Describe how to determine mass using an electronic balance

By understanding how error occurs and the factors that cause it, methods of minimizing error become apparent. The following describes some of the main factors that cause error, along with measures to minimize them.

1. Gravitational Acceleration

The important point here is not about how much of a sensitivity error occurs for a given movement of the balance, or by a given change in floors of a building, rather the point is that sensitivity should be readjusted whenever the balance is moved, even for short distances.

2. Temperature

In some cases, if the balance sensitivity is adjusted after the air conditioner is switched on and the room temperature has stabilized in the morning, it may be possible to adjust the sensitivity at that time, but it will immediately start changing again. Eliminating room temperature variations is best, but as a practical matter we recommend performing the most important measurements (those that require the most precision) in the afternoon (after the balance has thoroughly adjusted to the room temperature). Also, always remember to adjust sensitivity immediately before measurements.

3. Containers

Given a container volume of 100 cm3, a change in container temperature of 2 °C is equivalent to 0.82 mg. These conditions will cause measurement repeatability to increase (become worse). To avoid this situation, have the container adjust to the balance temperature as much as possible, such as by leaving the container next to the balance and not touching the container with bare hands.

4. Air Flow

There are many factors in our immediate surroundings that can cause air flow, such as air conditioners and the movement of people, but one factor that is often overlooked is the door to the room. If the door is a swinging type, think of it as fan that not only generates it own wind, but also changes the room air pressure, which disturbs the stability of the air inside the balance as well. These effects can be significantly prevented by taking measures with respect to the facility to prevent exposing the balance to air flow and having all personnel, including those not involved in using the balance, to cooperate together in being careful. Also, if possible, use a sliding type door.

Convection can be caused by factors such as the rise or fall of air due to temperature differences between items being measured and the weighing chamber or the disturbance of air from moving items being measured in and out of the weighing chamber. To minimize convection, it is necessary to keep in mind to let items being measured thoroughly adjust to the balance temperature, to avoid inserting hands into the weighing chamber, to place or remove items being measured in as short a time as possible, and to avoid opening the weighing chamber door more than necessary. In addition, to minimize temperature differences between inside and outside the weighing chamber, we recommend leaving the weighing chamber door open a few millimeters when the balance is not in use. However, no matter how careful you are, it is difficult to completely suppress convection. Therefore, to minimize its effect, do not let items being measured protrude from the pan. Success comes from paying attention to details, such as folding the edges of weighing paper.

5. Static Electricity

Static electricity can be discharged to the air and eliminated by humidifying the room, but using an ionizer is also effective. Shimadzu offers STABLO-AP ionizers.

Electronic balance error was discussed above. Hopefully, this will be helpful to those that read it. In addition to the causes mentioned above, there are a variety of factors that can cause errors, such as vibration and the density of the items being measured. Those will be discussed in a future article, as opportunity allows. Next, we will give a brief introduction to daily inspection of balances.

Updated April 24, 2017

By Kenrick Callwood

Electronic balances have become standard equipment for many high school and college chemistry departments. They allow the user to quickly and accurately measure the mass of a substance to a level of accuracy impossible for traditional balances to achieve. This is especially important in experiments that require precise amounts of each substance to achieve the desired results. The popularity of the electronic balance is also due to its extreme ease of use for any skill level.

Place the electronic balance on a flat, stable surface indoors. The precision of the balance relies on minute factors and wind, shaky surfaces, or similar forces will cause the readings to be inaccurate.

Press the "ON" button and wait for the balance to show zeroes on the digital screen.

Use tongs or gloves to place the empty container you will use for the substance to be measured on the balance platform. Fingerprints and other greases from your hands add mass and must be avoided for accurate measurements.

Press the "Tare" or "Zero" button to automatically deduct the weight of the container from future calculations. The digital display will show zero again, indicating that the container's mass is stored in the balance's memory.

Carefully add the substance to the container. Ideally this is done with the container still on the platform, but it may be removed if necessary. Avoid placing the container on surfaces that may have substances which will add mass to the container such as powders or grease.

Place the container with the substance back on the balance platform if necessary and record the mass as indicated by the digital display.

Mass measurements in chemistry and other sciences are performed using a balance. There are different types of scales and balances, but two methods can be used on most instruments to measure mass: subtraction and taring.

• A balance or scale is an instrument used to measure mass in the science laboratory.
• One common method of measuring mass is to tare the scale and measure the mass directly. For example, this is how people weigh themselves.
• The other common method is to place a sample into a container and measure the mass of the container plus sample. The mass of the sample is obtained by subtracting the mass of the container.

Before using a balance, it's important to take some preliminary steps. This will help ensure you get the most accurate and precise measurement.

• Be sure you understand how to use a balance before taking mass measurements.
• The balance should be clean and free of debris.
• The balance should be on a level surface.
• Never place a sample directly on the balance. You should use a weighted boat, weighing sheet, or another container to hold the sample. Some chemicals you may use in a lab could corrode or otherwise damage the surface of the weighing pan. Also, make certain your container will not chemically react with your sample.
• If the balance has doors, be certain to close them before taking a measurement. Air movement impacts the accuracy of mass measurements. If the balance does not have doors, make certain the area if free of drafts and vibrations before measuring mass.

If you place a container full of sample and weigh it, you're getting the mass of both the sample and the container, not just the sample. To find the mass:

mass of sample = mass of sample/container - mass of container

1. Zero the scale or press the tare button. The balance should read "0".
2. Measure the mass of the sample and container.
3. Dispense the sample into your solution.
4. Measure the mass of the container. Record the measurement using the correct number of significant figures. How many this is will depend on the particular instrument.
5. If you repeat the process and use the same container, do not assume its mass is the same! This is particularly important when you are measuring small masses or are working in a humid environment or with a hygroscopic sample.

When you use the "tare" function on a scale, you are making certain the reading starts from zero. Usually, there is a labeled button or knob to tare the balance. With some instruments, you need to manually adjust the reading to zero. Electronic devices do this automatically, but require periodic calibration.

1. Zero the scale or press the tare button. The scale reading should be "0".
2. Place the weighing boat or dish on the scale. There is no need to record this value.
3. Press the "tare" button on the scale. The balance reading should be "0".
4. Add the sample to the container. The value given is the mass of your sample. Record it using the proper number of significant figures.

Whenever you take a mass measurement, there are several potential sources of error:

• Air gusts may push the mass up or down.
• Buoyancy can affect measurements. Buoyancy is directly proportional to the air volume that is displaced and is affected by air density changes due to temperature and pressure fluctuations.
• Condensation of water on cold items may increase apparent mass.
• Dust accumulation can add to mass.
• Evaporation of water from damp items may change mass measurements over time.
• Magnetic fields may affect components of the scale.
• Temperature changes can cause components of the balance to expand or contract, so a measurement taken on a hot day may differ from one taken on a cold day.
• Vibration may make it difficult to obtain a value, as it will fluctuate.

Remember, a balance gives you a mass value. The mass would be the same whether you measured it on the Earth or on the Moon. On the other hand, the weight would be different on the Moon. While it's common to use the terms mass and weight interchangeably, they are only the same values on Earth!

• Hodgeman, Charles, Ed. (1961). Handbook of Chemistry and Physics, 44th Ed. Cleveland, USA: Chemical Rubber Publishing Co. pp. 3480–3485.
• Rossi, Cesare; Russo, Flavio; Russo, Ferruccio (2009). Ancient Engineers' Inventions: Precursors of the Present. History of Mechanism and Machine Science. ISBN 978-9048122523.