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A field instrument for water vapour measurements
Infrared
Phyws
1975. Vol. IS. pp. 79 R2. Pergamon
Press Prmtcd
in Great Bntam.
A FIELD INSTRUMENT FOR WATER VAPOUR MEASUREMENTS SVEIN SIVERIXN The Amoral (Receiced
and
Observatory,
12 Auyusf
JAN-ERIK
%LHEIM
9ooO Tromso,
Norway
1974; iti recisedjorm
18 Nowmher
1974)
Abstract-This paper describes a water vapour meter which measures the water vapour content in the air. The meter is easy to handle in the field and has been used for astronomical site testing. Its weight is about 25 kg and longest dimension 23.5 cm. The instrument can only be used with the sun as a light source.
INSTRUMENTATION
Figure I is a photograph of the instrument, showing its dimensions. Figure 2 shows the operating principle and the main components of the instrument*, which are: (a) two filters, one at 1.65 pm and the other at 1.91 pm, both with a half power bandwidth of 01 m; (b) chopper device; (c) PbS detector, not cooled; (d) two polarizers in the 1.65 pm channel. One of these has a fixed position, the other may be turned. When the instrument is directed towards the sun, which has to be used as source, sunlight will be detected in the two channels and the signals from them will be compared electronically. A needle will deflect if the detector does not respond with the same output from the two channels. The rotatable polarizer is now turned until there is zero deflection of the needle, i.e. equal signals occur from the detectors looking through the 1.65 and 1.91 pm filters.
Fig. I. This photograph
shows the dimensions
of the instrument.
l The instrument was constructed in collaboration with the Foundation Research at the University of Trondheim (SINTEF) and built at SINTEF.
79
of Scientific
and
Industrial
80
SVEW SWERWN and JAN-ERIK S~LHEIM
Fig. 2. Main components and operating principle of the water vapour meter. The different symbols mean: Fl. filter 1.91 pm; F2, filter 1.65pm; A, turnable polarizer; B, fixed polarizer; C, PbS detector; D. chopper; E, chopper engine; G, and H, gelatine filters; 1. mirrors; J, synchronous rectifier; K, focusing screen; L, ~nder/nuii indicator; P, pr~mpIifier; Q, amplitude limited amplifier; R, synchronization amplifter with amplitude limit; and T, phototransistor.
Knowing the absorption in the polarizing system (we have assumed zero water vapour absorption in the 165~ band, which is approximately correct for small water vapour contents), we are now able to determine the atmospheric absorption in the l-91 pm band. As soon as the instrument is calibrated, we can compute the wanted relation between the polarizing angle and water vapour content in the air. 2. THE
CALIBRATION
At the Kitt Peak National Observatory, a relation has been derived between the intensity of the atmospheric water vapour line 1 = 6943.803 A and the water vapour content in the path in absolute units, This absolute calibration is much better than 10 per cent according to Hall, Kitt Peak (private communication). Some years ago this relation was ~ansformed to the spectrograph at the Oslo Solar Observatory. The water vapour meter was calibrated at this observatory in 1972. The measurements started on May 24 and ended on August 27. During this period 81 measurements were performed and 66 of these were used in the calibration. The water vapour content in the atmospheric path varied from 65 to 42.4 mm precipitable water. The data are represented by the dots in Fig. 3, where the ordinates are co&?, 8 is the polarizing angle, and the abscissae are loglo (v Pos6), where v is the pre~ipi~ble water in millimetres and P the atmospheric pressure at the observing place in mbs. In the reduction procedure we have used a linear relation between the line depth and the water vapour content. This should be a working approximation as long as the line is not saturated i.e. for moderate water vapour contents. The next step forward was to get a functional relation between the water vapour content (v) and the polarizing angle (6). According to Murcray et al.,“’ which used the laboratory data of Howard et al (2) the experimental relations between the precipitable water and the absorption w& of the following forms vz A, dv = a + h log(vP”.6) (1) s vt for v > 2 mm precipitable water and v2 A, dv = c(LP*.~)*‘~ I *I for t; < O-04 mm precipitable water.
(2)
Field instrument for water vapour measurements
03s
81
-
036 -
ox E N B
-
032-
ii! f
030-
6 026E r VI 5 026E 6 OZLQ) -0 2
022-
020 -
016 -
016l
'
I
I
I
I
.
26
29
30
31
32
33
3L
35
logI0fv PO0
Fig. 3. Empirical fit to the calibration data. Ordinates are coszO where 0 is the polarizing angle. Abscissae are log,, (uP@~) where u is the precipitable water in mm and P the atmospheric pressure at the observing place in mbs.
