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Story 17: Things to Keep in Mind when Using Analytical Balances (Proper Handling Edition)

  • Analytical Balances
April 2, 2012

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

Analytical balances are very sensitive. Therefore, they are heavily affected by the environment in which they are installed and the way measuring personnel handle them. With regards to methods for assessing the environment, running AND-MEET (*1) will yield a judgment and assessment, and from there a concrete process for improving the environment can be proposed. In addition, in "Development Story 17", I explained a method for selecting a location for measuring instruments. So, for this edition of "Development Story", I will discuss proper handling with a focus on analytical balances.

The basic motto for weighing instrument operation is "quick and accurate". So in the case of someone taking time to slowly open and close a breeze break door, these words would tell us that such is not an optimal way of conducting measurement. It means that by increasing the time that the breeze break door is open, the air within the breeze break changes, and the weighing area's temperature will change. From amongst the analytical balances, I will use the microbalance, capable of measuring 1 millionth of a 1 yen coin (1 gram), as an example.

For instance, in bioscience research fields, many labs use micropipettes. Even with pipettes, we know that without accurate and experienced handling, random errors occur, and if there is a problem with the pipette itself then systematic errors can occur. The minimum capacity for a micropipette is around 1 to 2 μL. 1 μL is an extremely small amount compared to what we are used to in our daily lives. However, if 1 μL of water is converted into mass, it becomes 1 mg, and a sensitivity of 1 mg is a minimum display for general-purpose precision balances in the weighing instrument industry. Yet microbalances can measure 1/1000 of this unit – an ability to measure where 1 digit = 1 μg is stipulated. In other words, determining 1 μg is more difficult than using micropipettes, so it can be said that it's clear that experience and accuracy are demanded in measurement operation.

Electronic balances deliver weighing results via digital display: from general-purpose balances with a minimum display of 10mg or 1mg to analytical balances with minimum displays of 0.1/0.01/0.001mg. Because of this, people think that if the weighing sample is simply placed on the pan, an accurate weighing result will be displayed instantly. However, with readability that is orders of magnitude more precise than the minimum capacities of micropipettes, one must question whether the result displayed is really correct, and recognize that instability in the displayed result can be quite natural depending on how the instrument is operated.

Therefore, I will explain how errors in weighing occurred using actual examples from weighing sites.

1) Effects of static electricity

For weighing instruments used in production lines using automated machines or at sites conducting plastic injection molding, there have been instances of displays becoming unstable, or measurements changing in one direction with the passage of time. This phenomenon is called "drift" in the weighing industry. Currently, in the production processes for pharmaceutical manufacturing, primary and secondary batteries, electronic parts such as IC chips and LEDs, and resin molds, many weighing instruments are used for quality control. But on these production lines, the environment is usually like that of a clean room, and we have confirmed many areas with 24-hour air conditioning and humidity levels sometimes below 20% due to the undesirability of moisture. In other words, it is dry, and friction from insulated material caused by the moving around of objects causes static electricity to build up easily. Moreover, people working on the line or research can sometimes build up around 10,000 volts of electricity themselves. Under these circumstances, the effects of static electricity become greater, and errors of a few dozen milligrams can easily occur. (*2)

If the humidity in the weighing instrument's installation environment cannot be increased over 40% or if electrical build up occurs faster than electrical discharge, please introduce a static eliminator and conduct weighing operations after actively removing charges from the weighing sample.

2) Effects of temperature

Let's say you check the quality of a molded item right after resin molding it by measuring it on a weighing instrument, or you measure out some pharmaceuticals into a handheld vial and then weigh it, or you take a sample from another location and bring it in to measure it right away. In scenarios like these, a difference between the weighing sample's temperature and the weighing area's temperature will occur. This temperature difference will become a weighing error. The reason for this is that when the sample's temperature is higher than the room temperature, a layer of warmer air is created around the sample, and a slight upward air current is created. That air current has the effect of pushing the weighing sample up, and the weighing measurement will come up light at first. When the sample later reaches room temperature, the original weight will be displayed.

It depends on the temperature difference and the shape/material of the sample, but weighing errors on the order of a few dozen milligrams can occur.

                                              thermograph image
                                                Fig.1 Thermograph image of a container after being gripped by hand

Fig.1 shows the results of thermograph observations of a coffee can placed on an analytical balance. The can has been gripped for a few dozen seconds before being placed on the pan. Metal is especially conductive of heat, and a deviation from ambient temperature of a few °C can occur in a short period of time. It is known that the convection current generated by this temperature difference will affect weight measurements. (*3)

I experienced this personally more than 10 years ago, when we were setting up mass production of weights with tolerances conforming to the OIML Class E2 standard and compositions conforming to the Class F1 standard. In this instance, we found that our weight-adjusted 200 g weight had grown heavier on a 0.1 mg level the next day. We had touched the weight with gloves, but after conducting weight adjustment and screw tightening, we found that our body heat had warmed up the weight slightly. It was a good experience for me to understand why people say it is not good to touch weights directly with one's hand. In places where weighing instruments are being used, one may see people picking up weights with gloves and calibrating, but at least when it comes to analytical balances, we recommend doing calibration and performance checks using tools such as tweezers.

