Development of Volumetric and Manometric Hydrogen Gas Sensors for Measuring Gas Emission and Diffusivity in H₂ Charged Polymers

1. INTRODUCTION

Hydrogen is a clean and sustainable energy source [1-9] capable of replacing traditional fossil fuels and is gaining a critical position in the global renewable energy market because of its potential to reduce CO_x and NO_x emissions. However, hydrogen is highly explosive and flammable [10- 21] when present at concentrations between 4% and 75% in air; thus, safety management is essential in the areas of production, transportation, and storage applications. For the measurement of hydrogen gas concentration and leakage, the H₂ gas sensor is normally employed. Therefore, the performance of the H₂ gas sensor for the immediate and sensitive detection of low concentrations of hydrogen to prevent leakage from hydrogen infrastructure has become a critical challenge in real-time gas-sensing technology.

Polymer materials play a significant role in hydrogen refueling stations charged with high-pressure H₂ [22-43]. In particular, polymer-based materials such as O-ring seals, gaskets, liner materials, and nonmetallic pipelines are widely used in hydrogen environments. However, these polymers are subjected to harsh conditions, including wide temperature and pressure fluctuations. These conditions can lead to seal damage, insufficient contact with the groove, and gas permeation through the polymer seals, all of which can result in hydrogen leakage [44-50].

Additional hydrogen leakage is caused by the degradation of polymer materials [45,51,52]. Seals and gaskets made of polymer materials used in hydrogen refueling stations are subjected to harsh conditions, including high and low temperatures and repeated pressure cycles. Over time, these materials degrade, weakening their physical properties and increasing their gas permeabilities, which can lead to long-term hydrogen leakage. This can undermine the safety of the hydrogen infrastructure and requires highly reliable real-time gas detection in polymer materials.

Therefore, hydrogen gas sensors are vital for accurately measuring hydrogen concentrations and leakage across applications where hydrogen is produced, stored, and used, such as fuel cells, storage facilities, and transportation systems [53,54].

Hydrogen gas sensors rely on various sensing technologies that have unique advantages and limitations. Infrared (IR) sensing [55-63] measures hydrogen through its IR wavelength absorption properties, which are effective in specific environments but are often limited by their size, high cost, and power consumption, making them less suitable for compact and fieldbased applications. Semiconductor-based sensors [64-75] detect hydrogen by measuring changes in the resistance or conductivity on the sensor surface, which is often sensitive to temperature and humidity variations. These sensors may also respond to other gases, thereby posing selectivity challenges and potential long-term accuracy drifts. Catalytic combustion sensors [76-82] detect hydrogen by measuring the heat from hydrogen combustion on the catalyst surface. However, these sensors require oxygen for operation and lose sensitivity as the catalyst degrades, thereby limiting their use in oxygen-free or long-term applications. Optical spectroscopy [83-98] offers high-precision hydrogen detection in controlled laboratory settings. However, it is generally expensive, large, and requires frequent calibration, making it impractical for real-time or onsite applications.

Electrochemical hydrogen sensors [99-115] offer high sensitivity and selectivity, enabling the detection of low hydrogen concentrations. They are compact and energy-efficient, making them suitable for portable applications. However, these sensors can be influenced by factors such as temperature, pressure, and humidity. Periodic calibration may be required to maintain accuracy. Finally, gas chromatography [99,101,103,106,116-127] and mass spectrometry [128-140] provide highly precise analyses that are commonly used in research and laboratory settings. Mass spectrometry identifies gas molecules based on their mass after ionization, whereas gas chromatography separates and analyzes individual components in gas mixtures.

However, these sensing systems are expensive, require regular maintenance, and have a low processing speed. This renders them unsuitable for real-time field monitoring. In addition, conventional hydrogen gas-sensing technologies provide precise measurements but face challenges in terms of size, sensitivity, calibration, and environmental stability. These limitations highlight the need for advanced hydrogen sensors that are compact and exhibit rapid response times, high sensitivity, and robust performance across various field conditions to ensure safety in real-time hydrogen monitoring.