In these formulae P is measured in mm Hg,
is the integrated absorption between the frequencies v, and v2, and a, b and c are empirical constants. As the calibration data range from 6.5 to 42*4mm precipitable water, we have tried to fit the experimental data to a relation of the same form at (l), and got the following formulae: cos28 = 1.310 - 0.328 log,, (uP”~) f O-085 f 0.028
(3) with standard deviations shown. Here cos20 is proportional to the transmission through the rotatable polarizer. P is the atmospheric pressure in mbs at the observing place and u the precipitable water in mm. From equation (3) we can calculate the amount of precipitable water between the observer and the sun, i.e. for a specified zenith angular distance. In order to reduce the measurements to the zenithal direction, we have divided the observed water with the air masst3) which is approximately equal to set Z, where Z is the zenith angular distance. The field instrument described above can directly be used to measure the atmospheric transmission in the l-91 pm band when the polarizing angle which correspond to zero water is established. 3. CONCLUSIONS
A field instrument for atmospheric water vapour measurements has been described. The main problem with such a device is to control the zero point. Due to the fact that we do not have any portable reference source, the instrument should be frequently calibrated.
SVEINSIVERTSENand
82
It that than than
JAN-ERIK S~LHEIM
should be emphasized that this is an empirical calibration based on the fact the water vapour content during the calibration measurements always was larger 6.5 mm. The results are somewhat more uncertain for low water vapour contents for higher ones.
Acknowledgements-We would like to thank the staff of the Oslo Solar Observatory for performing the calibration measurements, especially 0. Engvold and T. Hansen and the staff at the Aurora1 Observatory for drawing the figures and writing the manuscript.
REFERENCES 1. 2. 3.
MURCRAY, D. G., F. H. MURCRAY and W. J. WILLIAMS, J. Geophys. Res. 67, 759 HOWARD, J., D. BURCH and D. WILLIAMS, J. Opt. Sot. Am. 46, 186, 237, 334 and HARDIE, R. H., Astronomical Techniques, Stars and Stellar Systems, Vol. 2, p. A. HILTNER)(1962).
(1962). 452 (1956). 178. (Edited by W.
Phyws
1975. Vol. IS. pp. 79 R2. Pergamon
Press Prmtcd
in Great Bntam.
A FIELD INSTRUMENT FOR WATER VAPOUR MEASUREMENTS SVEIN SIVERIXN The Amoral (Receiced
and
Observatory,
12 Auyusf
JAN-ERIK
%LHEIM
9ooO Tromso,
Norway
1974; iti recisedjorm
18 Nowmher
1974)
Abstract-This paper describes a water vapour meter which measures the water vapour content in the air. The meter is easy to handle in the field and has been used for astronomical site testing. Its weight is about 25 kg and longest dimension 23.5 cm. The instrument can only be used with the sun as a light source.
INSTRUMENTATION
Figure I is a photograph of the instrument, showing its dimensions. Figure 2 shows the operating principle and the main components of the instrument*, which are: (a) two filters, one at 1.65 pm and the other at 1.91 pm, both with a half power bandwidth of 01 m; (b) chopper device; (c) PbS detector, not cooled; (d) two polarizers in the 1.65 pm channel. One of these has a fixed position, the other may be turned. When the instrument is directed towards the sun, which has to be used as source, sunlight will be detected in the two channels and the signals from them will be compared electronically. A needle will deflect if the detector does not respond with the same output from the two channels. The rotatable polarizer is now turned until there is zero deflection of the needle, i.e. equal signals occur from the detectors looking through the 1.65 and 1.91 pm filters.
Fig. I. This photograph
shows the dimensions
of the instrument.
l The instrument was constructed in collaboration with the Foundation Research at the University of Trondheim (SINTEF) and built at SINTEF.
79
of Scientific
and
Industrial
80
SVEW SWERWN and JAN-ERIK S~LHEIM
Fig. 2. Main components and operating principle of the water vapour meter. The different symbols mean: Fl. filter 1.91 pm; F2, filter 1.65pm; A, turnable polarizer; B, fixed polarizer; C, PbS detector; D. chopper; E, chopper engine; G, and H, gelatine filters; 1. mirrors; J, synchronous rectifier; K, focusing screen; L, ~nder/nuii indicator; P, pr~mpIifier; Q, amplitude limited amplifier; R, synchronization amplifter with amplitude limit; and T, phototransistor.
