Pycnometry
Definition
Absolute density measurement using a calibrated-volume flask; the density of a solid is calculated from the mass of liquid displaced when the solid is introduced into the pycnometer.
- Measures
- Absolute (true) density of a solid or liquid
- Instrument
- Calibrated-volume glass or gas pycnometer
- Principle
- Fluid displacement by mass
- Common forensic use
- Glass fragment and fibre comparison
- Typical fluid
- Water or an inert gas such as helium
Common questions
Why is pycnometry preferred over simple volume-and-mass calculation for small fragments?+
Irregularly shaped or porous forensic items such as glass chips or paint flakes are difficult to measure by direct dimensions. Pycnometry sidesteps this by inferring volume from the mass of displaced liquid, which works regardless of the particle's shape.
How does pycnometry help link glass fragments to a source?+
Density is sensitive to a glass batch's exact composition, so fragments from the same pane cluster tightly around one density value. Comparing an unknown fragment's density to a known source can exclude or fail to exclude a common origin, though it cannot uniquely identify one.
What can cause an inaccurate pycnometry reading?+
Trapped air bubbles on the sample surface, incomplete wetting, temperature fluctuations that change the calibration fluid's density, and residual surface contaminants on the sample all introduce error into the displaced-volume measurement.
Related terms
- ASTM E2365
- The American Society for Testing and Materials standard for forensic comparison of glass by density; specifies temperature control, calibration, and precision requirements...
- Bromoform-Bromobenzene System
- The reference liquid mixture used in forensic density-gradient columns and sink-float tests; bromoform (2.890 g/cm³) and bromobenzene (1.495 g/cm³) mix in variable...
- Density
- Mass per unit volume (ρ = m/V), expressed in g/cm³ or kg/m³; a physical property used to compare and discriminate glass, mineral,...
- Density-Gradient Column
- Glass column filled with stepwise layers of liquids of decreasing density (commonly bromoform 2.89 g/cm3 and bromobenzene 1.50 g/cm3 mixed in known...
- DFSS
- Directorate of Forensic Science Services, India; issues the Forensic Laboratory Manual that governs glass and soil examination procedures, including the bromoform-based density...
- ENFSI ENG3-2013
- The ENFSI Glass Expert Working Group guideline for glass comparison casework; specifies the examination sequence, within-source variation requirements, statistical comparison framework, and...
- Hydrostatic Weighing
- Density measurement exploiting Archimedes' principle; the apparent weight loss of a solid submerged in a liquid of known density equals the weight...
- Neutral Buoyancy
- The condition in which a solid object's density exactly matches the density of the surrounding liquid; the object neither sinks nor rises...
- Oscillating-U-Tube Densitometer
- An instrument that measures liquid density from the resonant frequency of a liquid-filled U-shaped tube; precision of ±0.0001 g/cm³, widely used for...
- Sink-Float Test
- Qualitative density screening in which a solid fragment is immersed in a liquid of adjustable density until neutral buoyancy is achieved; the...
- Stokes Settling Formula
- The equation v = 2r²g(ρₛ - ρ_f) / 9η relating the settling velocity of a spherical particle to its radius, the density...