The RM9999 is a resistance tester used to test the resistance of filter canisters. The device provides a measurement in mm WS (milimeter wassersäule - milimeters of water). The measurement can be used to determine if the filter has absorbed excess moisture or contains internal blockages etc. The testing of the breathing resistance of a filter cartridge provides zero indication of its effectiveness against gases or dust particles. The device is housed in a wooden box that has been painted to imitate metal. The RM9999 has a standard 40mm threaded filter port and includes an adapter for circular plug-in filters. The instructions which are glued to the inside of the lid provides a table with acceptable resistance measurements for various Dräger filters.