Introduction

Growth and body composition are two different things. A weighing scale only shows total body weight, but it cannot tell whether that weight comes from fat, muscle, or bone. Air displacement plethysmography solves this problem. It is a whole body test that uses air pressure changes inside a sealed chamber to calculate body volume very accurately. Once volume and weight are known, body density can be calculated, and from density, the percentage of fat and fat free mass can be estimated.

This technology is not new. It has been studied and used in research and clinical settings for several decades. Two well known versions of this device exist today. One is designed for infants and small babies, and the other is designed for older children, teenagers, and adults. Both work on the same basic scientific principle but differ in size and design.

Unlike some other body composition tools, air displacement plethysmography does not use needles, radiation, or water immersion. It is considered one of the safest and most comfortable methods available for measuring body composition in children of all ages, including premature infants.

History of the Device

The idea of using displaced fluid or gas to measure body volume dates back to the ancient Greek scientist Archimedes, who discovered that a submerged object displaces a volume of water equal to its own volume. For many decades, underwater weighing, also known as hydrostatic weighing, was considered the gold standard for measuring body composition. However, this method required a person to be fully submerged in a water tank, which was impractical for infants, young children, elderly individuals, and those with physical limitations.

In the early 1990s, researchers began developing a system that would use air instead of water. The idea was simple. If a person sits inside a sealed chamber, the air they displace can be measured by detecting small changes in air pressure. This approach was far more comfortable and practical than water immersion. The first commercially available adult sized air displacement plethysmography system was introduced in 1994 by Life Measurement Instruments in the United States. It quickly gained acceptance in research universities and sports science laboratories.

By the early 2000s, the same underlying technology was adapted into a much smaller device specifically designed for infants. This infant version was introduced around 2004, allowing researchers and neonatal care teams to accurately measure body composition in newborns and small babies for the first time using a quick, safe, and non-invasive method. Today, the technology is manufactured by COSMED and is used in hospitals, research centers, and universities across the world.

Purpose of the Device and Where It Is Used

The main purpose of air displacement plethysmography is to measure body fat and fat free mass with high accuracy. This information is useful in many settings, including the following.

• Tracking healthy growth patterns in premature and full term infants
• Research studies on infant nutrition and feeding practices
• Monitoring body composition changes in children with growth or nutrition concerns
• Sports science and fitness assessment centers for older children and teenagers
• University and hospital research departments studying obesity, malnutrition, and metabolic health
• Clinical nutrition programs that need precise body composition data rather than weight alone

These devices are commonly found in hospital research units, neonatal care centers, university human performance laboratories, and specialized pediatric nutrition clinics. They are not typically found in a regular home or a small clinic because of their cost and the training required to operate them correctly.

Different Types of Air Displacement Plethysmography Devices

There are two main categories of air displacement plethysmography systems available today. Both use the same underlying air pressure science, but they are built for different age groups and body sizes.

1. Infant sized system

This version is built specifically for newborns and small infants, generally between 1 and 8 kilograms in weight, and is commonly used up to about 6 months of age. It has a small enclosed chamber shaped to safely and comfortably hold a baby. The chamber includes temperature control, ventilation, and a viewing window so the operator can watch the infant at all times during the test. The whole test usually takes only a few minutes, and the baby stays inside the chamber for one or two short periods.

2. Adult and child sized system

This is a larger, egg shaped chamber designed for older children, teenagers, and adults who are able to sit still independently. The person sits inside the chamber wearing tight fitting clothing such as a swimsuit or compression garment, and a swim cap to reduce the effect of trapped air in hair. This version is widely used in sports science centers, universities, and clinical nutrition departments.

Feature	Infant Sized System	Child or Adult Sized System
Typical user weight range	1 to 8 kilograms	Above roughly 12 to 15 kilograms, including teens and adults
Position during test	Lying inside a small enclosed unit	Sitting upright inside a larger chamber
Test duration	About 5 to 7 minutes total	About 5 to 10 minutes total
Clothing requirement	None, tested without clothing or diaper	Tight fitting swimwear and a swim cap
Common setting	Neonatal units, infant research centers	Sports science labs, nutrition clinics, universities

Parts and Components of the Device

Although the infant sized and adult sized systems differ in scale, they share many of the same core components. Understanding these parts helps explain how the device achieves its measurements.

Test Chamber (Subject Chamber)

This is the main enclosed space where the person or infant is placed during the test. In the adult version, it is a large, egg shaped fiberglass chamber with a seat inside and a door that closes to create a sealed environment. In the infant version, it is a smaller, crib like enclosure with a clear viewing window. The chamber must be perfectly sealed during a measurement to ensure accurate pressure readings.

