In modern biogas analysis, precise measurement of hydrogen sulfide (H2S) is crucial for plant protection and process optimization. As an operator, you are often faced with a choice between two established technologies: the electrochemical sensor (EC sensor) and non-dispersive ultraviolet absorption photometry (NDUV). While the EC sensor is considered a cost-effective classic, NDUV technology sets new standards in long-term stability and precision.
Electrochemical sensors: Proven and cost-effective
Electrochemical sensors (EC sensors) have been the standard solution for decades in ambient air monitoring and process gas analysis. Their operating principle is based on a chemical reaction: H2S molecules diffuse through a gas-permeable membrane to the working electrode and trigger an electrochemical redox reaction there. The resulting current flow is directly proportional to the H2S concentration.
Strengths of the EC sensor in practice
For many applications in biogas plants, EC sensors are a practical solution:
- High cost-effectiveness: EC sensors are unbeatable in terms of purchase price, making them ideal for simple monitoring tasks.
- Flexible measuring range: Thanks to modern methods such as micropulse operation or gas dilution, these sensors can also be used in raw biogas applications with concentrations of up to 10,000 ppm.
- Suitable for slow processes: Because anaerobic processes respond slowly from a biological standpoint, measurement intervals of several hours are often entirely sufficient—a scenario in which the EC sensor fully plays to its strengths.
Where the technology reaches its limits
Despite the advantages, operators must take the technological limitations of EC sensor technology into account. A key factor is sensor aging. Because the sensors are subject to continuous chemical wear, an annual replacement is mandatory—often significantly sooner under high loads. This results in recurring material costs, calibration effort, and operational interruptions.
In addition, the response time (t90 time) of ≥ 90 seconds is often too slow for fast control processes or for detecting sudden filter breakthroughs. Another risk is cross-sensitivities: While CH4, CO2, or NH3 usually have no influence, hydrogen (H2) and mercaptans in particular can cause massive interference signals. Even in the trace range (below 1–5 ppm), the EC sensor physically reaches its detection limit.
Technology selection by requirement: When NDUV, when EC?
The decision for a technology depends largely on the measuring point and the operating objective. The following overview serves as a guide:
| Requirement / measuring point | EC Sensor | NDUV |
|---|---|---|
| Raw gas (conc. < 5,000 ppm, interval > 3 h) | Cost-effective & proven | Technically superior, higher investment |
| Cyclic measurement (e.g., 3x daily) | Annual replacement mandatory | UV LED: years without replacement |
| After biological desulfurization (50–500 ppm) | Limited (slow, cross-sensitivity) | Fast (< 15 s), high selectivity |
| Fine desulfurization / trace range | Not suitable | Only option (DL 0.25 ppm) |
| Multiple measuring points (switching) | High calibration effort per sensor | One module for all measuring points |
UV LED technology: The economically superior alternative
When considering life-cycle costs, the picture often shifts in favor of NDUV technology, especially when using UV LED modules.
A practical example illustrates this: In a biogas plant with three measurements per day, an EC sensor must be replaced after no more than 12 months due to chemical aging. A UV LED module, by contrast, is active for only a few minutes per measurement. With a source lifetime of up to 180,000 operating hours, this results in a service life of several decades.
If the end of service life is nevertheless reached, it is not the entire module that needs to be replaced, but only the UV LED unit. The higher purchase costs of an NDUV unit often pay for themselves after a short time thanks to the elimination of annual sensor replacements and minimized calibration effort. Please feel free to contact us for further information!
Conclusion
For simple, time-spread measurements in raw gas, the EC sensor remains a solid choice. However, if high selectivity, trace-range measurements, or low-maintenance long-term operation are required, NDUV technology is the technically and economically superior solution. Fresenius Umwelttechnik offers the right expertise for both requirements to monitor your plant efficiently and safely. Contact us for your customized solution!
