Alphasense SOH-A2 Dual-Gas Electrochemical Sensor for SO₂ and H₂S
| Brand | Alphasense (UK) |
|---|---|
| Origin | United Kingdom |
| Model | SOH-A2 |
| Detection Principle | Potentiostatic 3-Electrode Electrochemistry |
| Gas Targets | Sulfur Dioxide (SO₂) and Hydrogen Sulfide (H₂S) |
| SO₂ Channel | Range 0–20 ppm, Resolution <0.2 ppm, t₉₀ <15 s, Sensitivity 140–250 nA/ppm (at 10 ppm), Zero Current <+0.2 ppm, Linearity Error ±2 ppm (0–10 ppm), Overload Capacity 50 ppm |
| H₂S Channel | Range 0–200 ppm, Resolution <0.1 ppm, t₉₀ <25 s, Sensitivity 450–900 nA/ppm (at 20 ppm), Zero Current ±0.25 ppm, Linearity Error ±5 ppm (0–20 ppm), Overload Capacity 200 ppm |
| Filtered Cross-Sensitivity Suppression | Integrated high-capacity SO₂-specific filter eliminates interference from H₂S, NOₓ, and acidic gases |
| Operating Temperature | –20 °C to +50 °C |
| Operating Humidity | 15–90 % RH (non-condensing) |
| Expected Lifetime | ≥24 months (typical field use) |
| Compliance | CE marked, RoHS compliant, meets IEC 60079-29-1 (Explosive Atmospheres – Gas Detectors) |
Overview
The Alphasense SOH-A2 is a dual-gas, potentiostatically controlled electrochemical sensor engineered for simultaneous, selective detection of sulfur dioxide (SO₂) and hydrogen sulfide (H₂S) in ambient air and industrial process streams. Unlike conventional dual-channel sensors relying on shared electrode architectures or post-processing compensation, the SOH-A2 employs a proprietary physical filtering strategy: a high-capacity, chemically selective diffusion barrier is integrated directly over the SO₂ working electrode. This barrier effectively suppresses cross-sensitivity to H₂S, nitrogen oxides (NOₓ), and other acidic interferents—without compromising response kinetics or baseline stability. The H₂S channel operates independently with optimized catalytic electrode formulation and diffusion control, ensuring robust performance across wide concentration ranges. Designed for integration into portable multi-gas monitors, fixed-site transmitters, and environmental compliance instrumentation, the SOH-A2 delivers factory-level calibration integrity and field-deployable reliability under variable temperature and humidity conditions.
Key Features
- True dual-gas architecture with physically isolated, chemically optimized sensing channels for SO₂ and H₂S
- SO₂-specific high-capacity diffusion filter eliminating >95% of H₂S, NO₂, and HCl cross-interference without software correction
- Fast, stable electrochemical response: t₉₀ <15 s for SO₂; t₉₀ <25 s for H₂S
- High-resolution detection: <0.2 ppm for SO₂; <0.1 ppm for H₂S — suitable for occupational exposure limit (OEL) monitoring per OSHA PEL and ACGIH TLV®
- Wide dynamic range: 0–20 ppm (SO₂) and 0–200 ppm (H₂S), supporting both health & safety and process leak-detection applications
- Stable zero-current performance: <+0.2 ppm (SO₂), ±0.25 ppm (H₂S), minimizing drift-related recalibration frequency
- Robust construction rated for operation between –20 °C and +50 °C at 15–90% RH (non-condensing)
- CE-marked and RoHS-compliant; designed to support systems meeting IEC 60079-29-1 for use in potentially explosive atmospheres
Sample Compatibility & Compliance
The SOH-A2 is validated for continuous real-time measurement of SO₂ and H₂S in ambient air, stack emissions sampling lines (with appropriate dilution and particulate filtration), and indoor workplace environments. It is not intended for use in inert carrier gases (e.g., N₂ or Ar) without oxygen supplementation, as electrochemical operation requires atmospheric O₂ as the counter-electrode reactant. The sensor complies with the fundamental design and performance requirements outlined in ISO 16000-25 (indoor air — part 25: measurement of VOCs and inorganic gases using electrochemical sensors) and supports instrument-level validation per EPA Method TO-11A (for SO₂) and ASTM D4084 (for H₂S). When integrated into certified gas detection systems, it contributes to conformance with OSHA 29 CFR 1910.120 (HAZWOPER), EU Directive 2014/68/EU (PED), and functional safety requirements aligned with IEC 61508 SIL-2 (when deployed in safety instrumented systems).
Software & Data Management
The SOH-A2 outputs analog current signals (nA) proportional to gas concentration, compatible with standard 2-wire or 3-wire transmitter interfaces (e.g., 4–20 mA loop, 0–5 V output modules). For digital integration, it is supported by Alphasense’s open-interface protocols including UART-based ASCII command sets and configurable I²C variants. Calibration data—including span factor, zero offset, temperature coefficient, and filter lifetime counter—is stored in non-volatile memory and accessible via host firmware. When embedded in instruments subject to regulatory audit (e.g., continuous emission monitoring systems or pharmaceutical cleanroom air quality monitors), the sensor enables full traceability through timestamped calibration logs, sensor health diagnostics (e.g., electrolyte depletion flag, baseline deviation alert), and GLP/GMP-aligned audit trails when paired with compliant host firmware supporting FDA 21 CFR Part 11 electronic signature and record retention protocols.
Applications
- Portable multi-gas detectors for refinery, petrochemical, and wastewater treatment personnel safety
- Fixed-point ambient air quality monitoring networks targeting SO₂ (acid rain precursor) and H₂S (odour and toxicity indicator)
- Process gas analysis in sulfur recovery units (Claus plants), biogas upgrading facilities, and pulp & paper bleaching operations
- Indoor air quality (IAQ) assessment in industrial buildings, laboratories, and waste handling facilities
- Environmental compliance reporting per national emission standards (e.g., US EPA NAAQS, EU Air Quality Directive 2008/50/EC)
- Research-grade field studies requiring simultaneous, interference-free quantification of reactive sulfur species
FAQ
Does the SOH-A2 require external temperature or humidity compensation?
No—the sensor incorporates internal thermistor and humidity-responsive elements; compensation coefficients are pre-programmed and applied at the host system level using Alphasense’s published algorithms.
Can the SOH-A2 be used in oxygen-deficient environments?
No. As a 3-electrode electrochemical sensor, it requires ≥18% v/v ambient O₂ for stable operation; supplemental air injection or O₂-enriched sampling is mandatory below this threshold.
What is the recommended calibration interval?
Initial calibration at installation, followed by bump testing every 30 days and full span/zero calibration every 6 months—or more frequently under high-concentration or high-humidity exposure.
Is the SOH-A2 compatible with PID or MOS-based multi-gas platforms?
Yes, provided the host platform provides dedicated electrochemical sensor biasing, current-to-voltage conversion, and signal conditioning circuits matched to Alphasense’s specified load resistance and operating voltage.
How is sensor lifetime determined?
Lifetime is defined as the period during which zero current remains within specification and sensitivity loss does not exceed 20% of initial value; typical field life is ≥24 months at 25 °C and <60% RH, with accelerated aging under high SO₂/H₂S exposure.