Thus, we developed two types of gas sensors with good performance. First, a sensor system for real-time hydrogen detection was proposed utilizing a volumetric measurement (VM) method based on an image-processing algorithm [5,141]. The system aims to provide real-time monitoring by connecting it to a computer via a GPIB interface and using a diffusion-permeation analysis program to accurately measure the H₂ concentration/diffusion with insensitivity to temperature and pressure changes.

Second, a simple H₂ sensor based on a manometric measurement (MM) method was developed using a portable USBtype data logger to measure and record the pressure/temperature in a sample container and a diffusion analysis program [142-144]. This portable detection system can easily measure the gas concentration/diffusion in a polymer specimen on-site in real time without any chemical reactions.

The developed H₂ sensor system is versatile for detecting and characterizing H₂ gas. It effectively measures the H₂ adsorption and diffusion in polymer materials in real time. The sensor exhibited reliable performance in terms of sensitivity, stability, and response time. The sensor technology presented in this study offers a compact, portable solution capable of replacing large-scale equipment with potential applications not only in hydrogen infrastructure safety management but also in the hydrogen gas industry for environmental monitoring.

The principles, procedures, results, and characteristics of each method were compared. Ultimately, these gas-sensing methods provide valuable insights into the H₂ transport properties, leakage, and sealing capabilities of rubber materials and O-rings under high-pressure conditions, which are applicable in H₂ fueling stations and H₂ infrastructure.

2. EXPERIMENTAL

2.2 . Volumetric Measurements for H₂ Concentration from Charged Polymers

A volumetric measurement technique to assess the hydrogen concentration and diffusion was developed. This method involves measuring the concentration of hydrogen released from a specimen after exposure to high pressures and decompression. Fig. 1 illustrates the volumetric analysis system used to quantify the hydrogen released at room temperature. The system consists of a chamber for hydrogen exposure and a graduated cylinder immersed in a water container.

Fig. 1.

Volumetric analysis system to measure the H2 gas released by a specimen after exposure to high-pressure gas and decompression. (a) Specimen charged with gas in a high-pressure chamber. (b) After decompression in the chamber, the specimen was loaded into upper air space of graduated cylinder. The cylinder was immersed in a water container, and H2 emission measurements were performed. Blue in cylinder represents water. The ● in the cylinder represents the H2 gas emitted from charged sample.

After exposure to the high-pressure gas and decompression, the specimen was placed in the upper air space of the cylinder, as shown in Fig. 1. Hydrogen released from the specimen caused a gradual decrease in the water level within the cylinder. Consequently, the pressure (P) and volume (V) of the gas inside the cylinder varied with time. The gas within the cylinder adheres to the ideal gas law, expressed as PV = nRT, where R is the gas constant (8.20544 × 10^-5 m³·atm/(mol·K)), T represents the gas temperature inside the cylinder, and n denotes the number of moles of H₂ gas released into the cylinder. The variations in pressure P(t) and volume V(t) of the gas in the cylinder can be described as [145]

$\[ P\left(t\right)=P_o\left(t\right)-\rho gh\left(t\right),V\left(t\right)=V_o-V_S-V_h\left(t\right) \]$

(1)

where P_o represents the external pressure surrounding the cylinder, g is the gravitational acceleration, and ρ signifies the density of distilled water. The h(t) describes the water level within the graduated cylinder over time, and V_o is the combined volume of gas and water inside the cylinder measured relative to the water level in the container. V_h(t) indicates the time-dependent volume of water in the cylinder, and V_s represents the volume occupied by the sample.

The amount of hydrogen gas released by the polymer specimen was quantified by tracking the water level position [V_h(t)] over time. Consequently, the total number of moles of gas emitted [n(t)] was calculated by measuring the total gas volume [V(t)] in the cylinder, which corresponds to the decrease in the water level, as expressed by [145]

