MI’s 9331 series of standard air resistors are small, light, and rugged resistance standards. They do not require a temperature-controlled oil or an air bath for their specification range. The 9331 series features an A version (up to 10 ppm yearly drift) and a B version (up to 5 ppm yearly drift) for operation in any working environment within the range of 18 °C to 28 °C.