Getting kids to understand the basics of mass measurement will help them effortlessly grasp the more complicated concepts in subjects like physics later on. Oftentimes people confuse mass and weight, but they are completely different measurements.

Mass deals with matter and inertia; weight is the force induced on an object by gravity.

The International Prototype Kilogram

For more than a century, scientists around the world have defined the exact weight of an object called a kilogram. It’s a small, metal cylinder made in 1889 of platinum and iridium. It’s so important that the International Bureau of Weights and Measures, or BIPM, keeps it under lock and key. It’s known as the “International Prototype Kilogram,” or IPK. And it’s so pristine that it can only be opened by three people who release their locks at the same time.

This artifact has underpinned four of the seven existing SI base units (gram, kilogram, ampere and Kelvin) and all their derived units, such as the mole, candela, volt and hertz. But, even though it serves metrologists — scientists who study measurement science — well, its dependence on physical objects limits its future usefulness.

In 2018, delegates from 57 nations meeting at the CGPM agreed to redefine the kilogram in terms of a formula that refers to Planck’s constant, a physical property tied to electrical current and voltage. The new definition will eliminate the need for the IPK, and most national “working standards” are expected to follow suit by 2024. NIST maintains two primary prototype national standards — K20 and K4 — and a number of stainless-steel working standards that are used to calibrate them.

The Metric System

The metric system is the universal decimal system of measurement that was developed during the French Revolution in the 1790s. Though the metric system has evolved over time, it is now the world’s most used system of measurement.

The base unit of the metric system is the gram, or g. It is joined by multiples and submultiples to create a set of units that are commonly used for measuring length, capacity, temperature, and force. The most familiar of these are the meter, kilogram, and kiloliter. Other useful metric units include the centimeter, milliliter and decimeter. Multiplication and division in metric units are done by using the number 10 and its powers, which makes conversions much easier.

The metric system is easy for people to understand and use. Its reliance on decimals means that simple calculations can be done in the head or with a calculator, and complex calculations can be easily written down. This ease of use and understanding makes it very popular around the world.

Measurement Errors

The difference between the true value of a physical quantity and its measured value is called a measurement error. There are two types of measurement errors: random and systematic. Random error affects the precision of your measurements, how consistently values are reproduced under equivalent circumstances; it is reduced by taking multiple measurements and averaging them. Systematic error skews your measurements away from their true value in a particular direction; this type of error can be reduced by carefully calibrating equipment, and observing the results of multiple tests to detect a trend.

Errors that are caused by the instrument or environmental conditions can usually be eliminated through a thorough investigation and appropriate countermeasures like recalibrating your scale or changing the location of your experiment. Errors caused by the operator can be more difficult to eliminate, but can be lowered through careful training and reevaluation of your measurement process. If these causes of error cannot be eliminated, then a greater margin of error must be accepted.

Measurement Accuracy

Measurement accuracy is the statistical conformity of a set of experimental data to a given normal distribution. It can be tested with a normal probability test such as the Kolmogorov-Smirnov test.

It requires that data is collected under the same conditions and over a short period of time. This includes the same instrument, same operator and, in some cases, the same day.

Achieving accurate mass measurement on a quadrupole orthogonal acceleration time-of-flight (q-oaToF) instrument is essential to a wide variety of scientific applications. The ability to measure ion masses and their distributions accurately helps scientists characterize structural characterization, early drug discovery and a wide range of other applications.

Modern mass spectrometers report accurate masses to a high level of precision. These values are reported to a few decimal places, typically up to four significant figures for masses between 100 and 999 Da. This level of precision is important to help ensure that the resulting data is free from rounding errors that may result in poor quality data.

Weighing processes are a critical part of any industrial application. The correct technique is essential to avoid errors and maximize accuracy.

Chemical and pharmaceutical industries often require measurement precision down to the microgram. Weighing by difference is the preferred method in these scenarios to ensure minimal errors.

Whenever possible, a weighing system should be installed at a level and within a structure that can resist flexing. This prevents unwanted horizontal forces on load cells that can impact weighing accuracy.

Workspace Preparation

Weighing workspaces must be sterile to prevent contamination of the sample or the balance. If the weighing process involves volatile chemicals, a fume hood or specialised isolator should be used. Otherwise, a sanitary weighing room should be maintained with an ISO 7 LAF (Large Area Filtration).

The work surface must be cleaned to remove debris and residue from previous weighing operations. Static charge can also build up on surfaces, especially with fine powders, and must be eliminated before a suitable weighing can be made. Using an antistatic device may help to minimize static charge, depending upon the sensitivity of the material.

If precision is the top priority, Weighing by Difference is preferred, but direct weighing can offer convenience and speed for recipes that don’t demand pinpoint accuracy. Whichever method is chosen, the resulting measurement must be equivalent to the original material.

Equipment Calibration

As time passes, equipment calibration can begin to drift. This can cause inaccurate test results that may impact important processes.

It’s especially important to keep up with calibrations when working with potentially dangerous materials or creating solutions for medical purposes. In these situations, small inaccuracies could lead to safety issues and other costly problems.

With advanced software solutions, managing the calibration process becomes a breeze. It helps reduce production downtime and facilitates seamless communication and collaboration across multiple company locations.

For mass calibrations, it’s vital to ensure that the instruments are in thermal and environmental equilibrium prior to weighing. Generally, the objects to be weighed and the reference standards must be placed in or near the balance for 24 hours in order to achieve this state. This will help minimize temperature fluctuations that could affect the calibration. The calibration process will generally require comparison weighing, which involves substituting the unknown instrument with an identically sized mass standard.

Sample Placement

Process weighing requires a combination of methods and careful attention to detail. Cutting corners with less quality weighing equipment can result in poor performance and inaccurate results.

It is important to always use the correct capacity load cell for a given application. COOPER Instruments & Systems can help with proper load cell selection and installation for process weighing applications.

When a sample needs to be transferred, the tried-and-true method is called “weighing by difference.” The empty balance is tared and then the solid is added to the weighing bottle with its cap off. The weighing bottle is then re-tared, subtracting the original mass to get the new weighed value.

After the weighing is complete, the final tare weight is recorded and the balance door closed. It is important to not touch or breathe on the weighing platform, since even slight air pressure changes can affect the measurement. The weighing results should be recorded directly into the laboratory notebook.

Data Recording

In this phase, the data is recorded in either a hard or electronic format. Whether it is in the form of notes, spreadsheets, or photos, this record serves as documentation of the work that has been performed.

It’s important to understand that even when the weighing process is done correctly, errors may still occur. These errors could be caused by improper balance operation, air currents, temperature changes, lack of thermal equilibrium, and magnetic or electrostatic fields.

Weighing methods are designed to eliminate these types of errors, ensuring that the results you receive are accurate and precise. When working with sensitive substances, such as pharmaceuticals and chemicals, precision is key. Weighing by Difference is the best method for these scenarios, providing a high level of accuracy while reducing contamination concerns. For more routine applications, Direct Weighing offers simplicity and speed for situations where precision is not a priority. Both methods can be optimized for the unique characteristics of your samples, allowing you to get the most out of your weighing process.