$\[ \begin{array}{l}\begin{array}{c}n\left(t\right)=\frac{P\left(t\right)V\left(t\right)}{RT\left(t\right)}=\frac{P\left(t\right)\left[V_A+V_H\left(t\right)\right]}{RT\left(t\right)}=\frac{P_0[1+\beta (t\left)\right]\left[V_A+V_H\left(t\right)\right]}{R{T}_0[1+\alpha (t\left)\right]}\\ \cong \frac{P_0}{R{T}_0}\left[V_A+V_H\left(t\right)+V\left(t\right)\left(\beta \right(t)-\alpha (t\left)\right)\right]\\ =n_A\left(t\right)+n_H\left(t\right),\end{array}\\ \begin{array}{c}with n_A\left(t\right)=\frac{P_0}{R{T}_0}{V}_A,\\ n_H\left(t\right)=\frac{P_0}{R{T}_0}\left[V_H\left(t\right)+V\left(t\right)\left(\beta \right(t)-\alpha (t\left)\right)\right]\\ \alpha \left(t\right)=\frac{T\left(t\right)-T_0}{T_0},\beta \left(t\right)=\frac{P\left(t\right)-P_0}{P_0}\end{array}\end{array} \]$

(2)

where T_o represents the initial temperature of H₂ in the cylinder, and P_o is the pressure of H₂ in the cylinder. V(t) is the total volume consisting of the remaining initial air volume (V_A) and the released H₂ volume [V_H(t)], such that V(t) = V_A + V_H(t). n_A denotes the initial mole number of air, and n_H(t) refers to the time-dependent mole number of H₂ corresponding to the increase in hydrogen volume due to its release. Therefore, n_H(t) was converted into the hydrogen concentration, [C(t)] emitted per unit mass from the specimen as follows:

$\[ \begin{array}{c}C\left(t\right)[wt\cdot ppm]\\ =n_H\left(t\right)\left[mol\right]\times \frac{m_{H2}\left[\frac{\mathrm{g}}{\mathrm{m}\mathrm{o}\mathrm{l}}\right]}{m_{sample }\left[\mathrm{g}\right]}\times {10}^6\\ =\frac{P_0}{R{T}_0}\left[V_H\left(t\right)+V\left(t\right)\left(\beta \right(t)-\alpha (t\left)\right)\right]\left[mol\right]\times \frac{m_{H2}\left[\frac{\mathrm{g}}{\mathrm{m}\mathrm{o}\mathrm{l}}\right]}{m_{sample }\left[\mathrm{g}\right]}\times {10}^6\end{array} \]$

(3)

The molar mass of hydrogen gas, m_H2[g/mol], was 2.016 g/mol, and m_sample is the specimen mass. In Eqs. (2) and (3), the time-dependent mole number of H₂, n_H(t), was converted into the H₂ mass concentration, [C(t)], using the factor . $$ k=\left[\frac{m_{H_2}}{m_{sample }}\right].$$n_H(t) and C(t) are affected by fluctuations in the temperature and pressure in laboratory environments. To ensure precise measurement, it is essential to compensate for these variations. This compensation can be achieved through automated programs according to Eq. (3). That is., the term V(t)(β(t)-α(t)) indicates the volume change in V_H(t), caused by the temperature and pressure variations. The compensation indicates the application of V(t)(β(t)-α(t)) calculation in Eq. (3). Thus, the insensitivity of the sensor to variations in temperature and pressure indicated the application of automatic compensation by the program.

The release of hydrogen gas was tracked by monitoring changes in the water level using an image-processing algorithm or digital camera.

2.3 . Pressure Measurement for H₂ Concentration from Charged Polymers

Fig. 2 shows a schematic of the manometric measurement system used to determine the concentration and diffusivity of the hydrogen gas released from the specimen at room temperature. The setup included a high-pressure chamber for H₂ gas exposure, as shown in Fig. 2 (a), and a rectangular specimen container equipped with a USB-type data logger and rubber seal, as shown in Fig. 2 (b). The ELP sensors used to measure the pressure and temperature are commercial data loggers capable of simultaneously recording atmospheric pressure and temperature.

Fig. 2.

Schematic of manometric analysis system used to measure H2 gas uptake and diffusivity in charged sample, utilizing a commercial pressure/temperature logger after high-pressure exposure and decompression. (a) The sample is charged with gas inside the high-pressure chamber. (b) After decompression, the charged sample is loaded in a rectangular specimen container. H2 gas emission is then measured using a data logger in the container. The ● in the specimen container represents the H2 gas emitted from charged sample.

