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Technology Used in the Development of a Microbalance
Naoto Izumo
R&D Division 5, A&D Company, Limited

We launched our BM-20/22 microbalances two years ago now, and its sales performance over the past two years has exceeded our expectations of that time. So we are now presenting some important information for stable weighing with the microbalance that we have learnt in the course of its usage.

Fig.1  ① BM-20, ② anti-vibration table, ③ tabletop breeze break and weighing environment logger, and ④ remote controller on a balance table	Fig.2  Temperature/humidity/barometric pressure/vibration monitor, OIML weight and special tweezers
Fig.3  Remote controller: reducing the necessity for key operation on the microbalance itself	Fig.4  Weighing environment logger: ensuring uniform management of temperature/humidity/air pressure/vibration and recording the results

Before the introduction of the BM series, the high price demanded for microbalances was clearly not being well received by the market. This is the impression we received anyway, from the many researchers who would approach our booth at trade shows asking for a microbalance to be commercialized and offered at a lower price by us. We also conducted market research on microbalances with an eye to commercialization. At the time, the many opinions we heard suggested that stable measurement results became a problem every time due to the installation environment, with researchers hence losing faith with microbalances. The equipment makers did not seem to want to pay attention to these problems and dealers were often stuck in the middle of these conflicting interests and did not want responsibility for selling these unreliable instruments.

The two causes of these problems have been determined to be (1) all imported microbalances were still circulating in Japan at prices reflecting the era of the weak yen, when USD1 was equivalent to JPY360; and (2) as Japanese manufacturers were not supplying microbalances themselves, the problems associated with the installation environment when performing microgram measurement could not be addressed individually or by any kind of systematic measures of offering technical market support.

Therefore, the BM-20/22 was offered at a price that was appropriate as a product manufactured in Japan. Further, a specialist tool for assessing the measurement environment at the customers' end, "AND-MEET",*1 was developed beforehand and offered to the market. As a result, the end users (researchers), as well as the dealers, came to use or sell microbalances with peace of mind in Japan. With the implementation of AND-MEET, A&D was also able to develop support technology relating to the installation environment of microbalances. What follows is the technical summary regarding micro measurement based on information we garnered on site in a working environment, which should serve as a further reference for those presently using microbalances or considering their introduction.

The BM-20/22 has a minimum display of d=1μg and a repeatability of 2-4μg. If the installation environment is properly arranged, it has the actual ability to go under 2μg in repeatability. As each device is tested for a 24 hour period to confirm repeatability before it is shipped, customers can be assured these figures are based on actual data from their microbalance. However, to ensure the catalog specification for repeatability is met, it is necessary to properly prepare the installation environment. The main causes of inaccuracy (uncertainty) in the installation environment are breezes, temperature, humidity, vibrations, foot traffic in and out of the measurement area, the construction of the building, the geographical conditions and weather. Problems caused by the people actually performing the measurements could also extend to improper handling of the microbalance or measurement sample, or static electricity they generate, etc.

Below, these different factors will be explained in order using examples from actual experience in the market.

Influences from temperature changes or breezes from air conditioning units (*1)

A microbalance would normally be installed in a specialized room for such measurement. The majority of earlier microbalance measurement rooms were sealed-off rooms of a size of about 3 Japanese tatami mats (approx. 5 square meters) with air-con units moderating the measurement environment. Ventilation from air conditioning can prevent any large fluctuations in temperatures in a room, but at the same time, in order to keep the temperature at a steady level it also generates a breeze. Also, as air-con units are repeatedly turned on and off to control conditions, a temperature change of about 0.5°C is constantly repeated. This breeze from the air-con unit and the slight temperature changes it causes repeatedly can be fatal for microbalance measurement.

As a countermeasure, the microbalance is arranged so that it is not directly hit by the breeze from the air-con unit. As the breeze break mounted on the microbalance as standard is not enough to prevent the unit being directly hit by air disturbance, a tabletop breeze break is often used that covers the entire microbalance unit. The effect of this is significant, and when there are no other causes of instability, repeatability can be reduced from 10μg to 3μg.

As a fundamental solution to these problems, it is necessary to prepare a wider measurement room to increase the thermal capacity of the room, limit the number of people entering and use a partition or tabletop breeze break to block the breeze from the air-con unit. By taking these countermeasures, it is possible to reduce the adverse effects of aggressive temperature control and realize more passive stabilization of temperature conditions ? a technique better matching the microbalance.

For creating a microbalance measurement environment, it is important to reduce the influence of air conditioning, which is the greatest external disturbance for a microbalance, by implementing the above-mentioned countermeasures.

