How Digital Scales Function

We often overlook the significance of weighing devices due to their commonality. As illustrated by the slightly inaccurate spring scale found in produce sections, scales have historically been essential for commerce, shipping, and even in pharmaceutical applications. From the tiniest, precise laboratory balance to massive pit-and-girder scales used for weighing train cars and trucks, scales are integral to modern life.

Scales, particularly balances, have been crucial to ancient builders, inventors, and economic advisors. Small balance weights from the early fourth millennium B.C. offer some of the earliest evidence of mankind's developing understanding of numbers. The oldest known balance scale in Egypt, which predates the dynastic period, dates back to before 3000 B.C. Like us, the ancient Egyptians used scales for trade and testing ores and alloys [sources: Encyclopaedia Britannica; Petruso].

Balances, which weigh objects by comparing them with reference weights, are finely tuned instruments still used in laboratories. Scales, on the other hand, operate on different physical principles and mechanisms to measure weight and other forces (which is simply the force exerted on an object by gravity). For example, spring scales measure weight through Hooke's law, which connects force (weight) to the stretching or compression of a spring made of a specific material.

While not all scales rely on springs, all scales measure weight using mechanical components. The key difference between mechanical and digital scales lies in how the weight is displayed—mechanically or electronically. In digital scales, an analog-to-digital converter is used to turn the continuous data into discrete digital information, similar to how a CD or MP3 encoding scheme converts music waveforms into digital format.

In general, digital scales are easier to use than their mechanical counterparts and offer greater precision and quicker processing. However, the specific capabilities of each scale can differ, especially when measuring extremely heavy weights, such as those in tons [sources: Camarda; Cox; Crowley].

"It depends," says Derrick Mashaney, director of product development at Fairbanks Scales Inc., known for specializing in large industrial scales. "Some master scales are mechanical. They’re adjusted to such high accuracy that there’s nothing else that can match them."

Just as a carjack lifts a vehicle by using mechanical advantage—like a lever or an inclined screw—while a mechanic's hoist utilizes hydraulic pressure, various scales operate based on distinct principles, including hydraulics, pneumatics, or bending beams.

Despite the different methods that make them stretch, compress, or sway, nearly all modern scales share one fundamental component: the load cell.

Turn and Face the Strain (Ga-ga-gauges)

Scales are available in a wide range of shapes, sizes, and designs, but the core component responsible for measuring is almost always a load cell.

A load cell is a type of transducer, a device that transforms one form of energy into another. In digital scales, load cells convert mechanical energy—the pressure or elongation caused by a weight—into an electrical signal. A common example is the strain gauge (also called strain gage), which detects changes in compression or tension as minute variations in electrical resistance within a Wheatstone bridge [sources: Mashaney; Omega].

Let’s break it down using the example of a compression strain gauge.

Compression occurs when a force is applied to reduce an object's volume, but it can also refer to a more general decrease in size in one or more dimensions. When an electrically conductive material is squished, its electrical resistance changes because longer, narrower wires have more resistance than shorter, wider ones [source: Craig]. Think of it like forcing water through a pipe: the longer and narrower the pipe, the harder it is to push the water through.

Different materials experience varying resistance changes when deformed, a property known as gauge factor. The gauge factor can also change with temperature. As a result, constantan alloy (55% copper and 45% nickel), which performs well at room temperature, has become the preferred material for strain measurements [sources: Encyclopaedia Britannica; Craig; National Instruments; Pratt].

To detect the resistance change caused by weight compression, one or more strain gauges are placed within a Wheatstone bridge. A Wheatstone bridge is an electrical circuit that can detect an unknown electrical resistance by balancing it against known resistances elsewhere in the circuit. Essentially, it functions like a balance scale for electrical resistance: the "weight" (resistance) on one side tells you the unknown "weight" (resistance) on the other [sources: Craig; National Instruments; Pratt].

A Wheatstone bridge can contain between 1 and 4 strain gauges. When multiple gauges are used, they are arranged in opposite directions to increase sensitivity and reduce temperature effects. Since the resistance change in a strain circuit can be incredibly small, the signal often needs to be amplified [sources: Craig; National Instruments; Pratt].