After exposure and decompression in the high-pressure chamber, the specimen was transferred to a rectangular container, as shown in Fig. 3 (b). As the hydrogen gas was emitted from the sample, the pressure inside the vessel increased over time. Consequently, both the pressure (P) and temperature (T) of the gas within the specimen container changed over time. The hydrogen gas inside the container followed the ideal gas law: PV = nRT, where R is the gas constant with 8.20544 × 10^-5 m³ ·atm/(mol·K), and n represents the number of moles of the released gas molecules inside the specimen container.

Fig. 3.

Diffusion parameters determined in cylindrical LDPE, HDPE, EPDM, and NBR specimen using volumetric method with image-processing algorithm and a digital camera. (a-d): Measured volume (blue filled circle) converted from the measured water level and H2 emission (black open square) derived from the measured H2 volume in units of wt·ppm for four specimens. The difference in patterns between the measured volume and H2 emission is due to temperature/pressure compensation applied using Eq. (2). The blue line represents the fitted results by Eq. (6), with gas diffusivity (D) and the total gas uptake (C∞) indicated by a blue arrow. Here, R is the radius and T is the thickness of the cylindrical specimen.

The number of moles of gas emitted from the charged specimen was obtained by measuring the increase in pressure [P(t)]versus time using manometric measurements at a constant container volume. Thus, the total number of moles [n(t)] obtained by measuring the increased gas pressure [P(t)] due to the gas emitted in the cylindrical container is described as follows [4,144]:

$\[ \begin{array}{l}n\left(t\right)=\frac{P\left(t\right)V_0}{RT\left(t\right)}=\frac{P\left(t\right)V_0}{RT\left(t\right)}=\frac{\left[P_0+\mathrm{\Delta }P\left(t\right)\right]V_0}{R{T}_0[1+\alpha (t\left)\right]}\\ \cong \frac{P_0{V}_0+\mathrm{\Delta }P\left(t\right)V_0}{R{T}_0}\left[1-\alpha \left(t\right)\right]=n_0+\mathrm{\Delta }n\left(t\right),\\ \begin{array}{c} with n_0=\frac{P_0{V}_0}{R{T}_0},\Delta n\left(t\right)=\frac{V_0}{R{T}_0}\left[\mathrm{\Delta }P\left(t\right)-\alpha \left(t\right)P_0-\right.\\ \alpha \left(t\right)\Delta P\left(t\right)]\end{array}\\ \mathrm{\alpha }\left(t\right)=\frac{T\left(t\right)-T_0}{T_0}\end{array} \]$

(4)

where T₀, V₀ and P₀ are the temperature, initial volume, and initial pressure of the air, respectively, at the initial time inside the cylinder. P(t) is the sum of remaining initial air pressure (P₀) and time-varying released gas pressure [ΔP(t)] from specimen, i.e., P(t)=P₀+ΔP(t), n₀ is remaining initial air mole, and Δn(t) is the time-varying gas mole corresponding to gas pressure increase from the released gas. α(t) is the change rate of temperature with respect to T₀. Thus, Δn(t) is transformed into the released gas concentration [ΔC(t)] per mass for specimen as [144]:

$\[ \begin{array}{c}\Delta C\left(t\right)[wt\cdot ppm]=\Delta n\left(t\right)\left[mol\right]\times \frac{m_g\left[\frac{\mathrm{g}}{\mathrm{m}\mathrm{o}\mathrm{l}}\right]}{m_{specimen }\left[\mathrm{g}\right]}\\ =\frac{V_0}{R{T}_0}\left[\mathrm{\Delta }P\left(t\right)-\alpha \left(t\right)P_0-\alpha \left(t\right)\Delta P\left(t\right)\right]\left[mol\right]\times \\ \frac{m_g\left[\frac{\mathrm{g}}{\mathrm{m}\mathrm{o}\mathrm{l}}\right]}{m_{specimen }\left[\mathrm{g}\right]}\times {10}^6\end{array} \]$

(5)

where m_g[g/mol] is the molar mass of the hydrogen gas used; for H₂, m_H2[g/mol] = 2.016 g/mol and m_specimen is the sample mass. The first term, ΔP(t) in Eq. (5), is the increase in pressure emitted by the gas from the sample. The second term, -α(t)P₀, is the pressure change caused by temperature variation [α(t)]. The third term, -α(t)ΔP(t) is the pressure change due to both temperature variation [α(t)] and [ΔP(t) ] pressure increase .

