Gas Pycnometer for Plastic Density Measurement

What is the Gas Pycnometer for Plastic Density Measurement?

A gas pycnometer measures the volume of a plastic sample by helium displacement in calibrated chambers, giving true density for powders, porous parts and irregular shapes. It avoids any contact with liquid and is fast and repeatable.

Overview

The instrument expands helium at a known pressure from a reference chamber into a sample chamber of known volume, and the resulting pressure ratio gives the volume of solid material displaced, from which density follows when the mass is known. Helium is used because its small atoms penetrate fine pores and its behaviour is close to ideal at the working pressure. Measurement takes a few minutes and repeatability of around 0.02% can be reached on homogeneous powders, which makes the technique suitable for polymer powders, compounds containing voids and materials that swell or dissolve in water. Sample chamber size should be matched to the specimen, since a chamber that is much larger than the sample reduces the volume resolution; some instruments offer several chamber sizes for that reason.

How does it work?

The classical method applies Archimedes' principle: the sample is weighed in air and again while immersed in a liquid of known density, and the difference gives the sample volume. This suits void-free solid specimens and returns density to three or four decimal places. A density gradient column contains a liquid column whose density varies linearly with height, so a small specimen sinks to the level matching its own density and is read against calibrated glass floats, which is useful for very small or thin samples. Gas pycnometry measures the volume of a sample by helium displacement in a calibrated chamber, so it handles powders and porous or irregular material. Bulk and apparent density methods instead measure the mass that fills a defined volume, which is the relevant quantity for granules, powders and cellular plastics.

What are the key features?

• Helium displacement measurement of true volume
• Multiple reference and sample chamber sizes
• Repeatability to about 0.02% on powders
• No liquid contact with the sample
• Automatic pressure and temperature logging
• Optional flow-through mode for purging

What are the technical specifications?

A typical gas pycnometer for plastic density measurement configuration offers measurement principle of Helium gas displacement, Boyle's law, sample chamber volume of 1–150 cm³ depending on module, volume accuracy of ±0.03% of chamber volume, repeatability of ±0.02% on homogeneous powder, analysis time of 2–5 min including purging and working gas of Helium, ≥ 99.99%. Values are indicative of the product class; the exact figures for each configuration are confirmed on the quotation and the calibration documents supplied with the instrument.

Which applications is it used for?

The Gas Pycnometer for Plastic Density Measurement is used in true density of polymer powders and masterbatch, void content assessment in moulded parts, density of materials sensitive to water and filler density checks in compounds. Across the wider Density Testing class, laboratories also deploy it for incoming verification of resin density, filler and mineral content estimation in compounds and crystallinity and formulation consistency checks.

Which standards and certifications apply?

Relevant standards and methods include ISO 1183-1, ISO 1183-2, ISO 1183-3, ASTM D792, ASTM D1895, ASTM D1622, ISO 845 and IEC 61010-1. Certification candidates for this model are CE and ISO 9001 manufacturing; each is to be confirmed for the specific configuration before shipment, and certificates are supplied only where they have been issued.

Temperature control is essential because liquid density and sample volume both change with temperature; record the temperature and apply the liquid's density-temperature table. Remove air bubbles from immersed specimens and from pycnometer chambers, as a single trapped bubble changes the volume materially. Weigh the specimen wet mass promptly to limit evaporation and use a covering liquid that does not swell or dissolve the polymer. Calibrate the balance and verify liquid density with a certified glass float or hydrometer at defined intervals. For bulk density, follow the specified funnel, orifice and cup, since changing any of these changes the result.

How do I select the right Gas Pycnometer for Plastic Density Measurement?

Choose a gas pycnometer when the sample is a powder, is porous, must not contact liquid, or when a fast repeatable true-density value is needed for process control. Confirm the chamber volume suitable for your specimen size and that helium supply and pressure regulation are available in the laboratory. Verify with a certified volume standard rather than an assumed value.

How does it compare with other Density Testing variants?

Match the method to the material form. Solid, void-free mouldings and pellets are measured by immersion balances or by density gradient columns when only a small specimen is available. Powders, porous parts and materials that must not contact a liquid are measured by gas pycnometry. Granules, flakes and powders that are handled in bulk are characterised by bulk or apparent density, and rigid cellular plastics by their own apparent density method with a defined specimen. For specification testing, follow the method named in the material standard: ISO 1183 parts and ASTM D792 give different procedures and tolerances for solid plastics, and results from different methods should not be mixed in one data set.