Reference Chamber

A second sealed chamber of known volume is located inside the device but is not visible to the user. The reference chamber is connected to the test chamber through a diaphragm. It provides a baseline pressure reference against which the test chamber pressure is compared.

Oscillating Diaphragm

A flexible membrane, known as a diaphragm, sits between the test chamber and the reference chamber. During measurement, this diaphragm oscillates rapidly, causing small, controlled pressure changes in both chambers. The device measures how the air pressure in the test chamber differs from the reference chamber when a body is present, and uses this difference to calculate the volume of the body.

Pressure Transducers

These are highly sensitive electronic sensors that detect the tiny changes in air pressure caused by the diaphragm oscillation. They convert these pressure changes into electrical signals that the computer can process. The accuracy of the entire system depends heavily on the quality and calibration of these transducers.

Precision Electronic Scale

Both versions of the device include a built in digital scale that is more accurate than a standard clinical scale. Body weight must be measured precisely because it is used together with body volume to calculate body density. Even a small error in weight measurement can affect the final result.

Calibration Cylinder

A solid metal or composite cylinder of known, precise volume is provided with every system. Before each testing session, this cylinder is placed inside the test chamber and a calibration test is run. The system compares its measured volume to the known volume of the cylinder to verify accuracy. If the readings do not match, the device must be recalibrated before any subject testing.

Computer and Software

A dedicated computer with specialized software controls the measurement process, collects raw data from the pressure transducers and scale, applies mathematical models to calculate body density and body composition, and generates a printed or digital report. The software also stores subject records and can track changes over multiple visits.

Breathing Circuit (Adult and Child System Only)

In the adult and child sized system, a breathing tube or circuit is used during a brief additional step called thoracic gas volume measurement. This step estimates how much air is inside the lungs during the test, since air inside the body needs to be accounted for when calculating true body volume. The infant system handles this correction differently using a predictive equation based on body weight.

Additional Components in the Infant System

• A clear viewing window so the operator can monitor the infant throughout the test
• An internal temperature control and ventilation system to keep the infant comfortable and safe
• A removable tray or bed insert that allows the baby to lie in a natural position

Component	Infant System	Adult or Child System
Test chamber shape	Small crib like enclosure	Large egg shaped seated chamber
Viewing window	Yes, always present	No, door closes fully
Temperature control	Built in, active	Not required
Breathing circuit	Not used, prediction equation applied	Used for thoracic gas volume measurement
Calibration cylinder	Smaller cylinder included	Larger cylinder included
Scale capacity	Up to approximately 12 kilograms	Up to approximately 250 kilograms

How the Device Works

The device works using a simple gas law principle. When a person or infant sits inside the sealed chamber, their body takes up space and slightly changes the air pressure inside the chamber compared to an empty reference chamber. The machine measures this pressure difference and uses it to calculate the exact volume of the body. Once the body volume is known, and the body weight is measured on an integrated scale, the machine calculates body density. From density, a mathematical model estimates the percentage of body fat and fat free mass.

This entire process does not involve any radiation, sound waves, or injected substances. It only depends on measuring air pressure and body weight, which makes it a completely non-invasive method.

Step by Step User Guide

The exact steps can vary slightly between models, but the general process for a standard test is described below. This procedure must always be carried out by a trained operator following the manufacturer instructions.

1. Preparation and calibration: The operator switches on the device and allows it to warm up. A calibration cylinder of known volume is tested first to confirm the machine is reading accurately.
2. Recording basic details: The child or infant's age, sex, height or length, and any relevant clinical details are entered into the connected computer system.
3. Preparing the child: For infants, a snug fitting swim cap is placed on the head to reduce trapped air, and the infant is tested without clothing or diaper. For older children, tight fitting swimwear and a swim cap are worn.
4. Weight measurement: The child is first weighed on the device's built in digital scale, which is more accurate than a standard scale.
5. Entering the chamber: The child or infant is placed inside the chamber, and the door is closed gently. For infants, the operator can see them clearly through a window at all times.
6. Volume measurement: The machine takes one or more short readings, usually lasting under a minute each, while the person sits or lies still and breathes normally.
7. Thoracic gas volume measurement (child and adult system only): For the larger system, the child is asked to breathe through a short tube while still seated in the chamber. This step measures the amount of air in the lungs so it can be subtracted from the total body volume, giving a more accurate result. The infant system skips this step and uses a prediction equation instead.
8. Repeat reading: A second reading is usually taken to confirm accuracy. If the two readings do not match closely enough, a third reading may be done.
9. Result generation: The software automatically calculates body volume, body density, fat mass, and fat free mass, and displays a printed or digital report.
10. Review with a professional: The results are reviewed and explained by the treating doctor, dietitian, or researcher, along with other growth information.