According to Eqs. (4) and (5), the time-varying hydrogen gas mole, , is transformed to hydrogen gas mass concentration, [ΔC(t)], by $$ k=\left[\frac{m_g}{m_{specimen }}\right].$$. Δn(t) and ΔC(t) are affected by both temperature and pressure variations. Thus, we compensated for the variations caused by the changes in temperature and pressure.

3. RESULTS AND DISCUSSIONS

3.2 . Manometric Measurement

After decompressing the specimens enriched with hydrogen under high pressure, the released H₂ gas concentration and diffusivity were determined using the MM method for the specimens loaded in a cylindrical container (Fig. 2). According to Eq. (5), the H₂ gas concentration in the MM method was obtained from the temperature/pressure using a commercial USBtype manometer. Similar to Fig. 3, Figs. 4 (a) through (d), the H₂ gas volume (blue filled circle) was transferred from the measured pressure and hydrogen emission content (black open square) in units of wt·ppm for the cylindrical LDPE, HDPE, EPDM, and NBR specimens. The right side of Figs. 4 (a) through (d) represents the diffusion parameters, D and, C_∞ derived using an analysis program by Eq. (6). The blue lines in Figs. 4 (a) through (d) represent the values fitted using Eq. (6), with the H₂ diffusivity and total hydrogen uptake marked by a blue arrow.

As shown in Figs. 3 and 4, single-mode hydrogen emission behaviors in LDPE, HDPE, EPDM, and NBR were observed in time-varying hydrogen emission measurements. The singlemode hydrogen emission in all specimens investigated in this study was attributed to gas diffusion into the amorphous phase. Moreover, the solubility S is obtained from the H₂ uptake versus pressure plot in Figs. 3 and 4 as follows [142,149-151]:

$\[ S\left[\frac{mol}{m^3\cdot MPa}\right]=\frac{\left(C_{\infty }/\mathrm{ }\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e} \right)\left[\frac{wt\cdot ppm}{MPa}\right]\times {10}^6\times d\left[\frac{g}{m^3}\right]}{m_g\left[\frac{g}{mol}\right]} \]$

(7)

m_g is the molar mass of H₂ gas (2.016 g/mol), and d is the sample density. The solubilities and diffusivities of the two sensors for H₂ gas in the LDPE, HDPE, EPDM, and NBR samples are listed in Table 1.

Table 1.

Solubility and diffusivity of hydrogen gas determined in four different polymers.

As shown in Table 1, the solubility and diffusivity in the four specimens for the two sensors exhibited a 10% relative expanded uncertainty for the measured value. The solubility and diffusivity results determined from both VM and MM were consistent with the estimated relative uncertainty.

3.3 . Performance Comparison between Volumetric and Manometric Gas Sensors

We evaluated the performances of the two H₂ gas-sensing systems using a manometric data logger sensor and a volumetric image sensor. The results of the performance tests, including sensitivity, resolution, stability, measurable range, response time, and figure of merit are presented in Table 2. The sensitivity of the volumetric and manometric sensors is defined as the change in mass concentration relative to the change in volume and pressure, respectively. The obtained sensitivities were 16.43 wt·ppm/mL for the volumetric sensor and 11.96 wt·ppm/hPa for the manometric sensor. A sensor with a higher sensitivity typically provides better resolution. The resolution of the volumetric sensor was reflected by a minimum measurable volume of 0.005 mL, which corresponded to a mass concentration of 0.08 wt·ppm. The resolution of the manometric sensor was reflected by the minimum measurable pressure (0.01 hPa), corresponding to a mass concentration of 0.12 wt·ppm. To further improve the resolution, it is possible to reduce the inner volume of the graduated cylinder and sample container or increase the number of specimens. This adjustment leads to better sensitivity and resolution.

Table 2.

