Article for the 28th Sensing Forum

Investigation of the Basic Performance of Analytical Balances
Proposal of a microbalance performance evaluation
method that includes the installation environment

Presented by: Naoto Izumo, Yuji Fukami, and Yoshikuni Sato
R&D Division 5, A&D Company, Limited
Organized by: The Society of Instrument and Control Engineers (SICE)

October 13 to 14, 2011
Hiyoshi Campus, Keio University
Yokohama, JAPANInvestigation of the basic performance of analytical balances
Proposal of a microbalance performance evaluation methodthat includes the installation environment
Naoto Izumo, Yuji Fukami, Yoshikuni Sato

A&D Company Limited

Higashi-Ikebukuro, Toshima-ku, Tokyo 170-0013 JapanAbstract
Operators of analytical balances sometimes question the accuracy of weighing data dependingon the ambient conditions of the location where the balance is installed. In recent years, it has
become necessary to provide evidence of weighing performance due to regulations such as fineparticle standards (PM2.5) and those of the USP defining minimum sample weights. We propose anew method to evaluate weighing performance in the various ambient conditions where themicrobalances are used and also report on the results of this method.

KeywordMicro-gram, weighing environment, minimum weighing
1. Introduction
Electromagnetic equilibrium digital balances were introduced to the market over 40 years ago.
Analytical balances with typical specifications (capacity: 200 g, minimum display: 0.1 mg) are usedin many weighing applications. Semi-microbalances (capacity: several hundred g, minimum
display: 0.01 mg), microbalances (minimum display: 0.001 mg (1 μg)), and ultra-microbalances(minimum display: 0.0001 mg) are now also available. The resolution (capacity divided byminimum display) of analytical balances continues to improve and now exceeds 10 million to 1.
One μg is a minute mass (1 millionth of a one yen coin), and it is clear that the weighing precision
is affected by environmental disturbances. This includes not only temperature and humidity
changes but also gravitational acceleration, atmospheric pressure, and vibration. However, evenwhen the causes of disturbances to the weighing environment were clear, determining the actual
effects of these disturbances on weighing performance was difficult and considered virtually
impossible in industry because the many disturbance parameters change over time, making themdifficult to quantify.1
Due to these issues, it was impossible to determine whether unstable weighing values are due
to a problem with the analytical balance itself or environmental disturbances. Obviously, if thecause cannot be determined, it is impossible to come up with a plan to deal with the problem. This
leaves the user of the balance feeling unsure about weighing and its results, and problem remains
unresolved. With this situation in mind, we proposed a measurement environment evaluation toolthat can help solve the abovementioned problems in the locations where analytical balances areused. Using this tool, we successfully revealed true weighing performance in the actual
environments where balances are used. We also made it possible to determine weighingperformance from a series of evaluation procedures and to propose improvements for the weighing
environment with the ultimate goal of improving weighing performance.
2. The new Measurement Environment Evaluation Tool (AND-MEET)
We propose AND-MEET2 as a procedure to evaluate the setup environment of weighing
instruments such as digital balances. In AND-MEET about 3,000 weighing data points arerecorded as an internal weight in the weighing instrument is loaded and unloaded repeatedly overa long period of time. The zero point of when the internal weight was unloaded is subtracted fromthe acquired internal weight value to calculate the span value over a day. The standard deviationof 10 adjacent span values acquired chronologically is calculated as the repeatability. When thedata is graphed with the repeatability value on the vertical axis and the time on the horizontalaxis, the weighing performance (repeatability) over 24 hours can be confirmed, for example. In
addition to weighing data, temperature, humidity, and atmospheric pressure data is recorded byembedded sensors. When a graph is created with time as the horizontal axis, interaction between
the abovementioned repeatability and environment changes can be evaluated.
3. The actual results of AND-MEET
3-1. An environment with temperature changes
Fig. 1 shows actual results from AND-MEET at a university. This graph shows data for ananalytical balance with a minimum display of 1 μg, with the zero point and span value (internalweight value minus the zero point) shown on the left vertical axis. Note that the internal weight ofthe balance is about 20 g and the span value indicates this value. The horizontal axis is the time
axis and represents a 24-hour period from 5 PM on February 17 to 5 PM on February 18 of thisyear. The temperature change is shown on the right vertical axis and shows that there was achange of about 3 °C between 19 and 23 °C over the day. Furthermore, one scale interval for thezero point and span value (the internal weight value minus the zero point) is 500 μg. The recordingof data for the graph started immediately after the digital balance was set up and powered on. Thegraph shows that the temperature rose 3 °C in 5 hours after the power was switched on. In thisperiod, range of up/down variation of the zero point was about 1.5 mg and the span value changed+0.5 mg. The temperature slowly dropped −3 °C over the next 12 hours in a linear fashion.
Accompanying this change, the zero point variation dropped and the span value also slowly
dropped −300 μg. It was determined that during the six hour period between 10 AM and 4 PM onFebruary 18 when the temperature variation was the smallest, the zero point dropped by −300 μg
but the span value variation was as small as about −100 μg

Fig. 1. Changes in temperature, zero point,Fig.

and span value over 24 hours

repeatability and temperature

2.

