The Gas Calibration Unit (GCU) is a dynamic gas dilution system that provides variable but known quantities of different standard compounds in a carrier gas stream, enabling accurate calibrations to be made. The GCU systems have been designed as standalone devices for use with many different analytical VOC instruments, but are particularly suited to PTR-MS systems, offering semi-automatic calibration functionality.   
   
GCU-standard (GCU-s)   
The GCU-standard (GCU-s) is an all-round device that allows calibrations to be made at varying humidities. A humidification chamber in combination with a gas cooler provides control of dilution gas humidify as desired, up to ambient temperature.   
   
GCU-advanced (GCU-a)   
The GCU-advanced (GCU-a) bases itself on the GCU-s but offers calibration at increased humidities (to 40 °C). This feature, plus an additional CO2 flow, makes the GCU-a particularly suited to applications (e.g. breath gas analysis or biotechnology), where high sample gas humidity and CO2 content can affect the PTR-MS sensitivity.