Stable Measurement Tips for Microbalances with A&D Weighing Part 2

Continuing with our series with A&D Weighing here are tips 4 thru 6 on stable measurements for microbalances. If you missed part 1, you can catch up here.

Tip #4:

Wait until the microbalance is sufficiently warmed up and acclimatized to the room temperature and humidity.

Why?

It usually takes from 8 to 12 hours for a microbalance to adjust to the room environment after being plugged in. During this period, the zero point drift becomes particularly large, worsening repeatability.

Solution

Install and plug in the microbalance at least a day before starting measurements while the environmental conditions of the room are kept constant.

The characteristics of the electronic components become more stable and the thermal distribution within the microbalance more even the longer the microbalance is hooked up to a power source. It is therefore advisable that the microbalance is constantly connected to a power source where possible.

Case Study

The two graphs below show data taken at a university research laboratory over 24 hours using BM-22 microbalance*. Its internal calibration weight (approx. 20g) was automatically moved up and down in 40-second cycles, simultaneously logging the balance temperature, zero point, and span value (Graph 1).

Further, repeatability (standard deviation) was calculated for every 10 consecutive span values (Graph 2).

As you can see, both the zero point and repeatability were clearly influenced by temperature changes.

*Note: The BM-22 is a smart-range model with 5.1g / 22g capacity and 0.001 mg (1μg)/0.01mg readability. Its repeatability specification for a 1g weight is 0.004mg (4μg).

Tip #5:

Prevent airflows and air pressure changes.

Why?

Breezes or air circulation hitting the microbalance destabilizes measurements. Flows from an air conditioner can not only sway the balance unit but cause temperature fluctuation, to which microgram measurements are especially susceptible (see also Tip 2: “Ensure that the ambient temperature is stable”).

Air turbulence or pressure change caused by people passing by, the opening and closing of doors and so forth, also lowers the stability of microbalances.

Solution

Here is a partial list of what you can do to minimize the effects of airflows and air pressure changes:

1. Keep the microbalance away from the source of any breezes, such as ventilators or air conditioners.

If that is difficult, use partitions to cut off direct breezes.

2. Put an external breeze break (draft shield) over the microbalance, such as A&D’s AD-1672.

In addition to protecting the microbalance from air movement, this can also mitigate the effect of temperature changes.

3. If possible, replace swinging doors with sliding doors as the former cause larger pressure changes. Also, avoid making the room airtight. Secure a vent to let air in and out if necessary.

Of course, avoid setting up the microbalance near a door, from which people and external air can enter and disturb the measurement.

4. Do not perform measurements when a rapid fluctuation in atmospheric pressure is observed.

Measurement is easily disturbed by rapid atmospheric pressure changes as well as swaying of the building due to strong winds caused by the passing of a low-pressure system (see also Tip: 1 “Avoid or isolate all possible sources of vibrations even when you cannot actually feel them”).

For stable weighing, it is desirable that daily fluctuation of atmospheric pressure be 10 hPa or less.

Monitoring the environment

As has become clear by now, controlling the environmental conditions is a prerequisite for stable measurement with a microbalance. For this purpose, it will be very convenient if you have a device that can simultaneously measure multiple key environmental parameters, such as A&D’s AD-1687.

The AD-1687 allows you to monitor and record with date-and-time changes in temperature, humidity, atmospheric pressure, and even vibration. Moreover, when connected to an A&D balance, it saves mass values sent from the balance together with these environmental data.

Tip #6:

Prevent or eliminate static electricity.

Why?

Static, while often overlooked, can seriously degrade precision weighing. An electrostatically charged object induces the opposite charge in nearby objects. The resulting attraction will make stable measurement extremely difficult.

Error when weighing a charged object

The object will appear heavier. The value then changes as static is dissipated into the air or via the weighing pan.

Error when a charged object approaches

Static attraction can pull the weighing pan in the opposite direction and cause values to drift.

Filter papers, disposable weigh boats and plastic centrifuge containers can all become charged just from normal handling. Charged powders can be displaced, causing cross-contamination.

Solution

Here is a list of what you can do to handle the effects of static electricity:

1. Keep the room humidity higher than 40% RH.

Static will easily occur when humidity drops below that level. This is especially common in dry winters, during which people and their clothing can also quickly become charged.

2. Select a microbalance with a weighing chamber whose glass panes are treated with conductive material, such as BM-20 or BM-22.

This will prevent the electrical lines of force entering the weighing chamber in case the operator’s clothing is charged, such as polyester lab coats.

3. Avoid using plastic or glass containers under low humidity conditions. Use metal containers instead.

Non-conductive materials such as plastic or glass are highly susceptible to static electricity.

4. Use an ionizer (static eliminator). Better yet, use a microbalance with a built-in ionizer, as the BM-20.

This is the quickest, easiest and surest way to completely remove static charge from weighing samples and containers. All you need to do is hold them in front of the ionizer for a second or two before starting the measurement. The ionizers supplied by A&D are fan-less, direct current (DC) type, which create no breeze that could blow away fine powder samples.

To explore MISUMI’s selection of A&D Weighing products, visit our dedicated page here. Stay tuned for the conclusion of the series!

About the Author

Carlicia Layosa

Carlicia is the Marketing Automation Manager at MISUMI. She holds a bachelor's degree in Mechanical Engineering and a master's degree in Energy Engineering from the University of Illinois at Chicago. She is a Certified SOLIDWORKS Associate, Marketo Certified Expert, and is passionate about education and training.

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