# Mass Measurement Instruments

Weighing objects is common in the science lab. However, the object’s weight may not always reflect its mass.

To determine mass, scientists use volume and density. To calculate mass, you must know the object’s volume and density and multiply them together. Mass is also determined by force and acceleration, so it’s important to keep these two factors in mind when measuring mass.

## How Mass is Measured

Although people use the terms weight and mass interchangeably, they are not the same. An object’s mass represents the amount of matter in it, while an object’s weight depends on the gravitational force acting on it.

In addition, mass is a property of matter and does not depend on the location of an object, while weight does. For example, an object with the same mass at the center of the Earth will have the same weight on Jupiter as it does on the International Space Station.

The simplest way to measure an object’s mass is with a balance, which is used in a similar manner to a scale. However, a balance can only work in no-gravity, low-friction environments. For this reason, most objects weighed on balances are actually measured in kilograms. The standard kilogram is a solid prototype made of platinum-iridium alloy kept at the United States National Institute of Standards and Technology. The kilogram is the primary unit for measuring mass in the United States and is based on the SI definition of a cubic decimeter of water.

## Gravity

Gravity is the force that pulls everything on Earth toward its center. It is the weakest of all the known forces in nature, but its long reach and universal action control the trajectories of bodies in our solar system and the universe as a whole.

Weight is a measure of the gravitational force of attraction between two masses. It varies depending on where you are, because the gravitational field of the Earth is stronger near the surface than it is at higher altitudes.

Scientists have tried to measure gravity for centuries. One of the earliest was 17th-century British scientist Henry Cavendish who used a device called a torsion balance. Today, scientists use instruments like torsion pendulums with a test mass on one end of a rod and an identical sphere on the other to make an oscillating measurement that can detect the smallest changes in gravity. The best-known of these are called gravimeters. Gravimeters must be incredibly sensitive, so they must be built with great care to be accurate.

## Transducers

A transducer takes a physical quantity, such as temperature, sound, light, pressure or motion and converts it into an electrical signal which can then be used by some type of information or control system. This process is known as transduction.

Different types of transducers exist for measuring different physical quantities. The choice of a particular type of transducer for an experiment is usually dependent upon the operating principle of the transducer as well as its range of operation and sensitivity.

For example, strain gauge based ultra-high pressure transducers are generally preferred over capacitive or piezo-resistive MEMS solutions for pressure measurement due to their greater sensitivity and overload resistance. It is also important that the input-output relationship of a transducer be linear and symmetrical. This can be achieved by constructing a calibration curve which relates Pin to powers of Vout. Examples of such curves can be found in the technical literature for Coriolis mass flow instruments.

## Balances

A balance (or scale) is an instrument for measuring mass. It works by measuring the force FF exerted by an object that is resting on it. It is not the same as a weightometer, which measures the force that an object applies to the earth’s surface and then converts this to weight.

In order to get accurate and repeatable mass measurements, it is necessary to follow certain rules. For example, all chemicals used in a weighing must be added to the tared container and not directly to the balance pan or even to a piece of weighing paper placed on the pan. It is also important to close the balance doors while a measurement is being made and to not handle objects with bare hands.

Lastly, it is essential to inspect the balance every day or before use to confirm that the value displayed by the balance is within its inspection tolerances. This can be done by weighing a known mass on the balance and then recording both the zero point and loaded weight values.

# The Weighing Process

Weighing is an essential process in any laboratory workflow. Good weighing practices can reduce or eliminate errors in the measurement of mass.

A creased square of glazed paper can be used as a ‘tared’ container for measuring solids directly on analytical balance pans (the first reading is the weight of the empty container). Taring can eliminate error caused by a difference in initial readings by subtraction.

## Workspace Preparation

In this phase, the area around the weighing system is cleared and prepared for the installation of equipment. This includes ensuring the system can be properly positioned, that no metal objects or ferromagnetic items are present in the vicinity and that any electrical shielding needed to prevent electromagnetic interference (EMI) is in place.

#### Process Weighing

This is when the weight of a product is critical to the production process and must be recorded at high levels of accuracy, such as within 0.02% and 2% of full scale capacity. Typically deployed in the STOCK, MAKE, PACK and SHIP manufacturing areas in plants, process weighing can help verify that a product is complete and accurate or that ingredients are correctly mixed.

This type of weighing also provides valuable statistical information that can be fed back into the source process, helping to reduce costs and improve quality. PCE can supply, install and maintain all types of industrial process weighing systems including hazardous area (ATEX) weighing applications for the mining, chemicals, oil and gas industries.

## Equipment Preparation

In process weighing, a powder material is measured continuously during production. This technology is used in a wide range of industries: monitoring inventory levels in silos, discharging material by weight or rate, batch mixing of multiple ingredients, and so on.

The most important aspect of the equipment that will measure the mass of your product is the load cell (also called a sensor or transducer). It’s a piece of machined metal that bends with the force exerted on it, and the strain is sensed by sensors bonded at points on the cell. This measurement produces a proportional electrical signal that is recorded by the balance.

Moisture in the weighing system’s junction box can wick into cables to the individual load cells, reducing the capacitance between signal lines and causing electronic noise. This can lead to inaccurate weighing results. For this reason, it is recommended to seal the junction boxes and plug any unused holes. It is also best to test automatic bulk weighing systems to their maximum capacity when new, using a combination of test weights and bulk material.

## Weighing

Weighing is a process that can be complicated and requires careful attention to detail. Having the right weighing instruments can make all the difference in production, and ensuring that they are installed correctly and regularly calibrated is essential for quality control.

When it comes to determining the correct mass of an object, the most accurate method is to weigh the item on a high-precision analytical balance that is kept in a clean room with windows closed to prevent air drafts from influencing the reading. Additionally, the weighing pan(s) should be placed inside a clear enclosure so that dust cannot collect and cause an error.

In many manufacturing applications, it is important to connect a weighing instrument to PLCs for data transfer and automation. COOPER Instruments & Systems offers a wide range of local displays with PLC outputs as well as DIN rail mount units to make integration easy. Contact us today for help with selection, installation and calibration of your weighing equipment.

## Recording

Getting a clear, precise measurement is only one step in the process. Taking action based on the measurements is equally important.

For example, if a pharmaceutical product fails quality inspections due to inconsistent blending of ingredients or weighing errors, that wastes time, money and resources while compromising human health and safety. That’s why it’s critical to design a process that improves efficiency and accuracy.

Choosing quality components that are specially designed for your application will go a long way toward achieving the kind of system weighing accuracy you need. Look for load cells with impressive worst-case specifications and a weight controller that can ignore plant and processing mechanical noise to provide accurate, repeatable results. Moisture that enters a weighing system’s junction box can wick into cables to each cell and reduce the capacitance between signal lines, resulting in erratic readings. Avoid this by using a NEMA 4-rated junction box and plugging unused ports. Also, be sure to keep moisture away from sensitive calibration standards that may oxidize.