# Assessing the Portable Gas Analyzer GA5000 for Accurate H Monitoring in Geological Exploration

Sarah Auclair, Nicolas Lefeuvre, Hugo Dutoit, Frédéric-Victor Donzé, Laurent Truche

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## To cite this version:

Sarah Auclair, Nicolas Lefeuvre, Hugo Dutoit, Frédéric-Victor Donzé, Laurent Truche. Assessing the Portable Gas Analyzer GA5000 for Accurate H Monitoring in Geological Exploration. GEONUM, 2024, GEONUM-NST-2024-01-ENG (1), pp.14. hal-04861335

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# Assessing the Portable Gas Analyzer GA5000 for Accurate H₂ Monitoring in Geological Exploration

S. Auclair, N. Lefeuvre, H. Dutoit, F-V. Donzé & L. Truche

Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 38000 Grenoble, France.

## ABSTRACT

This study aims to quantify the reliability of electrochemical sensors, specifically the GA5000 multi-gas analyzer from GeoTech®, in measuring H₂ gas concentrations under relevant outdoor conditions for soil gas surveys. We developed a specific experimental setup to evaluate the effects of defined and well-constrained environmental conditions, focusing on temperature, pressure, relative humidity, and CO interference. Assessing the metrological performance of electrochemical sensors for H₂ concentration measurements is crucial, especially given the growing interest in natural or geologic hydrogen, where such devices are widely used for exploration. We demonstrate the reliability and accuracy of the GA5000 device within a range of environmental conditions that match the diversity of situations encountered in the field. However, we also advise caution in interpreting H₂ concentration data in soil.

**Keywords:** Hydrogen Detection; Natural Hydrogen; Soil Gas Survey; Metrological Performance; Serpentinization

## INTRODUCTION

Soil gas analysis is part of the preliminary steps for geologic hydrogen (H₂) exploration, and can be considered as a key pathfinder (McCarthy et al., 1986; Truche et al., 2020; Mainson et al., 2022; Langhi et al., 2023; Lévy et al., 2023). This method has been recently used in many H₂ exploration studies targeting intracratonic basins (Larin et al., 2015; Zgonnik et al., 2015; Prinzhofer et al., 2019; Donzé et al., 2020; Moretti et al., 2021a and b; Frery et al., 2021), foothill basins (Lefeuvre et al., 2021, 2022), and ophiolites (Zgonnik et al., 2019). Using soil gas anomalies to explore minerals and petroleum is a well-established technique. It has been effective for finding ore deposits and hydrocarbons for decades (McCarthy and Reimer, 1986; Ball et al., 1990; Plet and Noble, 2023).

However, H₂ was never measured for exploration until it became a direct interest. Until recently, soil gas studies focusing on H₂ were mostly dedicated to active faults monitoring, as this molecule is known to be produced by mechanoradical processes induced by rock comminution during earthquake (Ware et al., 1984; Jones and Pirkle, 1981; Xiang et al., 2020;
Zhou et al., 2021). Note that mechanoradical processes generate reactive radicals by breaking chemical bonds in mineral crystal lattice through mechanical actions like grinding or crushing. This occurs when mechanical energy exceeds the bond dissociation energy, creating highly reactive species (Wakita et al., 1980; Kameda et al., 2003; Saruwatari et al., 2004; Narayanasamy and Kubicki, 2006; Delogu et al., 2011;). The operational requirements for H₂ exploration may be quite different from those for monitoring active faults. Soil gas surveys for H₂ exploration need to cover a large area, while earthquake monitoring requires long-term observation at specific sites. Therefore, the materials and methods must be adapted and carefully checked before use. Soil gas mapping for H₂ exploration can be guided by geological structures or done without prior knowledge, using a geometric grid to cover the targeted area from regional (>10,000 km²) to local (<1 km²) scales (Lefeuvre et al., 2021, 2022). This is crucial for greenfield exploration where there is no prior data indicating the presence of H₂. Thus, in situ and rapid analysis of H₂ is a prerequisite to carry out an efficient exploration campaign. Electrochemical (also termed galvanometric) sensors are the technology of choice to detect H₂ at the ppmv level in a matter of few tens of seconds in soil. These unique properties explain why this technology has been widely used in several studies (Larin et al., 2015; Zgonnik et al., 2015; Prinzhofer et al., 2019, 2024; Zgonnik et al., 2019; Lefeuvre et al., 2021, 2022; Frery et al., 2021; Lévy et al., 2023). However, it is also well known that these types of sensors are sensitive to temperature, pressure, relative humidity and cross interferences with other gas compounds (Korotcenkov et al., 2009).

