O-rings are measured in inches or centimeter.  The key measurements are inside diameter, outside diameter, and width.  Dash Numbers are used to references the o-rings sizes.

Many o-ring materials are available for IR spectroscopy applications, but the most routine o-ring materials are:

Viton: Viton (fluorocarbon) o-rings offer a wide spectrum of chemical compatibility and temperature-range adaptability. Use with petroleum products, diester lubricants, silicone fluids, solvents, and acids (except fuming nitric). They have good resistance to taking a set after compression, and low gas permeability. Great for hard vacuum service. Not for use with acetates, methyl alcohol, ketones, brake fluids, hot water, or steam. Durometer is Shore A: 75; temperature range is -31 to +400 F (except metric sizes, which have a temperature range of -15 to +400 F). Inch sizes meet ASTM D2000 MGHK810A1-10B38EF31Z1.

Kalrez® Perfluoroelastomer: Kalrez® features outstanding resistance to nearly all classes of chemicals except some fluorinated solvents. It has the resilience and sealing force of an elastomer with the chemical inertness and thermal stability of Teflon. Durometer is Shore A: 75; temperature range is -30 to +500 F. Each o-ring is packaged in a factory-sealed bag.

Chemraz® is a line of high performance o-rings that come in range of perfluoroelastomer and fluoroelastomer compounds designed for a range of applications that are meet by Teflon and Kalrez o-rings.

Teflon: Superior lubricity plus excellent temperature and chemical resistance make these o-rings perfect for static (stationary) sealing applications. For dynamic sealing applications where o-ring may be in motion, Teflon o-rings require mechanical loading. Durometer is Shore D: 55; temperature range is -300 to +450 F.

Teflon-Encapsulated Viton: Perfect for sealing corrosive fluids. Made with an FEP Teflon coating that makes them virtually impervious to most solvents and chemicals. Extremely low coefficient of friction and excellent resistance to taking a set after compression makes them ideal for static sealing application. The resin in FEP Teflon meets L-P-389A and ASTM D-2116 requirements, and is FDA compliant. Offers greater chemical resistance than Teflon-encapsulated silicone and is recommended for long-term sealing. Durometer is Shore A: 75; temperature range is -10 to +400 F.

Teflon-Encapsulated Silicone: Perfect for sealing corrosive fluids. Made with an FEP Teflon coating that makes them virtually impervious to most solvents and chemicals. Extremely low coefficient of friction and excellent resistance to taking a set after compression makes them ideal for static sealing application. The resin in FEP Teflon meets L-P-389A and ASTM D-2116 requirements, and is FDA compliant. Durometer is Shore A: 70; temperature range is -75 to +400 F.

Silicone: Excellent high- and low-temperature capabilities enable silicone o-rings to be used in long-term exposure to dry heat. They perform best in static sealing applications where the o-ring may be in motion. Silicone resists brake fluids and high aniline point oil. It is not recommended for ketones or most petroleum oils. Durometer is Shore A: 70; temperature range is -65 to +450 F.

FDA-Compliant Buna-N: Perfect for use in food-processing areas, this o-ring material complies with FDA Title 21 for contact with food. Durometer is Shore A: 70; temperature range is -35 to +250 F.