The terms “balance” and “scale” are often used interchangeably, and most of us would have a bit of trouble nailing down the characteristics of each. Indeed, various manufacturers even refer to some of their own single products as both balances and scales.
There are technical and practical differences between the two, including exactly what is being measured and the types of applications they are used for. However, the waters are still murky when it comes to nomenclature.
That being said, in this post, we’ll explain the major differences, the types of balances and scales out there, and some of the terminology that is used. This way, you can focus on finding a weighing solution that meets your needs, no matter what you call it.
What Balances and Scales Measure
Although the terms “balance” and “scale” are used interchangeably, technically they measure different things.
Balances Measure Mass
Balances are instruments that measure mass (basically the amount of matter in something). A true balance measures mass directly by comparing the unknown mass to a known mass, a process that is not affected by changes in gravity. A balance of this sort will give the same reading irrespective of location because gravity will act on both sides of the balance equally.
Scales Measure Weight
Scales measure weight, which is the force acting on a mass that is equal to the object’s mass times its acceleration due to gravity. A scale can’t measure mass directly, because the weighing mechanism and the weight of any given object are dependent on local gravity. On Earth, gravitational acceleration can vary by as much as 0.5%, changing with distance from the Earth’s core, as well as latitudinally.
From a practical viewpoint, once a scale has been calibrated at its location, gravity is assumed or ignored. Therefore, weights are reported in mass units like kilograms, even though weight is a measurement of force using the newton. This also allows for the use of the terms “weight” to refer to both weight and mass, and “weigh” can be the process of determining either.
Types of Scales and Balances and Their Nomenclature
While researching the various types of scale and balances available, you’re bound to get confused. As mentioned earlier, some manufacturers even refer to the same product as both a scale and a balance. Why this happens is unclear, but it certainly makes for some confusion.
What's more, there are many subcategories for both instruments, and the nomenclature for these is not always consistent. Generally, we hear about scales being used for larger masses and balances for smaller masses, although there can be some crossover. The below can serve as a general guide as to what to expect from each type.
Types of Balances
What springs to mind when many people think of a balance is a traditional model, such as a two-pan balance.
Here, two connected masses pivot on a “frictionless” fulcrum (knife-edge). The weighable material is placed in one pan, and calibrated metal weights in the other. The order in which they’re placed depends on whether you need weigh something out or determine the mass of something you already have. Other types of traditional balances include:
- One-pan balances: These types of true balances use only one pan and adjustable weights that balance it.
- Torbal Torsion balance: This uses steel tension bands as the fulcrum, and is the bane of pharmacy school students across the world.
However, in a modern laboratory setting, you are likely more used to using precision and analytical balances like the ones in the image below.
Modern lab balances work by Electromagnetic Force Restoration (EMFR) to determine the mass of the sample. The force of the sample is lifted by an electromagnet and the current required to oppose the weight is measured. Definitions for the different types of lab balances may vary but here are general guidelines for what constitutes a precision balance and an analytical balance:
- Precision balance: These can bear more weight but are less precise than analytical balances. They are usually open but some have closures to prevent ambient air movement affecting the reading.
- Analytical balance: These are typically suitable for masses of 0.1mg up to 200g. They are more precise than precision balances, but can’t bear as much load. These are usually closed models and applications include ensuring a safe product or reducing byproducts.
Other types available include semi-micro, micro, and ultra-micro balances. According to Sartorius, these are capable of reading the following units:
- Semi-micro balances: 1 µg
- Micro balances 0.1 µg
- Ultra-micro balances 0.01 µg
However there may be some discrepancy between manufacturers and models.
Bear in mind that all of these types of balances may be referred to as scales in some literature, even that coming from manufacturers. For example, Torbal has a Laboratory Balances category, but refers to all of its balances as scales. In addition, Mettler-Toledo, a worldwide industry leader in weighing solutions, has product literature for one of its analytical balances that refers to the product as a balance, a scale, and a balance scale.
Types of Scales
Scales determine weight by measuring the amount of force required to oppose an object’s acceleration due to gravity. As mentioned, scales are often used to weigh larger masses, such as raw materials, reagents, and products in a production setting. However, they may be used in commercial or household settings to weigh smaller masses, such as produce in a grocery store, or ingredients for baking. There are two main categories of scales: mechanical and digital.
- Mechanical scales: The mechanism of mechanical scales vary, but most commonly use a spring. The weight is applied and the measurement is shown by a moving dial. These scales allow for quick and easy-to-read measurements but are not considered very accurate.
- Digital scales: This type of scale employs the use of a special electrical circuit. As a weight is applied, the voltage within the circuit changes and a processor calculates the weight. Digital scales are more accurate than mechanical scales, can provide multiple units of measure, and can often be connected to a computer for easy processing of data.
Within these categories, there are a plethora of different types of scales, including bench scales, home weighing scales, kitchen scales, and so on. Below are just a few of the scales you might see in use:
A home weighing scale, an Ohaus Checkweigher, and a kitchen scale.
You might also hear the terms “compact scales” or “portable scales.” These just refer to models that can be moved around easily, and may be for household, commercial, or laboratory use. To add to the confusion, compact or portable balances are very often referred to as scales.
Technically, the main difference between a balance and a scale is what we’re actually measuring: mass versus weight. However, this also leads to practical differences such as the mechanisms via which the different models work, the accuracy of their measurements, and the types of applications they can be used for.
What’s more, because their uses are similar and there is often crossover in terms of application, there will likely always be cases where the two terms are used interchangeably.