Until now, electrochemical sensors have been used in H₂ exploration without fully assessing their accuracy, calibration, application range, and reliability in the field. Here, we aim to test the GA5000 electrochemical sensor, commonly used by geoscientists, to assess its performance under controlled conditions relevant for field exploration. We evaluated the effects of temperature, moisture, CO interference, and equilibration time using gas standard mixtures. This metrological assessment is essential to ensure the robustness of H₂ concentration measurements in the field.

# MATERIALS & METHOD

## The GA5000 multi-gas analyser

The GA5000 is a portable multi-gas analyzer from GeoTech® (Figure 1). It is easy to use and allows on-site gas measurements in ambient air or exhaust gases at temperatures from -10°C to 50°C and pressures from 500 to 1500 mbar. It integrates two types of sensors: electrochemical cells for measuring H₂, H₂S, CO, and O₂, and a dual-wavelength infrared detector for CO₂ and CH₄. The electrochemical sensor is notable for its ability to accurately measure low gas concentrations, making it ideal for field instruments. The electrochemical sensor operates when H₂ interacts with the sensing electrode material, initiating an electron transfer process. In this process, H₂ oxidizes at the anode, while O₂ from the air reduces at the cathode. Thus, the electrical signal from the sensor depends on the H₂ concentration. The device can also be equipped with a GPS sensor for geolocation and offers data retrieval via Bluetooth or USB connection. It has a built-in battery that lasts for 8 hours of continuous measurements.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** a) Soil gas measurement in the field. Immediately, after drilling a 1 m deep hole (1 cm OD) using a portable drill in percussion mode only (no rotation of the drill bit), a soil gas probe connected to the multi gas analyzer (GA5000 in the present case) is introduced in the hole. b) Picture of the GA5000 screen while measuring a 500 ppmv H₂ standard in argon.</figcaption></figure>

According to the operating manual, the device can measure H₂ concentrations from 0 to 1000 ppmv with a precision of ±2.5% full scale (Table 1). It has a flow rate of approximately 550 mL/min and a sensor response time of 30 seconds to 1 minute. Temperature, humidity, pressure, and interference from other gases can significantly affect these performances (Korotcenkov et al., 2009).

<table><caption>**Table 1.** Gas concentration, relative humidity, temperature and pressure ranges for accurate H₂ measurement using the GA5000 multi-gas analyzer as specified by the manufacturer (GeoTech®).</caption><thead><tr><td>H₂ concentration Range</td><td>Relative Humidity Range</td><td>Temperature Range</td><td>Pressure Range</td></tr></thead><tbody><tr><td>[0-1000] ppmv</td><td>15 to 90 %</td><td>-10 to 50°C</td><td>500 to 1500 mbar</td></tr></tbody></table>

* **Temperature:** Reliable measurements are obtained within the -10 to 50°C range according to the manufacturer. Temperature variations can lead to deviations from 0.5% to 1.0% per degree Celsius, with greater deviations above 25°C and smaller deviations below this threshold. Although temperature sensitivity affects sensor performance, internal compensation mechanisms reduce this effect. However, the temperature compensation function is not provided to the user and remains unknown to us.

