Mettler Toledo WXTS3DU, XP26, XP26DR, XP205DR, XP105DR Outstanding Weighing Performance Even Under Harsh Conditions—White Paper
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White Paper
Outstanding Weighing Performance
Even under Harsh Conditions
Reducing the impact of air drafts
This white paper describes the influences of air flow during the weighing process. Recent
innovations in the design of the weighing pan for precision balances and scales help to minimize any errors due to air drafts. The innovative new weighing pan makes it possible to
deliver results up to twice as fast, even under harsh conditions. What's more, the results show
an outstanding 86% improvement in repeatability (precision) for a 1 mg readability balance
under harsh conditions (inside a safety cabinet).
Content
1 Introduction 2
2 Considerations when Weighing with Precision Balances 2
3 Experimental Procedure 5
4 Summary of Results 10
5 Conclusion 16
6 Tips and Recommendations for Reliable Weighing 16
7 Further Information on External Influences 19
8 About the Author 19
1. Introduction
Weighing is one of the most common activities carried out in the laboratory. An electronic balance does not
directly measure a mass, rather the force of its weight. The uncertainty of this force depends not only on the
technical specification of the measuring instrument but also on many environmental effects such as temperature
variations and air drafts.
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The environmental conditions have an important impact on precision weighing. Air drafts from open doors or
windows, air conditioning units or fans, as well as air escape in a fume hood or safety cabinet when working
with malodorous, toxic or aggressive substances, and even movement of people around the balance, can
influence the weighing performance. Drafts can cause slower settling times and drift of measurements. In these
situations, the operators of a scale or balance cannot rely on accurate and reliable measurements. For more
than ten years, state-of-the-art analytical balances have been equipped with glass draft shields using motorized
opening for precise weighing, even in unstable environments. The user has access to the weighing pan via a
partially opening glass draft shield. The balance stabilizes faster because there is less disturbance from air
currents than in an unshielded environment. Additionally, there is the option to use cleverly designed hanging
weighing pans, SmartGrid™, with a grid structure. These pans present less surface area for air turbulence in the
weighing chamber to act upon, than conventional weighing pans. The result is shorter settling times and faster
availability of the results. But what measures can be taken to optimize weighing on precision balances?
2. Considerations When Weighing with Precision Balances
Precision balances are mainly used in open environments of laboratories and production sites where the
user has only limited possibilities to shield his equipment against air drafts.
I
n general, modern precision balances provide a high weighing performance. This means fast and reliable
weighing
from different sources like people moving, opening of doors, heating radiators, and fans from computer and
laboratory equipment influences this performance. The most critical metrics to determine the effect of these
external influences on a balance are the "average settling time" and the "average repeatability". The inherent
risk in this situation is that not enough care will be taken to prevent disturbing influences.
The air currents can be minimized using weighing pans which are specifically designed. An open grid
structure may seem, on the face of it, to address the issue of air drafts on precision balances. However,
there are inherent drawbacks to this approach. One issue, in particular, is the difficulty to clean and
maintain a weighing pan with an open grid structure. The latest innovation in design of weighing pan is a
sturdy quadruped platform. This simple, yet proven geometry, was developed based on consideration of
four primary design criteria:
results, coupled with a level of security that meets the demands of regulatory bodies. Air flow
1) Performance (speed and precision)
2) Placement of tare container
3) Weighing pan material and construction
4) Ease of cleaning
2
2.1 Performance
Air currents act as a force on the weighing pan and will influence the weighing results. Minimizing the effect of this
perturbing force with a clever geometry of the pan improves the above mentioned performance (Fig. 1). The pan is
primarily designed to minimize the negative effects of air currents without the need for any draft shield. The driptray beneath also includes a draft ring which provides an additional mechanical protection against air flow influence on the weighing pan. Experiments under different environmental conditions demonstrate the positive effect
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on the weighing performance (see Chapter 3). Speed and precision are key performance metrics of interest to the
balance user. The parameters used to measure these elements are settling time of the balance (speed) and repeatability of the balance measurement (precision).
Figure 1: Forces caused by air currents on standard pan versus
SmartPan™. The SmartPan™ displays less air resistance
2.2 Placement of Tare Container
The SmartPan™ is compatible with any kind of tare container without any danger of spillage (Fig. 2).
As precision balances are used in a wide range of applications in different industry segments and workplaces,
various sizes of tare container, such as vials, flasks, beakers, petri capsules, cylinders, boxes or buckets could
be used every day to contain and weigh samples.
Figure 2: The SmartPan™ is compatible with many different types
of tare container
Whether small, large or bulky tare containers need to be used, the SmartPan™ geometry offers a considerable
flat
support surface where any kind of tare container or sample can be easily placed in a stable position without risk
(Fig. 2).
To prevent or minimize eccentricity errors, the cross-shaped design of this weighing pan provides the user
with an easy and intuitive guide to correctly place the tare container or sample right in the center of the weighing pan.
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3
2.3 Weighing Pan Material and Construction
Weighing pan material and construction were carefully considered in the design of the innovative SmartPan™.
A zinc alloy was selected as the ideal material for the new weighing pan design, because of its better machinability
and superior tensile and impact strength, compared to the conventional aluminium weighing pan. This leads to a more
rugged construction.
Finite Element (FE) Analysis is a computer simulation technique that is often used in mechanical design processes to
predict an optimize the behaviour of complex objects. It allows a highly detailed analysis of any product or equipment
design, in order to carry out many physical tests, such as stress, vibration or heat transfer analysis, for example.
