A kilogram is a measurement of mass. It is one of the base units in the metric system.

People weigh stuff all the time with a balance, but that only measures weight, not mass. Your weight would change if you went to the moon, but your mass wouldn’t.

You can measure mass by using a variety of methods. This article will discuss some of these methods.

Definition

One of the best ways to understand how mass works is to teach it to kids. This helps them effortlessly grasp more complicated concepts like physics later on in their lives. Kids learn better in a relaxed environment, so try to start them off with something simple like counting the number of protons and neutrons inside a piece of fruit or a paperclip.

A balance (also known as a beam balance or laboratory balance) is the first device that was used for mass measurement. It consists of a pivoted horizontal lever with equal arms, with weighing pans attached to each end. The unknown object is placed on one of the weighing pans and standard masses are added to the other pan until the balance is at equilibrium.

Unlike spring scales, the accuracy of a balance is not affected by variations in gravity. This is why it can be used in space and places with no gravity.

Units

Several SI units are used in the measurement of mass. These include kilograms (kg) and gram (g).

The kilogram is the unit of mass in the metric system, commonly known as a “kilo.” It is defined by a platinum-iridium cylinder kept at NIST in the United States. A kilogram is equal to 2.2 pounds.

All other SI base units — the meter (base unit of length), second (base unit of time), ampere (electric current), kelvin (thermodynamic temperature), and mole (amount of substance) — are derived from the kilogram. They depend on the exact definition of this standard.

The kilogram, however, is the only one of these that is based on an artifact rather than an invariant of nature. This may not be ideal, especially for chemistry, which relies on physical constants. A group of scientists and engineers has proposed a redefinition of the kilogram based on an invariant of nature, such as the Planck or Avogadro constants.

Measurement techniques

The most common method of measuring mass is to use a balance. It compares the known value of an unknown mass with a calibrated standard weight and yields the value of the unknown mass in terms of the reference value. This technique works in space and other places without gravity because changes in gravity affect both masses on the balance equally.

To improve the accuracy of this measurement, a standard calibration report typically reports the apparent mass versus brass or a stainless steel scale bar. The “apparent mass” is determined for the weighings at normal temperature and in air of a conventional density of about 8.0 g cm-3.

Very small masses and forces have traditionally been measured using sets of carefully calibrated metal weights. However, this approach presents two inherent problems: contamination and wear on the weights over time. NIST scientists have developed a new technology that eliminates both of these problems. This new balance based on Kibbles balances can measure masses and forces down to the milligram level with an uncertainty as low as a few parts per billion, and it will be used to redefine the kilogram starting in 2018. NIST researchers are also developing other technologies for extremely accurate mass measurement, including photon pressure balancing for mass, force and laser power traceability.

Applications

In science, mass measurement is used to determine the properties of matter. It is also important for calibration of other instruments, including balances and atomic force microscopes.

Mass spectrometry is the preferred technique for measuring a sample’s composition and structure. Its ability to measure the mass of an ion allows it to separate ions from each other, yielding a spectrum that displays the m/z ratios and heights of their peaks. Each peak represents a specific element or biomolecule in the sample.

A m/z measurement is directly related to the molecular mass of the molecules being measured. This makes it a powerful tool for bioanalytical applications, such as peptide screening, drug discovery and ADME analysis (absorption, distribution, metabolism and elimination). It is also used to monitor wafer manufacturing processes, such as deposition, etch and clean. The precision of these measurements allows manufacturers to control critical process steps with A-level quantification and closed loop feedback. The latest mass measurement systems are capable of monitoring a range of different process parameters with high accuracy.