* **Humidity:** The sensor resists direct humidity impacts but may experience signal shifts due to prolonged exposure to extreme humidity conditions. Relative humidity below 15% or above 90% can change the water content of the electrolyte (H₂SO₄), affecting the sensor's signal.
* **Pressure:** The GA5000 can operate at atmospheric pressure ±500 mbar. At a vacuum level of 200 mbar, the internal pump stops functioning. This threshold is adjustable by the user. Pressure changes can cause variations in the analyte concentration, impacting the sensor's signal. Electrochemical sensors have minimal sensitivity to pressures within the normal ambient range (around 10%) and should be calibrated and operated under consistent pressure conditions for optimal accuracy.

* **Cross-gas Interferences:** Hydrogen measurements may be influenced by the presence of other gases like CO, H₂S, SO₂, and NO₂. The analyzer uses a compensation technique effective up to 2000 ppm of H₂ according to the operating manual (Table 2).

<table><caption>**Table 2.** Estimations of cross-gas effects on H₂ measurements. It represents how many ppm would be read on the H₂ cell if a given concentration of the interfering gas was applied, with no other cross-contaminates being present in the sample. Modified from the GA5000 operating manual.</caption><thead><tr><th>Type of Gas</th><th>Concentration Used (ppm)</th><th>Estimated impact on H₂ concentration (ppmv)</th></tr></thead><tbody><tr><td>Carbon Monoxide, CO</td><td>300</td><td>&lt; 60</td></tr><tr><td>Hydrogen Sulfide, H₂S</td><td>15</td><td>&lt; 3</td></tr><tr><td>Sulfur Dioxide, SO₂</td><td>5</td><td>0</td></tr><tr><td>Nitric Oxide, NO</td><td>35</td><td>≈ 10</td></tr><tr><td>Nitrogen Dioxide, NO₂</td><td>5</td><td>0</td></tr><tr><td>Chlorine, Cl₂</td><td>1</td><td>0</td></tr><tr><td>Hydrogen Cyanide, HCN</td><td>10</td><td>≈ 3</td></tr><tr><td>Hydrogen Chloride, HCl</td><td>5</td><td>0</td></tr><tr><td>Ethylene, C₂H₄</td><td>100</td><td>≈ 80</td></tr></tbody></table>

## Gas Standards and experimental execution

We have measured H₂ gas standards under different conditions of temperature, pressure, and moisture using the GA5000 device. The setup for an experiment at water saturation pressure (termed ‘saturated’) is shown in Figure 2. Two standard gas bottles (Messer®) were used: one containing 500 ppm of H₂, CO₂, and CH₄ in Ar, and another with 50 ppm of the same gases also in Ar. The gas standards are dry (contain less than 10 ppmv H₂O). The gas delivery setup involved connecting a thin (1/8' OD) PTFE tube from the standard gas bottle's manometer to the input port of the GA5000 device (Figure 2). A needle connected to the GA5000 sampling port was loosely inserted inside the gas delivery tube to create a gas split-splitless system. This connection ensures a smooth gas flow without compromising pressure or flow rate, allowing consistent and stable measurements. The GA5000 sampling port is always connected to a PTFE filter (0.2 µm) to prevent dust from entering the device, replicating field conditions. The gas filter induces a pressure drop that must be accounted for in the analytical chain. For experiments testing the effect of water moisture, a 300 mL gas cylinder (Swagelok®) filled with 50 mL of Milli-Q water (resistivity of 18.2 MΩ·cm) was used. The cylinder, held vertically, was flushed with the standard gas, entering from the bottom and exiting from the top after bubbling through the water. In this setup, the exhaust gas is fully saturated with water at every tested temperature. The cylinder and the GA5000 instrument were placed in a thermoregulated chamber, allowing temperature control to ±1 °C within the 25 to 80°C range. Some experiments were carried out outdoors at ambient temperatures of 10 and 17°C.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Experimental setup used in the present study to evaluate the effect of temperature, pressure, relative humidity on H₂ concentration values returned by the GA5000 multi-gas analyzer. When CO was present, the standard gas bottle was replaced by a 1 L gas bag filled with the CO-H₂-Ar mixture, and both the cylinder filled with water and the needle where removed. In such a case the GA5000 was directly pumping the gas contained in the gas bag through the PTFE filter.</figcaption></figure>

