Avancerede optiske metoder til måling af luftforurening i indeklima og udeluft “Upconversion detection for IR-spectroscopy of TCE and PCE detection”

Christian Pedersen Gruppeleder, DTU Fotonik

11. 05. 2015 2

• To map out the indoor climate at locations in the Copenhagen area.

• Investigate the potential of a new technology giving fast response and clarify potential sources and pathways of toxic gasses into building

• Todays technology relays on: “ATD-tubes” or “Orsa-tubes” – giving average concentrations over a 2 week period

• No simple approach exists for measurements of short time variationser

• No simple approach exists for leak detection

Motivation & background

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Expectations and perspective

• Real-time measurements of PCE and TCE

• Monitoring of changes in concentration (e.g. measurement per minute)

• Localization of leaks

• Measurements of many other gasses can be relavent in the future (e.g. PCB)

• Develop a fast reliable optical system for detection of PCE/TCE (in the range of 1 mg/m3

Target

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Mid-IR Wavelength

region: 2-15 µm

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Mid-IR: 2-15 µm is called the “fingerprint region”:

-Fundamental absorption bands of nearly all gasses

(NOX,CO2, CH4…., explosives)

-Chemical vibration bands of large molecules

- heat

Mid-IR Applications

Trichloroethylene

FOSS 29. 01. 2014 6 DTU Fotonik, Danmarks Tekniske Universitet

Origin of Mid-IR detector noise

Light emission from

room temperature “objects”

Planck radiation

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Upconversion

Upconversion detector

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A candle observed at 3 µm

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Detector benchmark A collaboration with Lund University

Cooled Indium-Antimonide detector

(Judson Technologies J10D)

First generation

upconversion detector

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x500

0.5 1 2 3 4 5 10 20 30

101

102

103

104

105

Acetylene concentration [ppm]

Dete

cte

d p

ho

tons (

VIS

)

Line integrated IR DFWM signal intensities

Fit to ax2

Acetylene measument at 3 µm using

Degenerate Four Wave Mixing

(collaboration with Lund University)

- 500 times improvement sensitivity compared to the cooled InSb detector deployed in the test

- Probe volume is appr. 10 mm^3 (!)

- Sensitivity= 0.5 ppm

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Concept and setup for TCE and PCE detection

Light source Multi-pass gas cell

Mixing laser Up-converter unit NIR camera

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Light souce

Multi-pass gas cell

Mixing laser

NIR camera

Up-converter unit

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Absorption spectra

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TCE spectrum measured at high concentrations

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TCE Measurements filled/empty cell at 944.3 cm-1

Measured absorbance in 36 m cell:

0.008 at 944.3 cm-1

Equivalent to a TCE concentration of:

0.31 ppm

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PCE measurement – 915 cm-1 (1400 µg/m3)

Average 1293 Concentration

1265

1297 0.196 ppm

Ratio 1.0238 1329 µg/m3

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PCE detection at 915.3 cm-1 - “dynamics”

Measured absorbance in 36 m cell:

0.557 at 915.3 cm-1

Equivalent to a PCE concentration of:

10.3 ppm

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Absorption on 1 % water vapor (example of “cross contamination”)

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Status & outlook • The absorption spectra of PCE

and TCE are well separated in

this spectral range.

• High concentrations of both PCE

and TCE can be measured with

short acquisition time.

• Addition work on noise/drift

reduction to facilitate averaging

-> improved signal to noise

• Cross contamination from e.g.

water or CO2 needs to be

removed for ppb measurements

using mathematical models.

• Make a compact robust system

for field test