Story 16: Things to Keep in Mind when Using Analytical Balances (Installation Environment Edition)

  • Analytical Balances
March 29, 2012

Things to Keep in Mind when Using Analytical Balances (Installation Environment Edition)
Naoto Izumo
R&D Division 5, A&D Company, Limited

In this edition, I want to talk about the installation environment for analytical balances, which have seen a lot of trouble on the market. In the next edition, I'll be talking about how to properly use an installed analytical balance and take measurements accurately, using knowledge gained from actual usage scenarios.

Because analytical balances are very sensitive, the environment in which they are installed will affect them a great deal. For the same reason, we know that the way in which operators handle the balances also has a large effect. As far as assessing the measuring environment is concerned, thanks to our measurement environment evaluation tool option "AND-MEET" (*1) and our market response, it's possible to get a clear idea of how to improve the measuring environment from a balance installation environment assessment. So for this edition, I've put together some general information about installation environments.

Since the March 11 Disaster, we continue to have frequent earthquakes in eastern Japan. This is a special concern for analytical balances capable of measuring on a microgram level, for not only do they pick up earthquakes, but also things such as movement of people, handcarts, and forklifts, as well as vibrations and changes in room air pressure from the opening and closing of doors.

As for weather effects, the force of wind from passing low pressure systems like monsoons and typhoons can cause problems due to buildings shaking, which becomes an even greater issue on higher floors. Buildings built with a quake-absorbing structure, which have become more common recently, are designed with shaking as a given. Such structures can shake for days due to wind pressure or earthquakes.

For situations like these, we have confirmed that passive anti-vibration tables such as the AD-1671 improve issues with repeatability. On the other hand, we have found that despite their high cost, active air suspension anti-vibration tables used for optical measuring instruments actually become a source of vibration, and negatively affect analytical balances.

Administrators of balances often ask us about the permitted specifications of an installation environment for an analytical balance. A&D recommends the following: (1) daily fluctuation of temperature of 4°C or less (within 10 – 30°C) and short term fluctuations of 0.2°C/30 minutes or less, (2) daily fluctuation of humidity of 10% or less, and (3) daily fluctuation of air pressure of 10 hPa or less. In particular, regarding the short term temperature fluctuations written about in (1), it is known that the repeated slight changes in temperature caused by air conditioning have an especially destabilizing effect on balances' zero-point display. To cite an extreme example, our data shows that even within the sort of windy environmental setup specified by the Ministry of the Environment's Manual for Continuous Monitoring of Air Pollution (a.k.a. PM2.5), using the AD-1672 tabletop breeze break (which surrounds the balance area) can have such an improving effect that catalog specifications for the microbalance can be met. (*2)

Allow me to explain proper installation of a balance using an actual example. Fig. 1 is a rough sketch using the seminar room in our R&D center's 2nd floor as a model. The seminar room is about 10 meters long on each side, and there are multiple air conditioning units in the central area of the ceiling. It would be rather large for a lab, but it resembles many labs in terms of the layout of things such as the air conditioners and lab tables. I've numbered the tables in this diagram from #1 - #16. I would like you as well to think about which spot is the best place in the seminar room (lab) to install a balance.

                                          laboratory layout
                                                                Fig. 1 - Diagram for evaluating balance placement

To select a location for the balance, first we must find a location that minimizes temperature fluctuations, which have the greatest effect on balances' performance. To be more precise, a place that is (1) out of direct sunlight and (2) far away from air conditioner vents. Next, we select a (3) corner of the room next to the wall. The center of a room has weaker construction, and the floor tends to shake more easily. However, there tend to be structural supports in the corners of a room, and they tend not to shake easily. In addition, even if room temperature is being controlled at a certain level, floors and walls often go below room temperature, especially during winter. Level temperature means that the temperature is evenly distributed (the flow of heat is even), but in the case of walls which have outside air on the other side, balances near that wall may be constantly subjected to outside temperature variations. For the same reason, installation should not be done near window glass. That is why it is best to (4) install the balance near a wall which has another room on its opposite side. As for the table on which it is installed, (5) a hard balance table with high heat capacity should be used, and (6) the balance table should be separated by a few centimeters from the wall and other tables in order to isolate it from heat and vibration coming from the wall and floor. A (7) dead end area with low foot traffic should be selected because people tend to come and go through the central part of a room. To further reduce people's influence, (8) an area far away from the door should be used, on a table where (9) only measurement is conducted in order to prevent vibration from people's actions from affecting the balance. Additional preconditions are that the room and wall where the balance is located should be (10) far away from routes with high traffic or heavy objects moving, and (11) on as low a floor as possible.

Using the above conditions, we can determine that within Fig. 1, the best areas in the room to place a balance are #3 as well as #2, the areas where the effects of direct sunlight are low, air conditioner vents and windows are far away, routes where people move and doorways are far away, and near an area where structural materials such as supports are installed. Issues with #3 include being near a wall to the outside, and near a wall to a hallway, but I believe that will not be an issue because only people pass through the hallway.

The above constitutes a general assessment of balance installation environments, but labs often have individual circumstances, such as having a heat-treating furnace, or there being a lot of people coming and going during the day, and so on and so forth. Ultimately, running AND-MEET in the locations where balances are to be placed, assessing the environment there, making any problems clear and developing concrete measures to deal with them is thought to be the best course of action.

To sum up the above, here is what is required of a balance installation environment.

 Be especially sure to consider the room temperature stability, and do not place a balance near an air conditioner vent in order to reduce the effects of temperature variations. If there is no other option, then utilize things such as tabletop breeze breaks or partitions to cut off direct wind.
 The balance should be placed in an area out of direct sunlight, away from routes people use and away from persons working on other things. To minimize the effects of vibrations, the central area of a wide floor should be avoided, and an area near the building's supports should be selected. At this point, the balance table should be separated a few centimeters from walls and supports in order to isolate it from vibrations and heat from the building.
 To reduce effects from vibrations, the balance should be placed in a location as far as possible from paths for moving heavy objects.
 The building will shake when low pressure systems cross the area, so install the balance on as low of a floor as possible. In addition, to reduce the effects of the building shaking due to earthquakes and vibrations, an anti-vibration table should be installed.

40 years have passed since the balance was transformed into an electronic device using microcomputers. Since then, digitalization has progressed, and the balance has come to be regarded as an instrument which can be easily used by anyone. However, at present, analytical balances have a resolution of 1/20,000,000 or more, and a certain level of skill and preparation is required to enact exact measurements. In particular, with regards to balance installation, there are many matters to be taken into consideration. I hope this article will help you understand the best environment, install the machine and set up the environment, and allow you to conduct reliable measuring work.

*1 Please refer to Development Story 12: Solutions Provided by the BM Series Part II
*2 Please refer to the 28th Sensing Forum: Investigation of the Basic Performance of Analytical Balances (PDF 1.28MB)

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