Precautions and Possible Dangers

Air displacement plethysmography is considered a very safe procedure with no known major health risks. However, some precautions and limitations should be kept in mind.

• The chamber is a small enclosed space, so children who are extremely anxious in closed spaces may find it uncomfortable, although the door is not locked and can be opened immediately if needed
• Accurate calibration before every session is essential, since incorrect calibration can lead to wrong readings
• Trapped air in hair, loose clothing, or jewelry can affect accuracy, which is why tight fitting clothing and a cap are used
• Excessive crying, movement, or an unsettled infant can reduce the accuracy of a single reading
• The device is a measurement tool, not a diagnostic tool, and should never be used as the sole basis for a medical decision
• Extremely premature or medically unstable infants may need clearance from their care team before testing
• The equipment must be used only by trained and certified operators, following the manufacturer's safety guidelines
• Recent food or drink intake can slightly affect results because of gas in the stomach and intestines, so testing is ideally done at a consistent time relative to meals
• The child's body temperature should be normal at the time of testing, since fever or hypothermia can influence body volume readings slightly
• The room temperature should be stable and within the manufacturer's recommended range, as temperature changes affect air pressure inside the chamber
• If the child has recently exercised vigorously, it is best to wait until breathing and body temperature have returned to normal before testing

How to Keep the Device Safe and Well Maintained

• Follow the manufacturer's calibration schedule strictly, usually before every testing session
• Clean and disinfect the chamber interior after every use according to the manufacturer's cleaning guidelines
• Keep the device in a stable, temperature controlled room away from direct sunlight, dust, and humidity
• Check door seals and ventilation systems regularly, since a damaged seal can affect pressure readings
• Schedule annual professional servicing and software updates as recommended by the manufacturer
• Store calibration weights and reference cylinders safely and handle them only as instructed
• Keep a maintenance and calibration log to track the history of the equipment's performance
• Ensure only trained staff operate or move the device, since incorrect handling can affect its sensitive components
• Install software updates provided by the manufacturer promptly, as these may include improvements to measurement algorithms and data handling
• Back up subject data and test records regularly according to your institution's data management policy
• Have the electrical connections and power supply checked periodically as part of routine biomedical equipment safety inspections
• Replace worn or damaged parts such as door gaskets, breathing tubes, and scale platforms only with manufacturer approved components

Interactive Tool: Test Readiness Checklist

Interactive FAQ

Other Methods Used to Measure Body Composition

Air displacement plethysmography is one of several tools used to assess body composition. It is often chosen because it is quick, comfortable, and does not involve radiation, unlike some alternatives.

Method	Basic Principle	Common Use
Air displacement plethysmography	Air pressure and volume changes	Infants, children, teens, adults, non-invasive
Dual energy X-ray absorptiometry	Low dose X-ray	Detailed bone and fat mapping, uses small radiation dose
Bioelectrical impedance analysis	Mild electrical current through the body	Quick screening, less precise in infants
Skinfold thickness measurement	Caliper measurement of skin folds	Simple field screening, depends on operator skill

Frequently Overlooked Points Worth Knowing

• The results are usually reported as percentage body fat and fat free mass, and must be compared against age and sex specific reference charts, not adult standards
• A single test gives a snapshot, but tracking changes over multiple visits often gives more useful information about growth trends
• Feeding, recent activity, and hydration status in infants and children can slightly influence results and are usually noted by the operator
• Results should always be discussed with a pediatrician or qualified healthcare provider rather than interpreted alone

How to Read and Understand the Results

After a test is completed, the software generates a report that typically includes the following values. Understanding what each value means helps parents and clinicians interpret the information correctly.

Result Parameter	What It Means
Body volume	The total space the body occupies, measured in litres. This is the core measurement from which everything else is calculated.
Body density	Body mass divided by body volume, expressed in kilograms per litre. Higher density usually means more lean tissue and less fat.
Percentage body fat	The estimated proportion of total body weight that is fat tissue. This is derived from body density using established mathematical models.
Fat mass	The total weight of body fat in kilograms, calculated as percentage body fat multiplied by total body weight.
Fat free mass	The weight of everything that is not fat, including muscle, bone, organs, and water, expressed in kilograms.
Fat free mass percentage	The proportion of total body weight that is fat free tissue, the complement of percentage body fat.

