,
6 – basis attached to the ground, 7 – ground
As observed, both in case of a balance and a seismograph, the same
principle is valid, stating that the structural elements are tending to reach
equilibrium status. In case of a balance, it is its weighing pan, and the
signal causing its deviation is presented as the weighing result. In
seismographs the inertial mass tends to reach equilibrium status, and any
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deviation of that mass, after being amplified, demonstrates vibrations of
the ground.
4. Influence of seismic on repeatability of electronic weighing
instruments
It is acknowledged, that vibrations cause instability of indications in
a balance, and thus prevent from carrying out any measurement. If the
vibrations are perceptible, sourcing from operating machines, then it is
easy to find the relation and possible means for eliminating this
phenomenon.
The situation is entirely different when the source of vibrations is
located in a large distance and activates in a random fashion with respect
to time and force. Therefore, practically no one is prepared for such
phenomena. There are also no means for decreasing or eliminating their
influence, bearing in mind the measurement is carried out with relatively
high resolution – more than 20 millions. In such case any disturbance is
immediately observed, and in particular if occurring in such a large scale
as in the Indian Ocean. The way these phenomena influence repeatability
of a balance is presented in below figures.
Reading interval d [1 µg]
Repeatability of indications expressed as a standard deviation
Time [h.]
The first diagram presents a reading of a seismograph located in
Książ that recorded the vibrations as the first one around 8:50. The
highest values were reached between hours 9 and 10 of the UTC time.
Please note that during the period from March to October, local time in
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Poland differs in relation to the UTC time by + 2 hours. Therefore, an
expected balance reaction to the disturbance should be delayed
by 2 hours with regards to the signal recorded by the seismograph.
The second diagram presents a repeatability test and weighing time
with use of balance function Autotest. The test was carried out on 10-11th
April on a microbalance MYA 2 series. The basic technical parameters of
the balance model and data related to the test are:
 Maximum capacity: 2 g
 Reading interval: d = 1 µg
 Mass used for carrying out the test: 1,8 g
 Time interval: 90 hours
A microbalance MYA 2 series
A typical metrological characteristics of a microbalance MYA 2 series
is presented in below table.
Technical data
MYA 2
Maximum capacity
2g
Reading interval
1 µg
Tare range
-2 g
Repeatability
1 µg
Linearity
±3 µg
Eccentricity
3 µg
1,5 x 10-6 x Rt
Sensitivity offset
1 × 10-6/°C × Rt
Sensitivity temperature drift
1 × 10-6/Year × Rt
Sensitivity time drift
Minimum weight (USP)
3 mg
Minimum weight (U = 1%, k = 2)
0,2 mg
Pan size
Ø 16 mm
Stabilization time
5s
Adjustment
Display
Interface
Internal (automatic)
5,7” colourful touch screen display
2 x USB, 2 x RS 232, Ethernet, 2IN/2OUT
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The testing procedure required cyclic loading and unloading the
weighing pan with test mass and automatic recording all measurements,
changes of zero point, temperature drift and humidity drift. It is also
possible to record pressure changes during the testing procedure. The
solution enables determining metrological parameters of a balance in
actual operating conditions. The Autotest function is available for
authorized RADWAG service points.
The test revealed considerable disturbance, resulting in significant
worsening of repeatability parameter, i.e. by 20 times. In practice it
means that carrying out any weighing process in this conditions is
impossible. The diagram enables differentiating 3 fields, i.e.:

Field A, demonstrating repeatability of measurements at the level of
approximately 1 µg, and this is the period before seismic occurred;

Field B, indicating substantial worsening of the repeatability of
indications. This is the time of seismic shocks and secondary shocks.

Field C, demonstrating repeatability of measurements at the level of
approximately 1 µg, and this is the period after the seismic activity
and extinction of the seismic.
reading interval d [1 µg]
Repeatability of indications expressed as a standard deviation
Time [h.]
The above described phenomena were observed in the RADWAG
Calibration Laboratory, Cracow University of Technology and most likely in
all places that use balances and mass comparators. According to our
measurements, the observed influence of disturbance took approximately
3 hours.
The vibrations originating from seismic are not to be confused with
the vibrations that are a result of operation of mechanical devices. These
are vibration of other characteristics.
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Summary:
1. Seismic shocks can be observed as any instability of indications
perceptible in high resolution weighing instruments (more than 2
million divisions). The phenomenon is particularly visible in mass
comparators used in measuring laboratories featuring instruments
with resolution reaching up to 100 million divisions. The issue
applies to all manufacturers applying similar solutions in the field of
designing weighing instruments.
2. Determining a precise relation between the locally designated
magnitude and an error of indication of a weighing instrument is
practically impossible. It is, however, possible the link the
occurrence of a seismic to an unexpected worsening of weighing
instrument parameters. In this particular case – 20 times worsening
of repeatability parameter.
3. In case of laboratories using high resolution weighing instruments,
there is real tread that an unexpected seismic activity may influence
the measurement results. If there is a suspicion that the
phenomenon may affect the measuring process, then it should be
verified using data obtained from automatic seismic stations. The
data is publically available on internet services.
During developing of this elaboration, the used information sourced from the
website of the Institute of Geophysics, Polish Academy of Sciences, Wikipedia
and the measurement results from Autotest function run on a RADWAG
microbalance MYA 2 series.
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