Table 3 shows all the experiments conducted, grouped by the variables tested. Each experiment was repeated three times, with a purge between each measurement to reset the system. Exceptions were experiments #5 to #8 and #16 (Tables 3 and 4), where the system was hermetically sealed (no split-splitless needle installed on the PTFE filter) and only pressure and gas composition were varied to provide specific insights into these parameters.

## Experimental Parameters

This section outlines the experimental parameters and conditions tested to assess the effectiveness of the GA5000 device (Table 3).

* **Temperature:** Measurements were conducted at different temperatures, including outdoor (10°C and 17°C), indoor (24.2°C), and in a thermoregulated chamber at 35°C and 45°C. Two experiments (#12 and #15) involved a temperature difference between the water in the cylinder and the GA5000 device, showing the device's response to high humidity, while kept at room temperature.

* **Humidity.** To evaluate the impact of humidity on H₂ measurements, a cylinder with 50 mL of Milli-Q water was used. By placing the cylinder vertically between the standard gas bottle and the GA5000 device, dry standard gases bubbled through the water, becoming saturated (100% relative humidity). The effect of water moisture was tested at different temperatures, resulting in different saturation vapor pressures. Conditions are termed "dry" when no cylinder is used and "saturated" when the cylinder is part of the system.
* **Pressure.** Tests were conducted to assess the impact of pressure on H₂ measurements by removing the gas splitting system (i.e. the needle). Pressure was tuned thanks to the manometer installed on the gas bottle. The pressure range at which the instrument returns erroneous concentration or ceases to function due to overpressure or under pressure was evaluated.

* **CO experiments.** A gas mixture containing 90% of the 500 ppmv H₂ standard and 10% of a 2010 ppmv CO standard was used to evaluate the accuracy of the GA5000 measurements. These measurements were cross-checked using a gas chromatograph. The gas mixture composition was measured with a Perkin Elmer® CLARUS 500 gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and a 2-meter-long column (RESTEK®, Shin Carbon ST 80/100) with Ar as the carrier gas. The GC was calibrated using standard gas mixtures (Ar + H₂ + He + CH₄ + CO₂ + CO + N₂ + O₂) of different concentrations injected with a calibrated gas-tight syringe. The estimated analytical error is ±3% within the 95% confidence level.