Changesinspanvalue

Fig. 2 shows repeatability (reproducibility: σn-1) of 10 adjacent span values calculated fromthe data of the graph in Fig. 1. The repeatability data for every 10 points is presented as a single
point, and a line connects the adjacent repeatability points. Repeatability immediately after setup
is poor at about 10 μg/20 g. When the temperature was rising, the value was 4 to 6 μg, which is arelatively poor value. Thereafter, the temperature dropped comparatively slowly and therepeatability was good at 2 to 4 μg. The temperature was most stable from 10 AM to until justafter 3 PM on Sunday, February 18, with the repeatability at 2 to 3.5 μg. This proves that theperformance achieved sufficiently satisfies the product catalog specifications of σ＝４ μｇ/１ｇ.
The balance was set up in a room in the engineering department of a national university inTokyo that conducts material research related to energy. When actually visiting the site, welearned that there is a large high-temperature treating furnace in the research room that operates
each day from 5 PM to 10 PM. This was why that room temperature rose every evening.
Of course, we could never recommend setting up a micro-balance in a room with an operatingheat treating furnace, but we explained the AND-MEET results and convinced the professors thatthe micro-balance performed up to standards during weighing in the afternoon when the furnace
was off.
3-2. When the building vibrates (with or without an anti-vibration table)
We will now describe the effects of a typhoon on repeatability. Fig. 3 and 4 are graphs showing
temperature changes and span value repeatability when a typhoon occurred. This data wasacquired on October 30, 2010. The rough weather that accompanied Typhoon 14 (975 hPa)
continued until dawn the next day. While the temperature change was slight (0.5 °C or less), Fig. 3shows that repeatability, which started at 3 μg, gradually worsened and reached an average of 14
μg. After this, it gradually improved. Data from another microbalance recording at the same time
nearby shows that repeatability was stable at about 3 μg excluding a few time periods over severalhours.

One balance was on an anti-vibration table and the other was not. Fig. 4 shows the data fromthe balance on an anti-vibration table.

Fig. 3. Passing typhoon, no anti-vibration table

Fig. 4. Passing typhoon, anti-vibration table used

While the following is speculation, we believe that the typhoon caused a gradual variation inatmospheric pressure so the repeatability of the span value was not affected, even though zerodrift was. However, the pressure changes from the typhoon were accompanied by winds that shookthe building. The people in the room did not feel the shaking of the building but the shaking
affected the highly sensitive microbalance greatly. The results of Fig. 3 and 4 show that theanti-vibration table protected the analytical balance from the minute vibrations of the building,which resulted in more stable weighing values.
3-3. The effect of air blown from an air conditioner
Most research rooms where analytical balances are set up use air conditioning to control theroom temperature. While this controls the temperature, this does not always produce positive
results for balances. If you use air conditioning to proactively stabilize the temperature and look at
the results carefully, you can see that in order to maintain a constant temperature the air
conditioner constantly causes minute changes in temperature. The analytical balance is sensitive
enough to track these changes and this produces small fluctuations in the weighing values.
Fig. 5 and 6 are graphs of data from microbalances set up in a room with an air conditioner.
The output from the temperature sensor inside the balance showed rippling of about 0. 1 °C andthe zero point changed along with this. The results here show that while the temperature change
for the day was small at 0.5 °C, the repeatability was about 5 μg and varied greatly. Conversely,
when a balance was covered with a table-top breeze break and the open space of the room wasnarrowed to prevent blown air from the air conditioner from directly hitting the balance (Fig 7 and8), even though the temperature change was large at 2 °C over a day, there were no ripples in thetemperature and the accompanying zero point changes, with the repeatability falling to an averageof 4 μg or less. In addition, it was determined that the poor repeatability at 6 AM on July 13 seenin Fig.6 was due to an earthquake (off the shore of Fukushima at 5:47 PM, Magnitude 5.3) over
200 km from where the weighing was performed in Saitama Prefecture.
This data is a good example by which we were able to understand the effects of blown air froman air conditioner on weighing values. Furthermore, it was determined that to further improve therepeatability value of 4 μg acquired for Fig. 8, it is necessary to strengthen measures to block
breezes.