3) Effects from work in the weighing area

Analytical balances come standard with a breeze break. It's there to stop drafts and maintain stability within the weighing area. However, if the breeze break's door is operated roughly, an impact will occur at the end of the swing and the force will reach the balance's weight sensor. This can result in variations in the zero point, and risks reductions in repeatability. But if the door is operated too slowly, the time when the door is opening and closing lengthens, and the air within the weighing area will be replaced. As a result, the temperature can become unstable and become another factor in the reduction of repeatability.

People's hands exceed room temperature, and placing one's hands in the weighing area can cause a disturbance in temperature. For this reason, the door should not be opened longer than necessary, the door should be operated accurately within a short period of time, and long tweezers need to be employed to avoid placing one's hands in the weighing area as much as possible.

As an aside, I've searched far and wide for an off-the-shelf set of long tweezers that are usable for calibrating weighing instruments. However, I was unable to find anything fitting the description. It was then that I independently drew up plans for an ideal set of tweezers, and gave the manufacturing contract to a manufacturer near Tsubamesanjo in Niigata Prefecture. In the production of these AD-1689 tweezers, special regional production techniques that made Japan the #1 producer of eating utensils such as spoons and forks have been used. The original "monotsukuri" (craftsmanship, artisanry) techniques found throughout Japan contain skills passed down by craftsmen for more than 150 years, and it is thought that these techniques have supported Japan's economic growth from the Meiji Restoration to the present day. I believe that continuing to support these techniques is absolutely essential to maintaining the Japanese economy going forward.

But I digress. I've summarized weighing instrument operation methods in a simple form below.

 When conducting weighing using a balance, special care must be taken with regards to the weighing sample's static charge and temperature.
 It is especially necessary to actively introduce a static eliminator to take care of static electricity trouble in dry environments with humidity of less than 40%.
 For the weighing sample as well, care in controlling the temperature is needed, including measures such as not touching the sample directly with one's hand. It is important to place the weighing sample in the weighing area beforehand, and allow it to adjust to the temperature there before commencing weighing.
 Weight measurement should be conducted quickly and accurately, the weighing area door should be opened as little as possible, and one's hand should not be inserted into the weighing area.

Reading the precautions above, one may feel heavy with the difficulty of operating an analytical balance. However, please rest at ease. Lately, multiple analytical balances with internal static eliminators are on the market. Additionally, there are models which feature a weighing preparation room where the weighing sample can be placed to allow it to adjust to the temperature. And there is also the set of long tweezers for weighing operations which I wrote about.

Regarding precautions aside from weighing operations, it is necessary to connect the balance to a power source the day before weighing to ensure that it is stable. In the case of weighing instruments at the semi-micro level and below, it can take from 6-8 hours for a connected machine to completely adjust to the room temperature. Additionally, one must do as much as they can to ensure that vibrations, pressure changes, temperature changes and humidity changes do not occur in the weighing room. As part of this, foot traffic in and out of the room should be reduced as much as possible.

Lastly, regarding handling of weighing instruments, the characteristics of electronic components of electronic balances become more stable the longer the instrument is hooked up to a power source. The thermal distribution within the device, including the weighing chamber, will become even. Since these instruments do not use much electricity, I recommend continuous connection to a power source if possible.

I believe that going forward, weighing instrument manufacturers shouldn't just conduct development that's all about how good the performance is or how many features there are, but that they should also come up with solutions which are easier to use on-site and which also include peripherals to display and reduce weighing errors. Moreover, this proposal means providing a comprehensive weighing and measuring service with everything from analysis to assessment, using environmental measurement, communication utilities, data management, graphing functions, and more. What is important to manufacturers at such a time is knowing the weighing and measurement market that forms the actual usage locations for these instruments. I would like to continue emphasizing market surveys and providing original products according to principles emphasizing the best results for all parties involved.

*1 Regarding AND-MEET: Please refer to the 28th Sensing Forum: Investigation of the Basic Performance of Analytical Balances (PDF 1.28MB)
*2 Regarding the effects of static electricity: Please refer to Training Material for Balances (1) (PDF 437KB)
*3 Regarding the effects of temperature on weighing samples: Please refer to Training Material for Balances (1) (PDF 437KB)

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