Performance comparisons for two sensor systems using volumetric and manometric methods

The stability of the two sensor systems was quantified by the standard deviation of measurements taken over 36 h after gas emission from the specimen ceased. This was less than 0.2% of the mass concentration for both sensors. The measurable range was defined as the maximum allowable concentration per specimen weight within a container with an inner volume. For both sensors, the measurable range was less than 1500 wt·ppm, which can be adjusted by varying the sample weight and specimen container volume. The volume and pressure responses of the gas sensor are almost instantaneous, occurring within 1 s of gas emission. The FOM is defined as the standard deviation between the measured data and theoretical value calculated using Eq. (3). An FOM value below 0.4% for the volumetric sensor and 0.7% for the manometric sensor indicated good agreement between the theoretical and measured values. Based on performance testing, the specifications (sensitivity, resolution, and FOM) of the VM sensor were superior to those of the MM sensor.

Fig. 4.

Diffusion parameters determined in cylindrical LDPE, HDPE, EPDM, and NBR specimens by manometric method using USB-type manometer and specimen container. (a-d): Measured pressure (blue filled circle) and H2 emission (black open square) transferred from measured H2 pressure in units of wt·ppm for four specimens. The difference in patterns between the measured pressure and H2 emission is due to temperature/pressure compensation applied using Eq. (4). The blue line represents the fitted results by Eq. (6), with gas diffusivity (D) and the total gas uptake (C∞) marked by a blue arrow. Here, R is the radius and T is the thickness of the cylindrical specimen.

4. CONCLUSIONS

Hydrogen gas sensors are important for ensuring the safety and protection of hydrogen infrastructure. To meet these requirements, we developed two hydrogen-sensing systems that utilize volumetric measurements with a graduated cylinder, image-processing algorithm, and digital camera. Changes in the pixel position of the water level in the cylinder due to the released hydrogen were correlated with the emitted H₂ gas volume, enabling precise H₂ content measurements. By incorporating a diffusion-permeation analysis program, this volumetric image hydrogen sensor can effectively detect H₂ concentrations and diffusivity from H2-enriched specimens under high-pressure conditions.

A simple manometric gas-sensing system was developed to characterize hydrogen gas. This technique, based on pressure measurements in a gas-charged specimen container and using a simple commercial data logger, leveraged the increase in gas pressure caused by H₂ gas released from high-pressure polymer specimens to determine the H₂ gas concentration and diffusivity via a diffusion analysis program.

The two effective and portable hydrogen sensor systems demonstrated several performance metrics: a low detection limit of H₂ content, a measurable range of up to 1500 wt·ppm, stability of 0.2%, and a rapid response time within 1 s. Furthermore, the insensitivity of the two sensors to temperature and pressure fluctuations makes them suitable for gas detection. High-performance systems with volumetric and manometric methods demonstrated successful real-time measurement and characterization of H2. They effectively measure the uptake and diffusivity of gases emitted from specimens with expanded uncertainty by considering the influencing factors. The main features of the two developed gassensing methods are as follows.

- Cost-effective and simple techniques: These two methods offer inexpensive and straightforward approaches for measuring the gas uptake and diffusivity in H2-charged polymers.

- Stability with respect to temperature and pressure: The two methods are stable against variations in temperature and pressure and are suitable for diverse specimen sizes and shapes.

- Precise calculation: Each method enables exact calculation by an analysis program.

- Adjustable sensitivity and range: Volumetric and manometric methods allow for adjustable sensitivity, resolution, and measurement range.

- Visible H₂ release monitoring: The volumetric method offers visual monitoring of gas release and leakage through changes in the water level.

The solubility and diffusivity results obtained from both the volumetric and manometric measurements were consistent with the estimated relative uncertainty. These two complementary methods are well suited for effectively measuring the hydrogen gas transport properties in sealing elastomers and can be applied to evaluate the permeation characteristics of rubber materials and O-rings under high-pressure conditions, making them ideal for use in hydrogen fueling station applications.

Acknowledgments

This research was supported by the Development of Reliability Measurement Technology for Hydrogen Fueling Stations funded by the Korea Research Institute of Standards and Science (KRISS-2025-GP2025-0014)

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