Scales and balances may be used for similar things, but understanding the differences in how they produce their weights tells you about their different uses. A lot of people use the words "scale" and "balance" to mean the same or similar things. This can cause confusion in determining what is precisely being measured through laboratory techniques that use scales and balances.
What Scales Do
Scales are generally used when measuring weight. They measure the force acting on a mass and use the formula for weight of an object on Earth to determine its weight. The types of a weighing scale can vary in how they work. Modern weighing scales sometimes use sets of springs arranged together so that the scale measures how much the spring compresses to determine weight.
Other weighing scales make use of strain gauge load cells. These are devices that, when a force is exerted upon them, slightly compress such that an electrical resistance in the strain gauge, devices that measure the electrical current through the load cell, can be measured. The resistance in this electrical circuit correlates with the weight placed on the scale so the change in this resistance can be measured and converted to weight.
Scales are generally used in applications where you don't need as much precision and complexity of a balance. This means you'll see use when stepping on a weighing scale at the gym or in your own home as well as areas of weighing food ingredients. Other types of a weighing scale include mechanical scales that measure the mass straightforward by how much a needle turns due to weight or digital scales that use a strain load gauge as described.
What Balances Do
Balances, on the other hand, tell you the mass of whatever you place on the platform of the balance. They calculate this based off the weight placed on the platform of the balance using the same principles that scales use. But balances in particular are generally built using a force restoration mechanism that opposes the force of weight of the material on the balance. This restoration force is what causes the object to return to equilibrium with a net force of zero.
In contrast to scales, balances are more complicated and are typically seen more frequently in laboratories, universities research centers, medical facilities and similar research environments. They can generally be more precise than scales as well.
Different types of a weighing balance can include microbalances that weigh mass samples to fractions of a gram, analytical balances which also measures minute changes in weight and precision balances, which have a larger range of weights than analytical balances but less precision. Precision balances can measure mass in grams with precision of up to two or three decimal places. Analytical balances can achieve greater precision, up to four decimal places, and microbalances can tell you mass in grams of up to six decimal places.
Despite these differences between scales and balances, the terms "scales" and "balances" are still used relatively interchangeably (as given by the term "scale balance"), even among scientists, especially given the mechanisms scales use may also measure mass and the ones balances use can also measure weight. Understanding these mechanisms in greater detail can help you discern the difference when necessary.
Weight on Scales and Balances
When people think of scales or balances, it's common they visualize two masses connected to one another on a pivot that weighs one against the other. This primitive form of determining mass or weight that has been with humans for centuries shows the physics of the gravitational force that many scales and balances use in determining weight or mass, respectively.
Scales and balances may measure weight and mass, respectively, but they rely on the same physical principles governing the gravitational forces on objects. Using Newton's second law, you can measure the force of an object F as a product of its mass m times its acceleration a using F = ma. Because the force of an object's weight W pulling towards Earth is this force that uses an acceleration of g, gravitational acceleration, you can rewrite the equation as W = mg for the mass m of the object.
In real-world applications, scales and balances should be calibrated based on the location at which they're being used because the gravitational acceleration can vary by as much as 0.5% across different parts of the Earth. After calibrating the scale or balance, the conversion between weight and mass is straightforward for the scientific instrument.
Scales and balances may sum this force alongside other forces such as the change in length of a spring in response to a weight placed on the instrument's surface. These springs expand and compress according to Hooke's Law, which tells you that the force acting upon a spring such as the weight of an object is directly correlated with the distance the spring moves as a result of it.
In a similar form to Newton's second law, this law is F = kx for an applied force F, the stiffness of the spring k and the distance the spring moves as a result x.
The spring scale can be as sensitive and precise to measure masses to fractions of pounds. When you step onto a bathroom scale, the springs inside of it compress such that the needle or dial rotates until your weight is shown. Spring scales can, unfortunately be subject to slackening as the spring is used routinely over long period of time. This causes the spring to lose its ability and expand and contract naturally. For this reason, they need to be calibrated appropriately and constantly to prevent this from happening.
In addition to Hooke's Law, you can use the Young's modulus (or elastic modulus) in determining how much a string will compress when you exert weight upon it. It's defined as the ratio of the stress to the strain, given by E = ϵ / σ for Young's modulus E, stress ϵ ("epsilon") and strain σ ("sigma").
For this equation, stress is given as force per unit area, and strain is the change in length divided by the original length. The Young's modulus measures the resistance of a material to being deformed, and more stiff materials have greater Young's moduli.
Young's modulus then has units of force per area, as does pressure. You can use this to multiply the Young's modulus by the surface area of the spring that receives the weight of the object to obtain the force exerted on the spring. This is the same force F in Hooke's Law.
Strain gauges that are used in weighing scales measure change in electrical resistance in the presence of the weight on the scale. The strain gauge itself is a piece of metal that surrounds a thin wire or foil arranged in a grid-like pattern of an electrical circuit such that, when it experiences a force in one direction, its resistance changes by even a precise, small amount in proportion to the weight.
When the weight makes parts of the wire or foil more tense and compressed, the resistance of the electrical circuit increases, and the strain gauge becomes thicker and shorter in response to this. Sending a current through the circuit, the scales calculate how this resistance changes due to the weight to determine the weight exerted upon them. The change in resistance is usually very minute and around 0.12 Ω, but this gives strain gauges all the more precision in determining weight.
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- LoadCellCentral: Load Cell and Strain Gauge Basics
- AdamEquipment: What are Analytical Balances?
- Khan Academy: What is Hooke's Law?
- Hunker: How Does a Digital Scale Work?
- ScienceStruck: How Does a Spring Scale Work? It's Comprehended Well Here