<table><caption>**Table 3.** List of experiments and experimental conditions tested</caption><thead><tr><th>#run</th><th>Initial Gas Composition</th><th>GA5000 Temperature (°C)</th><th>Water Temperature (°C)</th><th>Gas Pressure (mbar)</th><th>Relative Humidity</th><th>Water Pressure (mbar)</th></tr></thead><tbody><tr><td>1</td><td>50 ppm H₂</td><td>10.5</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>2</td><td>50 ppm H₂</td><td>22</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>3</td><td>50 ppm H₂</td><td>35</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>4</td><td>50 ppm H₂</td><td>45</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>5</td><td>50 ppm H₂</td><td>10.6</td><td></td><td>500</td><td>Dry</td><td></td></tr><tr><td>6</td><td>50 ppm H₂</td><td>10.6</td><td></td><td>750</td><td>Dry</td><td></td></tr><tr><td>7</td><td>50 ppm H₂</td><td>10.6</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>8</td><td>50 ppm H₂</td><td>10.6</td><td></td><td>1400</td><td>Dry</td><td></td></tr><tr><td>9</td><td>50 ppm H₂</td><td>22</td><td>22</td><td>1000</td><td>Saturated</td><td>26</td></tr><tr><td>10</td><td>50 ppm H₂</td><td>35</td><td>35</td><td>1000</td><td>Saturated</td><td>56</td></tr><tr><td>11</td><td>50 ppm H₂</td><td>45</td><td>45</td><td>1000</td><td>Saturated</td><td>96</td></tr><tr><td>12</td><td>500 ppm H₂</td><td>16.7</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>13</td><td>500 ppm H₂</td><td>20</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>14</td><td>500 ppm H₂</td><td>35</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>15</td><td>500 ppm H₂</td><td>45</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>16</td><td>450 ppm H₂ + 201 ppm CO</td><td>20</td><td></td><td>1000</td><td>Dry</td><td></td></tr><tr><td>17</td><td>500 ppm H₂</td><td>20</td><td>20</td><td>1000</td><td>Saturated</td><td>23</td></tr><tr><td>18</td><td>500 ppm H₂</td><td>20.2</td><td>45</td><td>1000</td><td>Saturated</td><td>96</td></tr><tr><td>19</td><td>500 ppm H₂</td><td>24.2</td><td>24.2</td><td>1000</td><td>Saturated</td><td>30</td></tr><tr><td>20</td><td>500 ppm H₂</td><td>24.2</td><td>45</td><td>1000</td><td>Saturated</td><td>96</td></tr><tr><td>21</td><td>500 ppm H₂</td><td>35</td><td>35</td><td>1000</td><td>Saturated</td><td>56</td></tr><tr><td>22</td><td>500 ppm H₂</td><td>45</td><td>45</td><td>1000</td><td>Saturated</td><td>96</td></tr></tbody></table>

## RESULTS

The results of the experiments are summarized in Table 4, showing the mean H₂ concentration and standard deviation for each tested parameter. Figures 3 and 4 illustrate the effects of temperature, water moisture, and pressure on the H₂ concentration measured by the GA5000 device for two different H₂ concentrations: 50 and 500 ppmv.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Effect of temperature, humidity (saturated or dry experiments), pressure and presence of CO (at 200 ppmv) on H₂ concentration returned by the GA5000 device using (a) the 50 ppmv H₂ standard, and (b) the 500 ppmv H₂ standard. Each point is labelled according to the corresponding experiment number. Error bars correspond to the standard deviation calculated for each set of measures.</figcaption></figure>

First, there is no clear effect of temperature on the H₂ concentration measured by the instrument within the range of 10.5 to 45.0 °C. This suggests that the temperature effect on the electrochemical cell is well balanced by the electronics or internal software. Second, relative humidity, with water saturation pressure varying from 23 to 96 mbar depending on temperature, also has no significant impact on the measurements. However, we observe a general tendency to overestimate the true H₂ concentration by about 3 ppmv for a 50 ppmv H₂ standard and 36 ppmv for a 500 ppmv H₂ standard. This positive offset is independent of temperature and relative humidity and falls within the measurement uncertainty. Third, pressure within the 500 – 1400 mbar range has only a minor effect on the measurements when below atmospheric pressure (Figure 4). Depressurization causes an overestimation of the H₂ concentration. At 500 mbar, the H₂ concentration is overestimated by 13% compared to ambient conditions: 62 ppmv versus 55 ppmv, respectively.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Effect of pressure on H₂ concentration measured with the GA5000 device using a 50 ppmv H₂ gas standard at room temperature (24.2°C).</figcaption></figure>

The time required for the instrument to return a stable H₂ concentration value as a function of temperature, relative humidity, and H₂ concentration is shown in Figure 5. The equilibration time clearly increases with H₂ concentration, from 110 seconds at 50 ppmv to 550 seconds at 500 ppmv. These equilibration times are much longer than the 60 seconds maximum stated in the GA5000 manual. Interestingly, experiments conducted under dry conditions tended to have longer stabilization times compared to those under saturated conditions.

