Reliability testing and verification of a portable CO₂ flux measurement device for geothermal and volcanic studies

Carbon dioxide flux (F_c) is an important indicator for resource exploration and potential assessment of eruption-related hazards. Accurate measurement of soil F_c values is essential for geothermal and volcanic studies. However, highly precise instruments for F_c measurements are often expensive, large, and require trained operators. To address these limitations, we developed a portable F_c measurement device (PCFD) by combining an accumulation chamber constructed in our laboratory with a generic nondispersive infrared CO₂ sensor (GasLab Pro CM-1000, CO2Meter). A strong correlation (R² = 1.00) was obtained for the PCFD accuracy in testing using pure CO₂ and a reference gas flowmeter. Comparison with an industry-standard meter also revealed a strong correlation for the F_c values (R² = 0.97); however, the PCFD tended to underestimate fluxes because of differences in the AC size and volume-to-area ratio. This bias was fully corrected by calibration. Controlled laboratory experiments verified the high precision and repeatability of the PCFD for measuring fluxes up to 100,000 g m⁻² day⁻¹. Field tests covering a wide F_c range in a geothermal area in Indonesia demonstrated that fluxes below 30 g m⁻² day⁻¹ were less reliable because of low sensor sensitivity. By contrast, higher fluxes were measured precisely (R² > 0.90). A close relationship was observed between high-flux zones and the major fault system in this area, which indicated that the fault lines acted as pathways for gas ascent. These results confirm the PCFD is suitable for applications in geothermal and volcanic areas where high F_c values are commonly observed.