As the load cell measures the change in compressive resistance, it sends a signal to the CPU, which converts it into input for a display board that then shows the result on a digital screen. This principle holds true whether a strain gauge or another type of measuring device is used.

You're Gonna Carry That Weight

The ability to measure weight accurately is crucial in research, industry, and commerce, where diverse environmental factors and spatial constraints must be considered while minimizing errors. While strain gauges are the most common type of load cell, they are not the only design currently in use [sources: Eilersen; Omega].

For industries that require enhanced safety and sterility, pneumatic load cells are a popular choice. These devices determine an object's weight by measuring the air pressure needed to balance it. Pneumatic load cells are ideal for environments like food processing or hazardous sites, as they do not contain fluids that could leak, spill, or contaminate the surroundings. They can handle a wide range of weights with high precision, although they require a clean and dry environment and are slower to respond [source: Omega].

Hydraulic load cells, which gauge weight based on changes in fluid pressure, are typically used for weighing tanks, bins, and hoppers. Since they operate without electricity, hydraulic load cells are well-suited for remote areas where power is unreliable. Though costly and complex, these robust fluid-filled devices can handle extreme loads—up to millions of pounds—without failing or degrading [sources: Eilersen; Omega; Takhirov et al.].

The terms "strain gauge," "pneumatic," and "hydraulic" refer to the operating principles, which are essentially the physical laws that govern each weighing method. However, a potential scale buyer may also need to take into account the size, shape, configuration, material, and other physical characteristics of the load cell, depending on the specific requirements of the task. For instance, someone who needs to measure in a wet environment might prefer a hermetically sealed canister cell, while a customer concerned with space constraints may select a more compact bending beam cell [sources: Mashaney; Omega].

Form factors are often chosen to minimize the impact of external forces. Scales generally measure force along a single direction known as the principal axis, which, in the case of weight, means downward (aligned with gravity). Errors in weighing typically arise due to off-axis forces, which act parallel to the load, and sideloads, which act at a perpendicular angle. S-beam load cells, with their zigzag design, are particularly effective at eliminating sideload forces [source: Novatech; Omega].

Within each category and subcategory, there are numerous variations in size, shape, and price to suit virtually every need. However, scales are not just about weighing; measuring force is riddled with technical challenges that must be addressed. This is particularly crucial because even slight differences, often just a few grams, can have significant consequences—affecting both human lives and substantial financial assets.

Economies of Scale(s)

There's an old saying that a man with a single watch knows the time, but a man with two watches can never be sure. Scales face a similar dilemma: If you weigh the same object twice, do you get the same result? What about on different days or in varying weather conditions? And if it does, can the scale continue to deliver consistent results despite wear and tear?

These issues -- known as repeatability, reproducibility, and adjustment -- are all crucial, but potential buyers must also keep in mind more fundamental questions, such as the scale's weight range, which includes its minimum and maximum capacities [source: AWS].

However, these statistics are meaningless if the device lacks the necessary accuracy and precision for the task at hand. Although the terms are often confused, they are not synonymous: Accuracy refers to how closely an instrument's measurement aligns with the true or expected value, while precision refers to how finely or specifically that measurement is made. A speed gun that clocks a 90.4213 mph fastball at 90 mph is accurate but not precise, while one that clocks the same fastball at 88.3246 mph is precise but not accurate.

Scales generally express accuracy as a percentage of full scale (FS) -- the total calibrated range the device can measure. For example, a 1 percent FS accuracy means +/- 5 pounds for a 500-pound scale but +/- 1 pound for a 100-pound scale. Alternatively, the error can be represented as a percentage of actual reading (AR): For an object weighing 100 pounds on such a scale, a 2 percent AR means a 2-pound error, regardless of whether the scale's range is 10 pounds or 10,000 [sources: AWS; Busch; Plint].

The precision of a scale varies as well. According to industry standards, scales, regardless of their total capacity, are divided into a maximum of 10,000 divisions: So, a 10,000-pound scale will break down into one-pound increments, while a 500,000-pound scale will divide into 50-pound increments [sources: AWS; Crowley; Mashaney]. These devices are categorized based on these divisions (see sidebar).

With this in mind, always opt for a scale that has a sufficient capacity to handle your measurements, but not excessively high. This ensures that you achieve the greatest precision, pound for pound.