# Sensing Hydrogen Seeps in the Subsurface for Natural Hydrogen Exploration
## 2 Materials and Methods
### Sensor Linearity and Effect of Methane  
Hydrogen gas standards were prepared by mixing with nitrogen using a flow controller and Figure 2 displays the experimental setup. To generate the reference gas, we used three cylinders each attached to its own mass flow controller (MFC). The MFCs were attached to a three to one manifold so the gas exiting the manifold was mixed according to the flow of each individual MFC.  
Table 1 shows the gases used for the sensor testing and calibration, which are as follows: pure nitrogen (supplied by Coregas as 5.0 grade) (A), 5% hydrogen in nitrogen (supplied by BOC as 5 \(\pm\) 0.25% H\({}_{2}\) in N\({}_{2}\) balance) (B), and 1% methane in nitrogen (supplied by BOC as 1.00 \(\pm\) 0.05% methane in N\({}_{2}\) balance) (C). The mass flow controllers were of the red-y series from Vogtlin. The MFCs are capable of very precisely controlling the flow of gas coming out of each cylinder in the range from 0 to 1000 mL/min. By regulating the flow rates on each of the three different MFCs, the concentration of methane and hydrogen at the manifold outlet is controlled (see Table 2). The gas mixture is then sent to a buffer chamber so that the methane concentration can be measured with the methane sensor (Aeris Pico natural gas analyzer, Hayward, CA, USA). The output from the methane sensor is sent to a second buffer chamber so the GA5000 can measure the hydrogen concentration. The buffer chambers are not hermetically sealed. Positive pressure inside the chamber ensures that there is no ingress of atmospheric air, which is achieved by setting the flow from the manifold greater than the flow coming into the methane sensor, and likewise the flow coming out of the methane sensor is higher than the flow going into the GA5000. The effect of methane on the hydrogen gas reading was tested by studying the GA5000 sensor response as a function of methane concentration. In addition, studies were performed to investigate the effect of sensor drift and stability.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Composition of Cylinder** & **A** & **B** & **C** \\ \hline Nitrogen & 100\% & 95\% & 99\% \\ Hydrogen & 0\% & 5\% & 0\% \\ Methane & 0\% & 0\% & 1\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Calibration gas mixtures with different amounts of nitrogen, hydrogen, and methane.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Experiment** & **Cylinder Flow Rates (mL/min)** & **Theoretical Manifold H\({}_{2}\)** & **Theoretical Manifold CH\({}_{4}\)** \\
**Number** & **A** & **B** & **C** & **Concentration (ppm)** & **Concentration (ppm)** \\ \hline
1 & 1000 & 0 & 0 & 0.0 \\
2 & 1000 & 0 & 50 & 0.0 & 469.8 \\
3 & 1000 & 0 & 100 & 0.0 & 897.5 \\
4 & 1000 & 0 & 200 & 0.0 & 1647.2 \\
5 & 1000 & 2 & 0 & 99.8 & 0.0 \\
6 & 1000 & 2 & 50 &  
Figure 2: (**a**) Experimental setup flow diagram showing different components of the setup and (**b**) photograph of the setup including: cylinders (bottom), mass flow controller in red, computer to control the mass flow controller, GA5000 (middle left), and methane sensor (far left).