Hydrogen Chloride in Ambient Air – Why Precise HCl Measurement is Indispensable
Hydrogen chloride (HCl) is a colorless, pungent-smelling gas that irritates the respiratory tract even at low concentrations and can cause serious health damage with prolonged exposure. In ambient air monitoring, it is not always about protecting individual workplaces – often the question is whether HCl from a neighboring industrial plant penetrates into one’s own operating environment and leads to a measurable burden there.
Reliable, continuous HCl measurement in the low ppm range is essential for this immission monitoring. Fresenius Umwelttechnik offers a solution for this based on the Gas Analyzer 210 (GA210) with an electrochemical HCl sensor.
Application: Immission Monitoring in Ambient Air
The application described here is the monitoring of ambient air for HCl inputs from neighboring industrial plants – for example, chemical production facilities, storage sites for hydrochloric acid, or other HCl-emitting operations. The expected concentrations typically range from 0 to a few ppm. The goal is early detection of elevated HCl concentrations to initiate protective measures if necessary.
Measurement Technology Challenges in HCl Trace Gas Measurement
Measuring hydrogen chloride in normal ambient air poses special requirements. Unlike emission measurements at chimneys, the expected concentrations in ambient air typically range from 0 to 100 ppm.
• Low detection limit: Concentrations from 0.5 ppm must be reliably detected.
• High resolution: A resolution of <0.1 ppm (RMS noise) is required.
• Fast response time: The tₐ₀ response time of our EC sensor is <200 s (from zero to 25 ppm HCl) – sufficient for immission monitoring.
• Long-term stability: For continuous monitoring tasks over months.
• Environmental conditions: Fluctuating temperatures, humidity, and air pressure must not falsify the measured value.
• Cross-sensitivities: Accompanying gases such as SO₂, NO₂, CO, or NH₃ must not significantly influence HCl measurement.
The Measurement Principle: Electrochemical Sensor Technology for HCl
For HCl measurement in the low ppm range, we rely on the electrochemical measurement principle. HCl molecules diffuse through a gas-permeable membrane to the working electrode, triggering an electrochemical reaction, and the resulting current is directly proportional to the HCl concentration.
Advantages of the electrochemical measurement principle
• High selectivity: Low cross-sensitivity to CO (<0.1%), NH₃ (<0.1%), CO₂ (<0.1%), and NO (<2%). Increased cross-sensitivity to Cl₂ and NO₂ must be considered in system design.
• Excellent resolution: RMS noise <0.1 ppm.
• Wide measuring range: 0–100 ppm HCl.
• Compact design for space-saving integration.
• Fast response time: tₐ₀ <200 s (from zero to 25 ppm HCl).
Our Solution: The GA210 with Electrochemical HCl Sensor
The Gas Analyzer 210 (GA210) from Fresenius Umwelttechnik is a flexible gas analyzer that can be equipped with different sensor principles depending on the measurement task. For HCl measurement, we configure the GA210 with a high-quality electrochemical HCl sensor and specific functions for trace gas analysis.
Technical specifications of the HCl measurement configuration:
Parameter Specification
Measuring component Hydrogen chloride (HCl)
Measuring range: 0–100 ppm
Resolution: <0.1 ppm (RMS noise)
Detection limit: 0.5 ppm
Linearity ≤6 ppm error at 40 ppm full scale
Response time :(tₐ₀) <200 s (from zero to 25 ppm HCl)
Measurement interval: Secondly
Pressure range: 0.7–1.1 bar (compensated)
Pump flow rate: 4 l/min (alternatively 2 or 1 l/min)
Sensor lifetime: 12 months (guaranteed up to 80% original signal)
The GA210 is manufactured in a compact 19-inch rack-mount enclosure. Signal output via galvanically isolated 4–20 mA analog outputs and RS232; optionally Profibus DP, Ethernet Modbus TCP, Ethernet/IP, or Profinet for seamless SCADA integration.
