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Figure 1 |
CS Edwards(1), GP Barwood(1),
P Gill(1), M Stevens(1), B Schirmer(2), R Besnyon(3)
and P Mackrodt(4)
(1)National Physical Laboratory,
Queens Road, Teddington, Middx TW11 0LW
(2)University of Erlangen-Nürnberg,
Cauerstrasse 4, D-91058 Erlangen, Germany
(3)Instituto Nacional
de Tecnica Aerospacial, Ctra. a Ajalvir, km4, E-28850, Torrejon de Ardoz Spain
(4)Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116
Braunschweig, Germany
There is an increasing demand for the measurement of trace humidity in air at levels down to a few parts in 109 (ppbv) at atmospheric pressure, for example in the microelectronics industry or in atmospheric chemistry research. This measurement process needs to be simple, robust and capable of producing a measurement on a time scale of a few seconds. This summary describes the development by a 6-member European collaboration of a diode laser absorption spectrometer capable of measuring trace humidity with a repeatability of 5 ppb. The diode laser was a distributed feedback device, tuned to the 3,0,3 ¬ 2,0,2 rotation transition in the n1 + n3 vibrational band of water vapour at 1393 nm [1-3].
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Figure 1 |
Three different spectroscopic techniques were investigated and two-tone frequency modulation spectroscopy (TTFMS) was found to be the most sensitive technique. It is also relatively straightforward to implement. TTFMS requires frequency modulation at two closely-spaced frequencies, in our case 9.19 GHz and 9.21 GHz. The signal is detected synchronously at the 20 MHz difference frequency. The signal size at line centre varies linearly with water vapour concentration, as discussed more fully in [1]. After a number of refinements to the optical layout, TTFMS features were obtained with no observable background interference effects. Figure 1 shows a scan with a signal to noise ratio on the strongest feature of 5700 in a 2.5 Hz detection bandwidth. This scan was obtained in 0.4 m of laboratory air at ambient pressure and humidity ( »50% relative humidity, equivalent to a concentration of 1.2 x 104 ppm).
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Figure 2 |
In order to provide a facility for trace humidity measurement, a multi-pass White cell was manufactured. The complete spectrometer, including the TTFMS electronics, is shown in figure 2. Both free space and fibre pig-tailed versions of the input optics were built. The mirror separation in the cell was 0.25 m, and a 10-m optical path length was obtained by passing the beam 40 times through the cell. The spectrometer performance was verified in the region above 1 ppm where certified humidity measurement is possible. The TTFMS signal size may then be extrapolated down to assess performance in the few ppb region. The diode laser is computer- controlled via an IEEE interface and current-scanned over the absorption feature to determine the peak TTFMS signal. The TTFMS results were read by the computer via an analogue to digital input card. Multiple scans over the line were used and a curve fitting routine implemented in order to improve on the sensitivity and reproducibility. The software outputs a voltage from the PC which varies linearly with the water vapour concentration. The concentration is also displayed on the screen and the data recorded as a function of time on disc. The fibre version of the spectrometer has undergone tests at the four humidity standard laboratories of the partners in the consortium and the results of this first comparison have recently been published [4].
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Figure 3 |
The most recent results obtained with the spectrometer have been obtained with the free space input optics and improved electronics. A typical computer controlled current scan over the line is shown in figure 3, at a concentration of 1 ppm. Initial results indicate that the repeatability of the instrument over a few weeks is 5 ppb (one standard uncertainty) in the range up to 1 ppm.
The authors acknowledge the support of the European Union under contract SMT4-CT96-2125.
1. CS Edwards, GP Barwood, P Gill, B Schirmer, H Venzke and A Melling, Applied Optics, 38, 4699-4704 (1999) and references therein.
2. JA Silver and DC Hovde, “A comparison of near-infrared diode laser techniques for airbourne hygrometry”, J Atmos. Ocean. Technol. 15, 29-35 (1998)
3. WJ Kessler, MG Allen, SJ Davis, PA Mulhall and JA Polex, “Near IR diode laser-based sensor for ppb-level water vapor in industrial gases”, SPIE Proceedings, 3537, 139-49 (1999)
4. B Schirmer, H Venzke, A Melling, CS Edwards, GP Barwood, P Gill, M Stevens, R Benyon, P Mackrodt, Meas Sci Technol 11, 382-91 (2000)