What Instrument Is Used to Measure Mass?
Mass measurements in chemistry, biology, and other sciences are obtained with a balance. All scales make use of the acceleration of Earth’s gravity to measure mass. The acceleration produces a downward force that has to be counteracted in some way. The way the downward force is counteracted depends on the type of instrument used.
Laboratory balances, considered general lab equipment, are essential measuring instruments because they can deal with such small measurement units.
Instruments for Measuring Mass
Balances and Scales
For the majority of everyday objects, a balance is used to determine an object’s mass. The balance compares an object with a known mass to the object in question. Different types of balances include digital scientific balances and beam balances, such as a triple beam balance. The standard unit of measure for mass comes from the metric system and is either grams or kilograms.
At home, you typically use a modern digital or spring scale to determine mass. This is how today’s bathroom scales work. A person can stand on the common bathroom scale to obtain their body weight. The digital scale calculates the person’s mass by taking the body weight and dividing it by gravity.
A lab balance, also known as an analytical balance, is designed to measure small mass in the sub milligram range.
Scales measure mass directly by counteracting the downward force by another equal downward force on the opposite side. On a beam balance with two pans, you have enough mass on the opposite pan to counterbalance the unknown mass with no force measured. This instrument compensates for differences in the acceleration of gravity at different locations on Earth.
While the most common type of instruments used to measure mass is the beam, there are also load cell instruments to measure mass. These are more sophisticated weighing machines that are highly accurate. The strain gauge load cells have a typical accuracy of one part in 2,000. Frequency shift technology instruments have even better accuracy using ceramic capacitive strain gauges, which can be made with accuracy up to one part in 30,000. One drawback to this type of measuring is the scale can be broken. Newer scales use technology that incorporates spring shock dampers to minimize this issue.
Force restoration instruments apply force restoration technology to weigh measurement. The latest electronic force rebalance transducers in today’s modern instruments can achieve accuracies in the range of one part in 10 million. More common bench scales range up to 25 lbs, with the accuracy approaching one part in 1 million. The maximum load rating of commercial instruments reaches up to 13,000 lbs. There are, however, special devices that have much larger capacities.
In situations where mass cannot be determined by using a balance, such as measuring the mass of a liquid in a tank, transducers are used. Transducers measure the mass properties of the liquid in a static state. The transducer works by sending a signal to a processor, which then makes the mass calculations. An indicator then displays the mass. Taking the measured mass of the liquid below the transducer and subtracting the mass of the vapor, the mass of a floating roof, and the mass of the bottom sediment and water yields the gross mass.
Vibrating Tube Mass Sensor
Dealing with measurements of physical properties on the microscopic level presents several challenges. One of the most effective methods for measuring fluid is to use the vibrating tube mass sensor. The censor first determines the buoyant mass of an object by using the fluid’s density. After this has been found, absolute mass can be determined by measuring the buoyant mass of the object in fluids of different densities. This is an ideal, affordable, and portable solution to measure biomaterials such as seeds, cells, and embryos.
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Mass or Weight?
There is a common misconception that mass and weight are the same things, which can lead to an incorrect choice of which instruments should be used to measure mass. The reason why weight and mass are used interchangeably is because they are proportional to one another. However, the mass of an object represents the amount of matter within it, while weight depends on the force acting on the object due to gravity. Measuring mass does not always measure weight. Mass will not change regardless of where an object is located because the amount of matter remains the same. In short, if you could count the number of protons, neutrons, and electrons within an object, you’d have a measurement of mass. It’s basically how much stuff is in an object.
Weight, on the other hand, refers to the interaction between objects that have a mass and Earth’s gravity. If the object is interacting with Earth, the force is known as the weight. The unit for weight is the Newton but it can also be expressed in pounds or kilograms.
Steps to Measuring Mass
Step 1: Prepare the Balance for Use
Once you know how to use the balance, make sure it doesn’t have any debris and is clean. Ensure the balance is on a level surface, and do not place the sample directly on the balance. Because some chemicals in a lab could damage the surface of the weighing pan, you should use a weighted boat, weighing sheet, or some other container such as a graduated cylinder, to hold your sample.
Before choosing the container for your sample, it’s crucial to make sure your sample will not chemically react with the container. If your balance has doors, close them before taking measurements because the air movement will affect the accuracy of the mass measurement. If there are no doors, make sure the area is free of vibrations and drafts.
Step 2: Tare the Scale
When you place your sample in a container, you end up with the combined mass of the container and your sample. To get the most accurate measurement of just your sample, you want to remove the mass of the container. The mass of the container can add extra metric units you weren’t originally wanting to measure. Using the tare function on your scale eliminates the mass of your container from your final measurement.
Some instruments may require you to manually adjust the reading to zero. Electronic devices will often tare automatically but require periodic calibration to remain accurate.
Step 3: Place Container on the Scale
Press the tare button again so the balance reading is 0 again. This removes the weight of your container.
Step 4: Add the Sample to the Container
Add the sample to the container. The value given is the mass of your sample.
Alternatively, you can measure the mass of your container, then measure the mass of the container with the sample, and subtract the container’s mass to arrive at the sample’s mass. For instance, if you find that your container weighs 5 grams, and together with the sample, it weighs 15 grams, you can safely determine the sample weighs 10 grams. Fortunately, most scales will come with a “tare” function and will do this calculation for you.
Sources of Error When Measuring Mass
Even after accounting for the mass of the container, you can still end up with an inaccurate measurement. Many instruments used to measure mass are susceptible to error. Taking mass measurements comes with the potential for error due to:
- Air gusts that push the mass up or down
- Dust accumulation on the measuring device
- Condensation of water on cold measuring instruments
- Magnetic fields affecting the components of the scale
- Evaporation of water from damp items
- Temperature changes that cause components to expand or contract (measurements taken on hot days may differ from measurements taken on cold days)
- Vibration (fluctuations will make it difficult to obtain a value)
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