5:15 PM - 7:15 PM
[SVC30-P01] Simple Volcanic Gas Monitoring Using Diffusive (Passive) Tubes
Keywords:volcanic gas, volcanic unrest, monitoring, fumarole
Various electrochemical and spectroscopic sensors have recently been utilized to monitor volcanic gases. However, in addition to their initial cost, their running cost can be a great burden because of frequent sensor calibration and parts replacement due to corrosion by volcanic gas. Since 2018, the relationship between the levels of volcanic activity and the ratios of volcanic gases has been examined by installing diffusive tubes near a fumarole at Owakudani, the center of the 2015 eruption of the Hakone volcano. The total number of gas observations had reached 188 by the end of 2024. Here, we present the findings from these observations.
Diffusive (passive) tubes are a type of gas detector tube made up of a glass tube and a detector agent sealed inside it. Unlike ordinary detector tubes, they use gas diffusion rather than suction for measurement. At the start of the measurement, one end of the glass tube is broken off to allow outside air to contact the detector agent. The target gas diffuses while reacting with the detector agent. The detector agent changes color after reacting, so the average concentration can be calculated from the length of the discoloration layer. They are widely used in factories and other working places to assess worker exposure, so they are mass-produced, stable in quality, and inexpensive (a few hundred JPY per tube). The tubes we use are manufactured by GASTEC and are called Passive Dositubes. The gases we measure are SO2 (No. 5D), HCl(No. 14D), and H2S (No. 4D) (in parentheses are the product codes). The measurement times in the catalog are 1 to 10 hours (1 to 48 hours for H2S). We have been conducting simultaneous measurements using these three tubes bundled with vinyl tape. The bundles are placed (1) in direct exposure to the atmosphere and (2) in an open container with silica gel. The measurement period is approximately one hour. Our findings are as follows.
Time-series changes: At the Owakudani 15-2 fumarole, where the diffusive tubes have been installed, conventional gas measurement (Ozawa, 1968), in which all the gas species contained in the fumarole are quantified. The time-series changes in the ratios of volcanic gases (SO2/H2S) for both methods agree. The SO2/H2S ratio consistently increased whenever volcanic inflation was detected. The diffusive tube is equally effective as the conventional method for monitoring volcanic gases.
Simultaneous measurement: We compared SO2/H2S values of both methods measured within 7 days. In 2019, the SO2/H2S ratios by the conventional method were systematically higher than those of the diffusive tube, while in 2023, they were notably lower. Such a difference can be attributed to the difference in the vent that both methods measured. The 15-2 fumarole is a group of steaming vents. While the conventional method collects gas only from a minor vent named 15-2E, the diffusive tubes appear to be significantly affected by the steam from a predominant vent named 15-2R. The variation between the two methods likely reflects differences in the gas composition from the measured vents. Consequently, while a positive correlation was noted between the values of the diffusive tube and the conventional method, the correlation coefficient r was just 0.56, indicating a weak correlation.
Effect of silica gel: Water vapor from the fumarole often condenses on the surfaces of diffusive tubes and the detection agent, hindering measurements. However, such condensation is highly unlikely in a container with silica gel. Conversely, there are rare instances where the observed concentration is significantly low due to insufficient steam being introduced into the container. Additionally, when the concentration of volcanic gas is low, the ratio may be influenced by silica gel’s absorption of volcanic gas. Therefore, parallel observation of direct exposure is recommended.
Diffusive (passive) tubes are a type of gas detector tube made up of a glass tube and a detector agent sealed inside it. Unlike ordinary detector tubes, they use gas diffusion rather than suction for measurement. At the start of the measurement, one end of the glass tube is broken off to allow outside air to contact the detector agent. The target gas diffuses while reacting with the detector agent. The detector agent changes color after reacting, so the average concentration can be calculated from the length of the discoloration layer. They are widely used in factories and other working places to assess worker exposure, so they are mass-produced, stable in quality, and inexpensive (a few hundred JPY per tube). The tubes we use are manufactured by GASTEC and are called Passive Dositubes. The gases we measure are SO2 (No. 5D), HCl(No. 14D), and H2S (No. 4D) (in parentheses are the product codes). The measurement times in the catalog are 1 to 10 hours (1 to 48 hours for H2S). We have been conducting simultaneous measurements using these three tubes bundled with vinyl tape. The bundles are placed (1) in direct exposure to the atmosphere and (2) in an open container with silica gel. The measurement period is approximately one hour. Our findings are as follows.
Time-series changes: At the Owakudani 15-2 fumarole, where the diffusive tubes have been installed, conventional gas measurement (Ozawa, 1968), in which all the gas species contained in the fumarole are quantified. The time-series changes in the ratios of volcanic gases (SO2/H2S) for both methods agree. The SO2/H2S ratio consistently increased whenever volcanic inflation was detected. The diffusive tube is equally effective as the conventional method for monitoring volcanic gases.
Simultaneous measurement: We compared SO2/H2S values of both methods measured within 7 days. In 2019, the SO2/H2S ratios by the conventional method were systematically higher than those of the diffusive tube, while in 2023, they were notably lower. Such a difference can be attributed to the difference in the vent that both methods measured. The 15-2 fumarole is a group of steaming vents. While the conventional method collects gas only from a minor vent named 15-2E, the diffusive tubes appear to be significantly affected by the steam from a predominant vent named 15-2R. The variation between the two methods likely reflects differences in the gas composition from the measured vents. Consequently, while a positive correlation was noted between the values of the diffusive tube and the conventional method, the correlation coefficient r was just 0.56, indicating a weak correlation.
Effect of silica gel: Water vapor from the fumarole often condenses on the surfaces of diffusive tubes and the detection agent, hindering measurements. However, such condensation is highly unlikely in a container with silica gel. Conversely, there are rare instances where the observed concentration is significantly low due to insufficient steam being introduced into the container. Additionally, when the concentration of volcanic gas is low, the ratio may be influenced by silica gel’s absorption of volcanic gas. Therefore, parallel observation of direct exposure is recommended.