MultiGAS (Multi-component Gas Analyzer System) is an instrument that measures the concentrations of multiple volcanic gas species simultaneously, typically H₂O, CO₂, and SO₂. By sampling gas directly from volcanic plumes or fumaroles, MultiGAS provides real-time ratios and fluxes of key volatile species. Changes in gas composition and emission rates are among the most important precursors to volcanic eruptions, as they reflect processes occurring in the magmatic and hydrothermal systems at depth.
How it works
A MultiGAS unit draws ambient air through a set of electrochemical and infrared sensors. Each sensor is tuned to detect a specific gas species. The system logs concentration data at regular intervals, and ratios such as CO₂/SO₂ can be calculated to track how the gas signature evolves over time. These ratios are particularly diagnostic: an increasing CO₂/SO₂ ratio, for example, may indicate deeper magmatic input.
MultiGAS is distinct from other gas measurement techniques used in AVERT.
Scanning DOAS measures SO₂ remotely by analysing UV absorption in the plume
from a distance, while soil CO₂ probes measure diffuse degassing through the
ground. MultiGAS directly samples the gas mixture in the air near vents or
fumaroles.
AVERT deployments
At Poas volcano, a MultiGAS station was already operated by OVSICORI prior to the AVERT partnership. It provides continuous measurements of the gas plume composition near the active crater. The AVERT project did not deploy new MultiGAS instruments, but the existing MultiGAS data are an important part of the multi-parameter dataset at Poas, complementing the seismic, GNSS, magnetic, and soil CO₂ records contributed by AVERT. 
