Operating Balances and Scales

Why is Maintaining your Balances Important?

In order to achieve accurate weighing results, balances and scales need to be calibrated regularly, otherwise measurements become inaccurate. Normal wear and tear from regular use and external factors like mechanical shocks can render the accuracy of balances and scales to be less reliable over time. Due to the potential for both quick degradation or deterioration of accuracy over time, depending on factors relating to the surrounding environment and frequency of use, regular scheduled calibrations in conjunction with routine testing can greatly improve the equipment's overall lifetime and weighing accuracy. Put simply, calibration is a quantitative comparison. To evaluate your equipment's ability to weigh a sample, a known reference weight is placed on the pan. The error is defined as the difference between the measured value (the reading) and the true value (the reference weight).

Objectives

This protocol teaches users how to:

• Handle, set-up, and clean a scale
• Level and calibrate the A&D GF-1200 scale

• Properly weigh chemicals and samples

Materials

• A&D GF-1200
• Calibration weights (at least one of 500g)
• Spoon or scoopula
• Weigh boats
• Weighing paper
• Beakers
• Sample Containers

Handling, Setup, and Cleaning

Handling

Scales should always be held upright when transporting, minimizing any tilting.

If available, lock scale during transit using the lock on the bottom of the scale.

Pressure should not be applied to the weighing platform, especially when travelling with the scale.

Attention should be taken to avoid any shocks or impact to the scale (dropping, banging, ....)

Set-up

Make sure the balance is placed on a stable level surface (such as a cement table), devoid of vibration, shock, drafts (away from doors and windows), and out of the sun. These external factors can contribute to unstable measurements.

Warm up the balance for at least 30 minutes before use and do your best to eliminate temperature differences between your sample and the environment.

After moving the scale to a new location or before each use, check that the balance is:

1. Level
2. Properly calibrated

After you have set-up the scale in its new location, you can tape around the scale so that if the scale is accidentally moved it will be more noticeable, and can be re-calibrated in needed.

Cleaning

Any chemicals or sample that spill onto the weighing platform can be wiped away with a damp paper towel or kimwipe.

Always read the SDS for the chemicals you are using to properly assess how to clean up any spills.

If chemicals or sample spill underneath the weighing platform, you can carefully remove the platform to pick up the spilled sample using tweezers, or wipe with a damp paper towel or kimwipe.

Levelling and Calibration

The need for leveling and calibration can vary depending on the usage of your scale.

1. Leveling and calibration should be completed whenever you move the scale from one location to another.
2. Weekly spot checks should be performed to ensure that your scale is properly calibrated (simply place the calibration weight on a tared scale and ensure the reading is accurate).
3. Monthly calibration is recommended to ensure stable and accurate readings.

Levelling

The GF-1200 balance is level when the bubble on the left side of the interface sits in the middle of the circle (Figure 1). The bubble can be moved by turning the two (or more) levelling feet on the front of scale, to lower or raise each side.

Figure 1: Illustration of the levelling foot and bubble

Once you have it levelled, check that it is measuring accurately by weighing different calibration weights (Figure 2). Make sure to place them on the center of the metal plate. If it is measuring accurately, carry on with your weighing. If not, proceed to calibrating it.

Calibration

Figure 2. Check that the scale is weighing accurately by using standard calibration weights.

The GF-1200 scale has a plastic tab beneath it, with instructions on how to conduct a calibration (Figure 3). The instructions are simple and quick to complete.

Once you have calibrated it, check again that it is accurate by using the standard calibration weights from our lab.

Figure 3. Pull the tab under the scale to access the calibration instructions.

Operating the Scale

Ensure you are using a scale within the correct range for the amount of chemical or sample you are weighing. The range of weight and precision of measurement are typically shown on the top right of the scale display. For example, the A&D GF-1200 scale is only accurate with a minimum of 0.5g to a maximum of 1210g, has an error of 0.1g, and gives a decimal point to 0.01g.

Taring between sample measurements, closing the doors of scales if present to obtain a correct measurement.

Some balances allow you to connect to a computer which automatically records the weights of your samples. For more details, refer to your user manual.

Weighing Chemicals

Use a scoopula or spoon to weigh out your chemical.

Use a disposable weighboat or weighing paper for each different chemical.

A weighing paper can be folded prior to being tared, this can provided a useful point to grab onto the weigh paper and help funnel the chemical into your vessel.

Remember to tare the weighboat or weighing paper before placing chemical inside.

Clean your scoopula or spoon between different chemicals and putting the scoopula into the chemical jar to prevent contamination.

If possible use water or solution to remove the chemical from the weighing vessel to ensure you have used all of your sample.

Weighing Soil Samples

Do not weight a sample while it is still warm from the oven, the heat the soil sample will cause air around it to rise, leading to a lighter measurement, it can also transfer heat to the scale and platform, which can similarly affect your measurement.

If you want to avoid hygroscopic water content in the sample, place it into a desiccation chamber with a dessicant (drierite, calcium sulfate) until they reach room temperature.

Use a scoopula or spoon to weigh soil samples into your vessel (beaker, flask etc.), weigh boat, or weighing paper.

Clean the spoon or scoopula between uses with either water or ethanol depending on what your protocol requires.

When using a test tube or falcon tube, place the tube into a beaker first to tare the weight of the beaker and test tube to provide a more stable container to add soil to.

References

1. Why is Balance and Scale Calibration Important? (2020). Retrieved August 24, 2020, from https://www.chemeurope.com/en/whitepapers/1126599/why-is-balance-and-scale-calibration-important.html
2. A&D Company Limited. (2014). GF Series Precision Balance Instruction Manual. 1-84. https://www.aandd.jp/products/manual/balances/gf.pdf