Case Study CS-TC09 Rev. A

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The DHR is equipped with condenser microphones instead

of tuning forks or cantilever beams to detect molecular

light absorption. Quartz tuning forks with enhanced

photoacoustic spectrum work at lower pressure, but

they can be difficult for beam collimation. The condenser

microphones, although not as sensitive as cantilever

sensors, avoid much of the temperature and vibration-

sensitivity issues found in tuning fork and cantilever beam

interferometers.

Figure 3 shows the line strengths of the three gases to

measure: methane, ethylene, and ethane. A single ICL

can cover this wavelength range of absorption signals in

3344-3350 nm. Each gas has absorption peaks at different

wavelengths, but minimizing cross-interference will be

important.

Figure 3. The line strengths of methane, ethylene, and

ethane in 3344-3350 nm simulated using the HITRAN

database.1

The commercial ICL used for the multi-component

detection has a center wavelength of 3345 nm and a

dynamic current range of 20-100 mA, corresponding to a

wavelength range of 3344-3355 nm. Researchers used

Wavelength Electronics' WTC3293 temperature controller

to regulate and maintain the laser temperature at 20ºC

with high precision. The typical output power of the laser is

17.2 mW. Because the system uses microphones instead

of tuning forks and cantilever sensors, a collimating lens is

used to create a beam 3 mm in diameter with a Gaussian

power distribution. The microphones have a sensitivity of

-26 ± 3 dB. The main gas tubes are only 50 mm in length,

and the photoacoustic cell holds approximately 2.5 mL in

volume.

The photoacoustic cell is filled with sample gas for gas

detection measurements and is controlled to ambient

pressure. The output of the ICL is modulated to cover the

METHOD

Researchers from the College of Chemistry and Molecular

Engineering, Nanjing Tech University, China have

developed a photoacoustic cell to detect traces of CH

4,

C

2H4, and C2H6 using an ICL source and a differential

Helmholtz resonance cell under ambient pressure. The

setup, including the laser, photoacoustic cell, and gas flow,

can be seen in

Figure 1.

Figure 1. Schematic of the experimental workflow.

(MFC: Mass flow controller; ICL: Interband cascade laser;

PA cell: Photoacoustic cell; DAQ card: Data acquisition

card; PC: Personal computer)1

One of the main changes to the PAS design is the addition

of the differential Helmholtz resonance (DHR) cell. The

simple and low-cost technique is acoustically similar to a

mechanical oscillator composed of two mass blocks and

springs.1 The gas cell contains two identical channels with

gas moving like a piston: compressing gas in one channel

and releasing it in the other channel. Only one channel is

irradiated by the laser, but both gas channels produce

acoustic signals of the same size and opposite phase

because of the laser's Helmholtz resonance frequency.1 The

two channels, and the DHR cell, can be seen in

Figure 2.

Figure 2. (a) Schematic and (b) photo of the DHR

photoacoustic cell.1