In this case FE was employed to determine the force distribution, and therefore strength and robustness, of the new
weighing pan design.
Figures 3 and 4 show a finite element simulation with the static force distribution of the conventional pan compared
to the new weighing pan design. The finite element method calculates component displacements, strains, and
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stresses under internal and external loads. Thus it is ensured that the geometry remains in the linear elastic range.
The simulations were executed with a 5 kg weight put in the middle of the balance XPE6003SD5
(6100 g maximum capacity and 5 mg readability).
Figure 3: Finite element simulation of the force distribution of the conventional aluminium pan geometry
Figure 4: Finite element simulation of the force distribution of the new zinc weighing pan geometry
The observed maximum stress on the SmartPan
cal and negligible value. This value is far below the critical proof stress value of 300 Mpa which means irreversible deflection of the material. One can easily observe, from the colour maps in Figs. 3 & 4, that the quadruped
pan has the same force distribution as the standard pan; therefore the SmartPan™ is very robust and the factor
of safety is retained.
2.4 Ease of Cleaning
This innovative new weighing pan with quadruped design (SmartPan™) has many advantageous features for
improving the weighing performance and the ease of cleaning. In order to allow an excellent cleaning performance the pan is made of die cast zinc. The choice of material offers a high resistance to acid and aggressive
cleaning agents. The design incorporates a drip-tray underneath, similar to that on the analytical balances, to
collect any substances which are accidentally spilled. The rounded edges and the gap-free design of all components enable convenient cleaning operations.
™
is approximately 7.45 Mpa (Megapascal) which is a non-criti-
4
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3 Experimental Procedure
The experiments described in Table 1 were carried out to demonstrate the performance of the new geometry of
weighing pan compared with the conventional weighing pan design (standard pan). The key performance metrics measured were settling time and repeatability of the balance.
Each test was carried out was repeated with (a) the balance fitted with a standard weighing pan and (b) the
same balance fitted with a SmartPan™. For every test carried out, three replicates were each weighed 20 times
and the reported results are the average values.
Tests were made in different environmental situations (standard – on open lab bench; harsh – under air conditioning unit or in safety cabinet), in order to represent specific environmental situations that may be realistically
encountered in a laboratory environment, as well as with different balance models (5 mg readability and 1 mg
readability).
The balance settings used during these experiments were:
• Weighing mode: universal
• Environment: standard
• Value release: reliable and fast
Experiment 1 2 3 4
Conditions Standard Harsh Standard Harsh
Description On open
lab bench
Weighing pan (a) Standard pan
(b) SmartPan™
Under air
conditioning unit
(a) Standard pan
(b) SmartPan™
On open
lab bench
(a) Standard pan *
(b) SmartPan™
In safety
cabinet
(a) Standard pan *
(b) SmartPan™ *
Balance model XPE6003SD5 XPE6003SD5 XPE12 0 3 S XPE12 0 3 S
Capacity of balance 6100 g 6100 g 1210 g 1210 g
Readability of balance 5 mg 5 mg 1 mg 1 mg
Test weights 400 g
2 kg
5 kg
400 g
2 kg
5 kg
200 g
500 g
200 g
500 g
Number of weighings 60 60 60 60
*
Tabele 1: Description of Experiments comparing Standard weighing pan with new SmartPan™ design
equipped with draft shield
3.1 Experiment 1: Weighing on a 5 mg readability balance under standard conditions
Experiments 1(a) and 1(b) were carried out using a 5 mg readability precision balance (XPE6003SD5) on an
open lab bench. The experimental set-up is illustrated in Fig. 5. All necessary factors were taken into
account to ensure that the balance was working in the best possible environment, such as positioning the balance: on a stable lab bench; in a laboratory temperature of 20 – 23 °C; in a relative humidity of 35 – 45%; in
vibration-free surroundings (i.e. not located near any vibration sources such as fans, vacuum pumps or generators); and in a suitable location (i.e. not too close to doors and with minimal passing foot-traffic).
This weighing environment was defined as being under "Standard conditions".
For recommendations on the ideal environment for the best weighing results, see Chapter 6.
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5
In Experiment 1(a) the balance was equipped with a standard weighing pan. Test weights of 5 kg, 2 g, and 400 g
were placed repeatedly on the weighing pan of the balance under test. Three replicates were each weighed 20
times, with a total of 60 measurements made. In Experiment 1(b) the balance was equipped with the new SmartPan™ design and the same tests were carried out. The settling time and the first stable digit were automatically
recorded and average values are reported, as shown in Fig. 6.
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Figure 5: Experiment 1(b): 5 mg readability precision balance with SmartPan™ on
open lab bench
Weighing on a 5 mg readability balance with a SmartPan™ compared to standard
weighing pan in a standard environment (on a open lab bench)
20
15
10
StandardPan
5
Repeatability (First Stable) [mg]
SmartPan™
0 1 2 3 4 5 6
Setting Time [s]
l Smart 400 g
l Smart 2 kg
l Smart 5 kg
n Std 400 g
n Std 2 kg
n Std 5 kg
6
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Figure 6: Results of Experiment 1: Effect of SmartPan™ on settling time (Speed) and repeatability (Precision) of a 5 mg readability precision balance on open lab bench (Standard Conditions)
The results of Experiment 1, shown in Fig. 6, demonstrate a decrease in the global average settling time and an
improvement in the repeatability achieved using the new weighing pan design. This clearly shows that the SmartPan™ offers faster and more precise weighing performance than a standard weighing pan under standard laboratory conditions for weighing on a 5 mg readability precision balance.
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