Intelligent Functions for Stable Measurement Results
Automatic zero point adjustment via activated carbon filter
At regular intervals (e.g., daily), ambient air is passed through an integrated activated carbon filter, which adsorbs HCl and other reactive gases. The purified air serves as a zero gas reference. Zero point drift is automatically corrected – without external intervention and without test gas cylinders.
Sensor replacement instead of calibration
Our HCl sensor is guaranteed for 12 months (at least 80% of the original signal). Since the sensor in immission monitoring does not see continuously high HCl concentrations, it hardly ages. Our maintenance concept therefore provides for an annual sensor replacement – more reliable than a calculated calibration, without complex test gas calibration.
Integrated pressure and temperature compensation
The GA210 automatically compensates for weather-related air pressure fluctuations (0.7–1.1 bar) and the temperature-dependent sensitivity of the EC sensor. Operating temperature range: −30 to +50°C.
Powerful internal sample gas pump
Flow rate up to 4 l/min – ideal for longer sample gas lines when the measuring point is not directly at the analyzer.
Application Scenarios and Flexibility: Also Available as a Rental Unit
HCl monitoring is often a temporary measurement task. Therefore, we also offer the GA210 as a rental unit – pre-configured, calibrated, and ready for use, from a few weeks to several months.
Typical application scenarios:
• Immission monitoring: Protection against HCl inputs from neighboring plants.
• Incident analysis: Rapid deployment in case of unforeseen HCl releases.
• Remediation support: Continuous monitoring during dismantling work.
• Quality assurance: Verification of exhaust air purification systems.
• Chemical industry: Monitoring of hydrochloric acid and chlorine processes.
• Waste management: Air quality control for chlorine-containing waste.
• Metal processing: Monitoring of pickling and electroplating processes.
Why Electrochemical Instead of NDIR or FTIR?
For precise monitoring in the range of 0–100 ppm, the EC sensor (GA210 HCl) is the ideal technology, as it convinces with a fine detection limit of 0.5 ppm and requires a low investment with low maintenance (annual replacement). In contrast, the NDIR method is not suitable for this purpose due to its insufficient detection limit of >10 ppm. FTIR spectroscopy offers excellent accuracy (0.1–1 ppm), but is oversized for this application and associated with a very high investment.
Conclusion: Reliable HCl Immission Monitoring with a System
With the GA210, we offer a solution that combines electrochemical precision sensor technology with automatic zero point adjustment, pressure and temperature compensation, and a powerful sample gas pump: detection limit 0.5 ppm, resolution <0.1 ppm, second-by-second measured values – long-term stable and without complex test gas calibration.
Whether as a permanently installed monitoring system or a flexible rental unit – the GA210 in the HCl configuration adapts to your requirements.
Are you planning an HCl measurement in ambient air or do you need a rental unit? Contact us – we will be happy to advise you.
Frequently Asked Questions (FAQ)
Why electrochemical instead of NDIR or FTIR?
NDIR does not achieve a sufficient detection limit for HCl in the ppm range. FTIR would be technically suitable but is oversized and significantly more expensive. The EC sensor offers an optimal balance of sensitivity, compactness, and cost-effectiveness.
How does automatic zero point adjustment work?
Daily, ambient air is passed through an integrated activated carbon filter that adsorbs HCl and reactive gases. The purified air serves as a zero gas reference; zero point drift is automatically corrected.
Why sensor replacement instead of calibration?
The sensor is guaranteed for 12 months (80% original signal). Since it is hardly stressed, it ages little. Annual replacement is more reliable than calculated calibration; automatic zero point adjustment complements this.
Can the GA210 also be rented?
Yes – pre-configured and calibrated, flexible from a few weeks to several months.
What interfaces does the GA210 offer?
Standard: galvanically isolated 4–20 mA outputs and RS232. Optional: Profibus DP, Ethernet Modbus TCP, Ethernet/IP, or Profinet.
