Vapor pressure refers to the force of a material’s vapor phase that exists in equilibrium with its pure solid or liquid phase at a given temperature inside of a closed container. It reaches this equilibrium point when molecules are evaporated into the gas phase at the same rate as they are condensed back into a liquid or deposited as a solid. Since the kinetic energy of both phases will increase when heated, vapor pressure increases with rising temperature.

As the different intermolecular forces within different materials require different energies to come apart, this relationship between vapor pressure and temperature will vary by material. The property is usually expressed in kilopascals (kPa), but other units may be used depending on the context of the measurement, including torr, psi and atm. As an indicator of a substance’s volatility, the property is often determined in a laboratory using a vapor pressure tester.

Samples commonly tested in the lab with a vapor pressure analyzer include oils, natural gases, fuels, and various volatile chemicals and solvents. Many test methods exist in order to standardize the methodology, most notably though ASTM. All commercial vapor pressure analyzers conform to one or more of these test methods.

The measurement process depends on the method, but it usually involves placing the sample in a piston chamber. When the piston is moved to expand the volume of the chamber, part of the sample volatilizes to fill the void. When equilibrium is reached, the pressure generated by the vapor is measured. The measurement can be made as a single data point for a given temperature, or it can be made as a series of data points at various temperatures.

Vapor pressure testers help to ensure that a volatile material has the correct properties for its intended use. Fuel testing is one of the most important applications, since the measurement is used to indicate how well a particular fuel will perform under various conditions, as well as ensure that it meets environmental pollution regulations.

Crude oils and fuels are also routinely tested for vapor pressure before they are transported by pipeline, oil tankers, rail or other means in order to prevent costly damage to the transportation system. In fact, customers generally require a guarantee of maximum vapor pressure before accepting a fuel delivery. The analyzers are also crucial for determining the quality of oil and petrochemicals during refinement and blending.

Major suppliers of benchtop vapor pressure analyzers include Grabner Instruments (AMETEK), PAC (Roper Technologies) and Stanhope-Seta. The leader in the market is Grabner Instruments, which offers the MINIVAP series, including a 12-position autosampler option. PAC offers the HVP 972, while Stanhope-Seta sells the SetaVap 2 and SetaVap 3 analyzers. Other suppliers include Anton Paar, Normalab, Koehler Instrument and Tanaka Scientific. The total market is currently worth about $13 million. Driven by transport regulations in the oil and gas industry, sales are projected to increase modestly in the next few years.

Vapor Pressure Analyzers at a Glance:

Largest Suppliers:

• Grabner Instruments (AMETEK)
• PAC (Roper Technologies)
• Stanhope-Seta

Largest Markets:

• Petroleum
• Natural Gas
• Fuels

Instrument Cost:

• $2,000–$15,000