Below is a general guide to typical body fat percentage ranges in children. These values are approximate and vary by population, ethnicity, and the reference standard used. They are provided for general orientation only and should not be used for clinical decisions without professional guidance.

Age Group	Typical Range for Boys	Typical Range for Girls
Newborn (full term)	About 10 to 15 percent	About 10 to 15 percent
Infant (3 to 6 months)	About 20 to 30 percent	About 22 to 32 percent
Child (5 to 10 years)	About 12 to 22 percent	About 14 to 25 percent
Adolescent (11 to 17 years)	About 10 to 20 percent	About 16 to 28 percent

Advantages and Limitations

Like every measurement tool, air displacement plethysmography has both strengths and weaknesses. Understanding these helps set realistic expectations about what the device can and cannot do.

Advantages

• Completely non-invasive, no needles, no radiation, no water immersion, and no physical discomfort
• Very quick, with the actual measurement taking only one to two minutes per reading
• Highly repeatable, meaning that two tests done one after the other on the same person will usually give very similar results
• Considered a reference or gold standard method for body composition assessment, alongside underwater weighing and dual energy X-ray absorptiometry
• Safe for all ages, including premature and medically fragile infants, because it involves no ionizing radiation or chemical exposure
• Comfortable for the subject, especially compared to methods that require water submersion, injection, or prolonged stillness
• Provides a permanent digital record of body composition data that can be tracked over time for growth and nutrition monitoring
• Can be performed repeatedly at short intervals without any cumulative risk, unlike methods that involve radiation exposure

Limitations

• High equipment cost limits its availability to specialized research centers, hospitals, and universities
• Requires a trained and certified operator, so it cannot be used at home or in most primary care clinics
• Results can be affected by trapped air in clothing, hair, or body cavities if not properly managed
• Excessive infant movement or crying can reduce the accuracy of individual readings
• The device divides the body into only two compartments, fat mass and fat free mass, and cannot separately measure bone, muscle, or organ tissue within the fat free mass category
• Body fat calculations rely on mathematical models that assume certain densities for fat and fat free tissue, and these assumptions may not be equally accurate across all populations and clinical conditions
• Weight limits apply to both the infant and adult versions, so some subjects may fall outside the testable range
• A child with severe edema, large casts, or unusual body shapes may not produce accurate results

Troubleshooting Common Problems

Operators may occasionally encounter issues during testing. The following are some of the most common problems along with suggested solutions. For any persistent or unexplained issue, always consult the manufacturer's technical support.

Problem	Possible Cause	Suggested Solution
Calibration fails or gives an error	Chamber door not fully sealed, calibration cylinder not placed correctly, or ambient temperature has changed significantly	Ensure the door is properly closed and the seal is intact. Reposition the calibration cylinder. Allow the system to stabilise at room temperature before retrying.
Two consecutive readings do not agree	Subject moved during measurement, clothing or cap shifted, or door seal is compromised	Repeat the measurement after ensuring the subject is settled and properly prepared. Check the door seal for damage.
Software displays an error message during measurement	Communication error between hardware and computer, sensor malfunction, or software glitch	Restart the software and, if needed, restart the computer. Check all cable connections. If the error persists, contact the manufacturer.
Results seem unusually high or low compared to previous tests	Incorrect calibration, subject not properly prepared (clothing, hair, feeding status), or equipment drift	Recalibrate the system. Review subject preparation against the standard checklist. Compare with known quality control data.
Infant becomes very unsettled during the test	Hunger, discomfort, overstimulation, or unfamiliar environment	Pause the test, comfort the infant, and retry when settled. Schedule future tests at times when the infant is usually calm, such as after a feed and nap.
Temperature warning or fluctuation alert	Room temperature outside recommended range or the device has not warmed up sufficiently	Ensure the testing room is within the manufacturer's specified temperature range. Allow the device to warm up fully before beginning calibration.

When to Contact the Manufacturer or Service Provider

Most routine issues can be resolved by trained operators using the troubleshooting steps above and the information in the user manual. However, some situations require professional technical support.

• Calibration repeatedly fails despite following all recommended steps and checking the door seal
• A hardware component such as a pressure transducer, diaphragm, or scale appears to be malfunctioning or giving erratic readings
• The software crashes repeatedly or displays persistent error codes that are not covered in the user manual
• The device has been physically damaged, dropped, or exposed to water, extreme heat, or power surges
• Annual or scheduled preventive maintenance is due, as recommended by the manufacturer
• A software update or firmware upgrade is available and needs to be installed by an authorised service engineer
• The institution is relocating the device and needs guidance on safe transportation, reinstallation, and recalibration

Suggested Reading and Official Resources