Knowing the absorption in the polarizing system (we have assumed zero water vapour absorption in the 165~ band, which is approximately correct for small water vapour contents), we are now able to determine the atmospheric absorption in the l-91 pm band. As soon as the instrument is calibrated, we can compute the wanted relation between the polarizing angle and water vapour content in the air. 2. THE
CALIBRATION
At the Kitt Peak National Observatory, a relation has been derived between the intensity of the atmospheric water vapour line 1 = 6943.803 A and the water vapour content in the path in absolute units, This absolute calibration is much better than 10 per cent according to Hall, Kitt Peak (private communication). Some years ago this relation was ~ansformed to the spectrograph at the Oslo Solar Observatory. The water vapour meter was calibrated at this observatory in 1972. The measurements started on May 24 and ended on August 27. During this period 81 measurements were performed and 66 of these were used in the calibration. The water vapour content in the atmospheric path varied from 65 to 42.4 mm precipitable water. The data are represented by the dots in Fig. 3, where the ordinates are co&?, 8 is the polarizing angle, and the abscissae are loglo (v Pos6), where v is the pre~ipi~ble water in millimetres and P the atmospheric pressure at the observing place in mbs. In the reduction procedure we have used a linear relation between the line depth and the water vapour content. This should be a working approximation as long as the line is not saturated i.e. for moderate water vapour contents. The next step forward was to get a functional relation between the water vapour content (v) and the polarizing angle (6). According to Murcray et al.,“’ which used the laboratory data of Howard et al (2) the experimental relations between the precipitable water and the absorption w& of the following forms vz A, dv = a + h log(vP”.6) (1) s vt for v > 2 mm precipitable water and v2 A, dv = c(LP*.~)*‘~ I *I for t; < O-04 mm precipitable water.
(2)
Field instrument for water vapour measurements
03s
81
-
036 -
ox E N B
-
032-
ii! f
030-
6 026E r VI 5 026E 6 OZLQ) -0 2
022-
020 -
016 -
016l
'
I
I
I
I
.
26
29
30
31
32
33
3L
35
logI0fv PO0
Fig. 3. Empirical fit to the calibration data. Ordinates are coszO where 0 is the polarizing angle. Abscissae are log,, (uP@~) where u is the precipitable water in mm and P the atmospheric pressure at the observing place in mbs.
In these formulae P is measured in mm Hg,
is the integrated absorption between the frequencies v, and v2, and a, b and c are empirical constants. As the calibration data range from 6.5 to 42*4mm precipitable water, we have tried to fit the experimental data to a relation of the same form at (l), and got the following formulae: cos28 = 1.310 - 0.328 log,, (uP”~) f O-085 f 0.028
(3) with standard deviations shown. Here cos20 is proportional to the transmission through the rotatable polarizer. P is the atmospheric pressure in mbs at the observing place and u the precipitable water in mm. From equation (3) we can calculate the amount of precipitable water between the observer and the sun, i.e. for a specified zenith angular distance. In order to reduce the measurements to the zenithal direction, we have divided the observed water with the air masst3) which is approximately equal to set Z, where Z is the zenith angular distance. The field instrument described above can directly be used to measure the atmospheric transmission in the l-91 pm band when the polarizing angle which correspond to zero water is established. 3. CONCLUSIONS
A field instrument for atmospheric water vapour measurements has been described. The main problem with such a device is to control the zero point. Due to the fact that we do not have any portable reference source, the instrument should be frequently calibrated.
SVEINSIVERTSENand
82
It that than than
JAN-ERIK S~LHEIM
should be emphasized that this is an empirical calibration based on the fact the water vapour content during the calibration measurements always was larger 6.5 mm. The results are somewhat more uncertain for low water vapour contents for higher ones.
Acknowledgements-We would like to thank the staff of the Oslo Solar Observatory for performing the calibration measurements, especially 0. Engvold and T. Hansen and the staff at the Aurora1 Observatory for drawing the figures and writing the manuscript.
REFERENCES 1. 2. 3.
MURCRAY, D. G., F. H. MURCRAY and W. J. WILLIAMS, J. Geophys. Res. 67, 759 HOWARD, J., D. BURCH and D. WILLIAMS, J. Opt. Sot. Am. 46, 186, 237, 334 and HARDIE, R. H., Astronomical Techniques, Stars and Stellar Systems, Vol. 2, p. A. HILTNER)(1962).
(1962). 452 (1956). 178. (Edited by W.