Finally, experiments with CO alongside H₂ yielded reliable results, with H₂ concentration averaging 558 ± 33 ppmv without CO (run #13) and approximately 560 ppmv when CO (200 ppmv) was introduced (Figure 3, Table 4).
<figure><img src="image_7.png" /><figcaption>**Figure 5.** (a) Stabilization time plotted as a function of temperature for saturated/dry experiments using the 50 ppmv H₂ standard. Each point is labelled according to the corresponding experiment number. (b) Experiments #13: dry experiment with the 500 ppmv H₂ standard, representing the time it takes for a H₂ measurement to stabilize using the GA5000 device.</figcaption></figure>
<table><caption>**Table 4.** Experiments grouped according to conditions, with the total average and standard deviation calculated. N corresponds to the number of measures conducted for each set of data.</caption><thead><tr><th>#run</th><th>Initial Gas Composition</th><th>Relative Humidity</th><th>Variables</th><th>Total Average</th><th>N</th><th>Standard Deviation</th></tr></thead><tbody><tr><td>1-4</td><td>50 ppm H<sub>2</sub></td><td>Dry</td><td>Temperature</td><td>52</td><td>12</td><td>2</td></tr><tr><td>5-8</td><td>50 ppm H<sub>2</sub></td><td>Dry</td><td>Gas Pressure</td><td>58</td><td>12</td><td>3</td></tr><tr><td>9-11</td><td>50 ppm H<sub>2</sub></td><td>Saturated</td><td>Temperature, Water Pressure</td><td>53</td><td>9</td><td>2</td></tr><tr><td>12-15</td><td>500 ppm H<sub>2</sub></td><td>Dry</td><td>Temperature</td><td>540</td><td>28</td><td>14</td></tr><tr><td>16</td><td>450 ppm H<sub>2</sub> + 201 ppm CO</td><td>Dry</td><td>Gas Composition</td><td>560</td><td>1</td><td></td></tr><tr><td>17-22</td><td>500 ppm H<sub>2</sub></td><td>Saturated</td><td>Temperature, Water Pressure</td><td>510</td><td>30</td><td>42</td></tr></tbody></table>

DISCUSSION

The results show that the GA5000 multi-gas analyzer performs well for H₂ concentration measurements within its specified range. The pressure, temperature, relative humidity, and H₂ concentration ranges are compatible with conditions encountered during soil gas exploration for geologic H₂ targeting. However, the temperature correction remains unknown to the user. Even though this study confirms the accuracy and performance of the instrument within its application area, the inability to fully control the data processing procedure prevents careful assessment, calibration, and tuning of the device. Thus, while electrochemical sensors are suitable for measuring H₂ concentration in soil, we recommend using open and transparent signal processing for reliable data control.

Indeed, the variation in pressure has a slight impact on H₂ measurements, especially when below atmospheric pressure. It is important to keep the entire sensor within the same pressure since rapid differential pressure changes within the sensor can damage the sensor's diffusion membrane. As Korotcenkov et al. (2009) mentioned, sudden changes in pressure can force more gas into the sensor, producing a current transient. These transients rapidly decay to zero as normal diffusion conditions are re-established, but some transients could trigger false alarms. At 500 mbar, the H₂ concentration is overestimated by 13%, illustrating this effect. Maintaining consistent pressure conditions is vital for accurate measurements. This discrepancy highlights the need for transparent signal processing to better understand and adjust for these variations.

