Gas Spectrometer Wavelength Modulation and Direct Absorption Integration
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Solution Overview
Problem
Current gas concentration determination methods, such as direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS), face challenges in achieving high functional safety due to non-random errors from external influences like intensity fluctuations and pressure changes, which can lead to increased costs and effort when using both methods simultaneously.
Innovation Solution
A method that generates an artificial measurement curve by shifting stored measurement points in time, allowing for evaluation using both DAS and WMS methods with a single laser tuning through a wavelength range, thereby eliminating the need for high-frequency modulation and enabling independent error reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If both DAS and WMS methods are used simultaneously to improve functional safety, then measurement reliability is improved, but device complexity and cost increase
Solution Approach 1:
The patent combines DAS and WMS methods into a single integrated measurement system that performs both evaluation methods sequentially using the same hardware components. The control unit executes DAS evaluation first, then WMS evaluation on the stored measurement data, merging two previously separate measurement systems into one unified device that achieves enhanced functional safety without proportionally increasing complexity
Solution Approach 2:
The measurement system is designed to perform multiple functions using the same hardware infrastructure. The single laser source, measurement volume, and detector are used for both DAS and WMS evaluations, making the system universal. The control unit dynamically switches between evaluation methods, allowing one system to serve multiple measurement purposes and reducing the need for separate dedicated hardware for each method
2Measurement precision
If high-frequency modulation is applied in WMS to improve measurement precision, then detection sensitivity is improved, but device complexity and potential error sources increase
Solution Approach 1:
The measurement data is stored in memory during the laser tuning process, creating a recorded dataset that can be evaluated multiple times with different methods. This preliminary storage action allows the WMS evaluation to be performed on pre-captured data without requiring real-time high-frequency modulation during the actual measurement, simplifying the modulation requirements while maintaining precision
Solution Approach 2:
The patent creates an artificial measurement curve by copying and processing the stored measurement points. The WMS evaluation works on this copied representation of the original measurement data, applying derivative-based analysis to the recorded intensity values. This copying approach allows WMS precision to be achieved without the complexity of real-time high-frequency physical modulation, as the modulation effect is applied computationally to the data copy
3Measurement precision
If laser wavelength is tuned slowly across absorption line for DAS, then measurement accuracy is improved, but measurement speed decreases
Solution Approach 1:
The laser wavelength is tuned continuously across the absorption line at a constant speed, and the intensity is recorded continuously throughout the tuning process. This continuous recording approach ensures that no measurement data is lost, allowing the full absorption line profile to be captured in a single pass. The continuous action maintains measurement accuracy while enabling faster tuning speeds compared to step-by-step scanning methods
Solution Approach 2:
The measurement process is structured as a periodic cycle where the laser tuning and data acquisition are repeated at regular intervals. Each cycle captures a complete absorption line profile, and the periodic repetition allows for systematic data collection that maintains accuracy while improving overall measurement throughput. The periodic structure enables efficient use of the measurement system and facilitates consistent data processing
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances functional safety by providing two concentration values from a single measurement, reducing errors and costs, and allowing for higher repetition rates without sinusoidal modulation, thus achieving a higher Safety-Integrated-Level (SIL) without the drawbacks of simultaneous DAS and WMS application.
Implementation Method 1
Light is absorbed by the measurement gas in accordance with Lambert-Beer's law on tuning the current ramp when the laser passes through the range of the absorption line
Implementation Method 2
I=I0*e−α(λ)cL where α(λ) is a wavelength dependent absorption, c is the gas concentration, and L is the path via which the gas is absorbed
Implementation Method 3
The wavelength is slowly tuned across an absorption line of the measurement gas and is additionally modulated slightly with a high modulation frequency f with respect thereto, typically in a sinusoidal manner
Implementation Method 4
the absorption measurements of small frequencies (close to DC) in which the light source has large noise is displaced to high frequencies in which the shot-noise is the limiting factor. This frequency shift can improve the measurement sensitivity by three to five orders of magnitude
Implementation Method 5
When the light beam wavelength modulated in this way propagates through the measurement path an amplitude modulation of the light results from the intensity change of the laser and through the absorption of the measurement gas
Implementation Method 6
One of the AC components can be selected for the evaluation and can be evaluated in a phase sensitive method e.g. with a lock-in method. This method is also referred to as demodulation
Data Source
AI summary
The invention relates to a method of determining the concentration of a gas component comprising the steps:generating and guiding a light beam having a wavelength variable in a wavelength range through a measurement volume in which the gas component having an absorption in the wavelength range is present;tuning the wavelength range;detecting the intensity of the light beam after passage through the measurement volume;storage of measurement points during the tuning that respectively consist of a point in time and an associated intensity value, to obtain a direct absorption line;generating an artificial measurement curve from the stored measurement points by shifting the measurement points on the time axis;wherein the shift takes place so that an artificial modulation results in the wavelength time extent; andevaluating the artificial measurement curve in accordance with the method of the wavelength modulation spectroscopy and determining a first concentration value therefrom.