Fig. 5 Before countermeasures for air conditioner Fig. 6 Before countermeasures for air conditioner

Fig. 7 After countermeasures for air conditioner

Fig. 8 After countermeasures for air conditioner

3-4. The effects of humidity changes
Fig. 9 and 10 show the data when investigating the introduction of a micro-balance as a masscomparator at a location where the business of calibrating weights is planned.
The balance was set up in a room covered in thick concrete. After the balance was set up,AND-MEET was started along with the air conditioner. The temperature change was a mere 1 °C,
and the data had a very good span value repeatability of 1.8 μg. However, the zero point variation
was high at +5 mg/day, which caused the user some uneasiness. This zero point drift was due to asudden humidity change caused by switching on the air conditioner, which can be seen in Fig. 10.
The humidity changes occurred because it was thought that it would be better to switch on the air
conditioner when AND-MEET started. As the air conditioner continued to run, the zero point drift
abated and ideal repeatability data was also obtained.

Fig.9 Effect of humidity

Fig. 10 Effect of humidity

4. Discussion○ The environmental causes affecting the repeatability of microbalances
The results of AND-MEET clearly indicated the causes of weighing value errors affecting theperformance of micro-balances. All the data cannot be presented here due to limitations of space,but concrete examples of the effects of earthquakes, passing low pressure systems, temperatureripples and wind and humidity changes from air conditioner use, the coming and going of people(vibrations, pressure variations), and heat sources near the balance have been presented. Using
AND-MEET clearly makes it possible to determine these influences quantitatively and understandthem.
○ Determination of the minimum sample weightClinical testing and contracted research in the pharmaceutical and food product fields often
involve work that requires validation of weighing. In the specimen sampling stage at the start ofanalytical work, analytical balance repeatability and the minimum sample weight3, which is
calculated from the repeatability, must be determined. To determine the minimum sample weight,
it is necessary to confirm the extent to which the balance is influenced by disturbances in the setup
environment. AND-MEET is an effective means to verify these influences. Analyzing theAND-MEET results makes it possible to improve the weighing environment and prove theminimum sample weight that can be achieved in that environment.

○ The expanding needs of environmental weighing (PM2.5, etc.)
The problem of lung cancer deaths due to air pollution is increasing. The Manual for
Continuous Monitoring of Air Pollution announced by the Ministry of Environment in February oflast year describes the measurement of minute particulate matter (PM2.5)4. The manual statesthat a special filter is used to trap particulates, which are then measured at the μg level. The
measurement environment must have a temperature of 21.5 ± 1.5 °C and a relative humidity of 35± 5%, so a constant-temperature room is required. Such requirements mean that weighing must bedone with the micro-balance set up in an environment with a strong air circulation. We haveconfirmed that AND-MEET is an effective way to evaluate the effects of the blown air. By assessing
and evaluating the setup environment of a micro-balance using the AND-MEET results, it has
become possible to improve the setup environment and meet the PM2.5 specifications required forthe micro-balance.
5. Summary
We believe that a manufacturer that simply makes micro-balances and other special
instruments under a certain environment is not fulfilling its market responsibilities. Instruments
are strongly influenced by their setup environment and manufacturers of weighing instruments
must take proactive measures to deal with this issue. Consequently, A&D offered its measurement
environment evaluation tool (AND-MEET) to users, advancing methods to acquire weighing datausing automated machines in production lines utilizing balances. By using AND-MEET, users candetermine the basic performance of analytical balances in their setup location and infer andinvestigate the causes for repeatability issues. With AND-MEET, users can also ultimately develop
plans to improve weighing environments.
In this way, AND-MEET can be used to assuage the worries of researchers unsure about
introducing more sensitive weighing instruments. At the same time, it can be used to investigate
the basic performance of weighing instrument products while considering the setup environment.
As with other industrial product markets, developing countries are assuming greaterprominence in the weighing market, and the “Made in Japan” brand is reeling from the effects.
Nevertheless, Japan still maintains prominence in the markets it has long dominated: “small andlight” markets, such as new materials and electronic components, and instrument markets that
require greater precision and higher quality. This is evidenced by the wide use of production
equipment made in Japan for μg-level weighing on production lines.
Our newly proposed AND-MEET is a fundamental tool to support these still growing
industries going forward. We will work to ensure that it is recognized as a way to support locations
using analytical balances in various fields ranging from research to production and contributes tofurther advancements in these fields.
References
1. Yearly Report, No. 31, 5-7, 2007, Okayama Prefecture Environmental Health Center
2. “Development Stories No. 12”, A&D website
3. “USP & Minimum Sample Weight”, A&D website

2008 USPC Official 5/1/08 General Chapters:<41>WEIGHTS AND BALANCES
4. Manual for Continuous Monitoring of Air Pollution, 6th edition, March 2010, Air Quality Office,
The Ministry of Environment