Relative humidity can also change the water content of the electrolyte, affecting the sensor's signal. In high-humidity environments, the electrolyte (H₂SO₄) in the electrochemical sensor may absorb water, altering its properties and the sensor's response. This study observed that relative humidity, with water saturation pressure varying from 23 to 96 mbar depending on temperature, had no significant impact on the measurements.
Moreover, the time required for the GA5000 device to return a stable H₂ concentration value increases with H₂ concentration, from 110 seconds at 50 ppmv, up to 550 seconds at 500 ppmv. These equilibration times are much longer than the 60 seconds maximum stated in the GA5000 manual. Experiments conducted under dry conditions tended to have longer stabilization times compared to those under saturated conditions. The equilibration time may have a significant impact on soil gas concentration measurements carried out in the field. Indeed, prolonged pumping time increase air dilution. This is because a probe connected to the GA5000 has to be loosely inserted in the soil in order to sample the gas (Figure 1). This probe is generally 1 m long with tiny apertures at its tip. Pumping induces a depressurization inside the probe that tends to drive air downward along the surrounding free space in the hole. Thus, H₂ concentration in soil as measured through this technique cannot be quantitative. The higher the concentration of H₂, the longueur the stabilization time and the higher the air dilution. Generally, it is the peak H₂ concentration value that is reported as stable value are rarely, if ever, measured. Therefore, the peak concentration value recorded by the GA5000 is lower than the true H₂ concentration in the porous network of the soil

Finally, experiments with CO alongside H₂ yielded reliable results, with H₂ concentration averaging 558 ± 33 ppm without CO and approximately 560 ppmv with 200 ppmv CO. This suggests that the GA5000 can accurately measure H₂ in the presence of CO, though the overall reliability and accuracy depend on the precise control of all influencing factors.

While the GA5000 multi-gas analyzer is suitable for measuring H₂ concentrations within its specified range, users must be aware of the limitations and potential sources of error. Transparent and open signal processing is essential for reliable data control. Furthermore, consideration of all factors impacting gas composition, including drilling-induced H₂ generation and the source of H₂, is crucial. Future research should focus on robust diffusive gas flux measurement techniques and integrating other proxies such as radon or noble gases to improve the accuracy and reliability of soil gas surveys for geologic H₂ exploration. When measuring soil gas composition, particularly H₂ concentration, it is important to use a reliable and accurate instrument, but also to consider all processes that can affect gas composition. Drilling in soil and rocks to introduce sampling probes, for example, may artificially generate H₂. Although outside the scope of this study, a clear assessment of the impact of artificial H₂ generation is crucial to avoid inaccurate measurements and misinterpretations of hydrogen concentrations and fluxes.

It is also evident that H₂ concentration alone does not provide information about the source of H₂. Care must be taken not to conclude that locally encountered concentrations indicate potentially recoverable geologic H₂ resources, whether as flux or stock. Like hydrocarbon gas seepage, defining surface H₂ concentration or flux levels that indicate potentially economic H₂ resources is misleading. Therefore, soil gases must be used with caution, and other indicators are definitely needed to decipher microbial and abiotic sources. Despite the importance of geologic H₂ as a clean fuel or energy source, the current understanding of geologic H₂ occurrences is limited, and recoverable and economic resources of geologic H₂ have not been adequately quantified.

## CONCLUSION

This study aimed to evaluate the GA5000 device's reliability in measuring H₂ gas concentrations under controlled conditions, considering temperature, pressure, and humidity variations. The results highlight the device's ability to deliver accurate H₂ measurements within specified temperature and pressure ranges, with minor fluctuations. However, longer
stabilization times than those stated in the user manual, potential underestimation of gas concentrations in high-humidity environments, and the impact of rapid pressure changes on measurements were noted.

Despite challenges in assessing the GA5000's reliability for soil gas surveys under relevant environmental parameters, the user cannot control every stage of the signal processing, especially temperature correction. It is recommended to use transparent and open electrochemical sensors that allow full control over data processing. Additionally, the artificial generation of H₂ during drilling must be considered when measuring soil gas concentration.

Above all, it is important to remember that the presence of H₂ in soil can result from various processes and cannot be used as definitive proof of deeply sourced H₂ degassing. Moving forward, future research on soil gas surveys for H₂ exploration should focus on developing robust diffusive gas flux measurement techniques and integrating other proxies such as radon or helium.

## ACKNOWLEDGEMENTS

This project has received funding from the French National Research Agency (grant agreement ANR-20-CE01-0020). L.T acknowledges support from the Institut Universitaire de France.

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