Influences from changes in humidity (*1)

It is necessary to plug in microbalances for at least one day prior to commencing use for measurement. This is in order to equalize the internal temperature within the device so they can deliver accuracy as a microbalance. Moreover, while it is not a problem where air conditioning is running on a continuous basis, if the air conditioning is switched on immediately prior to measurement a change in humidity will happen. If the humidity of the measurement room drops, the microbalance will start moisture release from its sensor unit, and that change will be expressed as a slow drift in the measurement value due to a change in the zero point. Microbalances respond to temperature or humidity changes over the course of several hours, but as the change in temperature and humidity is greatest at the point immediately after starting air conditioning, it is particularly important to act with caution at that time. Further, if there is a heating furnace in the room where the microbalance has been installed, while the furnace is active there will be a slow change in room temperature and during that period the repeatability of the microbalance will worsen. Particularly, during heating there will be dramatic change in temperature and a big influence on measurement, so it is necessary to be careful to separate measurement time and operation of the furnace.

Influences from vibrations and foot traffic (*1)

In the research laboratory, the stand the microbalance is placed on can also double as a work desk. In situations like this, if the operator is there at the time of measurement, vibrations from their work can lead to instability in the microbalance. To address this factor, please do not perform other operations at the time of measurement and use an anti-vibration table for microbalance use recommended by the maker.

If someone is walking behind at the time measurement is being conducted, as even air is fluid to some degree and therefore has viscous properties, the act of someone walking past will cause the air to move. As measurement in the scale of micrograms will be affected by any kind of movement of the air, the microbalance should be set up in an area that does not receive any passing foot traffic. By the same token, if measurement is conducted when people are entering or leaving the room repeatability will worsen, so care should be taken in this respect.

Besides the influences above, it is necessary to install the microbalance in a position where it does not receive any direct sunlight, is far from any doors where people will enter or exit, in a building which does not shake easily, and near a wall or column.

Influences from construction of the building, geographical location and weather

If measurement is performed in a building near a large estuary or on the coast, near a highway or road which is used by heavy vehicles, or in an area with neighboring high-rise buildings built on weak ground, even a normal analytical balance with minimum display of d=0.1mg will on occasion have unstable display. In buildings recently constructed as seismically isolated structures, when an earthquake occurs it may take several days for the balance to achieve stability again. Further, anti-vibration stands employing costly air suspension are designed on the premise that shaking is constant, so these will also lead to instability for the balance. The cause of instable measurement values in bad weather is shaking of the building due to strong winds or high tides caused by low pressure systems or proximity of typhoons. There is presently no established method of stabilizing low frequency vibrations of just a few dozen Hertz like these. When microbalances perform calibration with their internal weight and more time is needed than usual to complete calibration, this could be due to vibrations in the building. This method is actually recommended as a reference to determine whether the time is good for the balance to be used. (*2)

Influences from static electricity and method of using microbalances and measurement samples

Measurement work with microbalances requires "swift accuracy". If the air changes in the weighing chamber, a convection current is created. This convection current accompanies faint changes in temperature, which destabilize measurement at a microgram level. For similar reasons it should be strictly forbidden to put one's hand into the weighing chamber. Instead, special long tweezers should be used. Further, the door to the chamber should only be opened to a minimum degree and the measurement sample should be placed very gently on the weighing pan. It is important to perform these steps swiftly.

As body heat or breath from the person performing the measurement has a negative impact, it is also important to only go near the microbalance as necessary and cover one's body with white robes or other appropriate clothing.

Static electricity can't be seen but its impact is very serious. If humidity drops below 40%, people can easily become charged with up to 10KV of static electricity. Also, weighing paper, a weighing pan made from plastic or the sealing and removing of the cap on a vial can cause static electricity which results in measurement errors of more than 1mg. In order to minimize the effect from static electricity it is necessary to prevent the electrical line of force from charged people entering the weighing chamber. To do this, an analytical balance should be used whose weighing chamber is constructed with glass treated with conductive material to make the balance resistant to static electricity. Also, the measurement container or sample should be neutralized using a static eliminator before measurement is performed.

As can be seen above, stabilizing microbalance measurement is not an easy task. Therefore, at A&D the use of the "AND-MEET" installation environment evaluation method for analytical balances including microbalances is recommended.

If you have concerns on the introduction of new weighing devices, or if you would like to improve the measurement environment and thereby increase the quality and productivity with regards to measurement, please do not hesitate to contact your nearest A&D representative.

References

(*1) 28th Sensing Forum "Investigation of the Basic Performance of Analytical Balances"

(*2) Roundtable Research Conference on Organic Microanalysis, "1st Electronic Microbalance Seminar ? Towards Accurate Measurement"

(*3) A&D Development Story No. 17 "Things to Keep in Mind when Using Analytical Balances (Proper Handling Edition)"

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