VOC Zeroing Tubes
VOC Zeroing tubes are single-use charcoal filters intended for zero calibration, especially for the ppbRAE 3000, where a zero gas with <5 ppb isobutylene-equivalent response is required. Other charcoal tubes with higher capacity might also be used for this purpose, but once they have
been opened for some time, they tend to absorb VOCs from the ambient air and then release ppb levels of VOCs back into the zero calibration
stream. The VOC Zeroing tubes ensure a clean background by virtue of being sealed in glass until just before use. These tubes could also
be used to remove organic vapor interferences in other sensors such as electrochemical CO sensors or unfiltered CO tubes.

VOC/CO2 Zeroing Tubes
These tubes are identical to the VOC Zeroing Tubes, but with an additional layer to remove CO2 for multi-gas meters having both PID and CO2 sensors.

(Source: RAE Systems)