Acoustic method for determination of the effective temperature and refractive index of air in accurate length interferometry

Korpelainen, Virpi; Lassila, Antti

doi:10.1117/1.1787834

Cited by 21 publications

(25 citation statements)

References 20 publications

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“…However, if measurements are done in an industrial environment or outdoors, fluctuations of ambient conditions can be several orders of magnitude larger, thus significantly increasing the measurement uncertainty. The acoustic method based on speed-of-sound measurement of an ultrasound burst signal can be also used to determine the effective air temperature with low uncertainty [7]. Echoes, beam spreading, and absorption, however, make it more suitable for shorter distances compared to the developed spectroscopic method.…”

Section: Introductionmentioning

confidence: 99%

Spectroscopic Measurement of Air Temperature

Hieta

Merimaa

2010

Int J Thermophys

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Optical dimensional measurements have to be corrected for the refractive index of air. The refractive index is conventionally calculated from parameters of ambient air using either Edlén or Ciddor equations or their modified versions. However, these equations require an accurate knowledge of ambient conditions and especially the temperature of air. For example, to reach an uncertainty of 10 −7 in dimensions, the air temperature has to be known at ∼100 mK level. This does not necessarily cause problems in a stable laboratory environment. However, if measurements are done outdoors or in an industrial environment, variations in temperature can be very rapid and local temperature gradients can cause significant error if not taken into account. Moreover, if the required distance is long, the temperature over the whole measurement path can be impractical or impossible to determine at sufficient temporal or spatial resolution by conventional temperature measurement techniques. The developed method based on molecular spectroscopy of oxygen allows both lateral spatial and temporal overlap of the temperature measurement with the actual distance measurement. Temperature measurement using spectroscopy is based on a line intensity ratio measurement of two oxygen absorption lines, previously applied for measurements of high temperatures in flames. The oxygen absorption band at 762 nm is a convenient choice for two-line thermometry since the line strengths are practical for short-and long-distance measurements and suitable distributed feedback lasers are commercially available. Measurements done on a 67 m path at ambient conditions demonstrate that the RMS noise of 22 mK, or 7.5 × 10 −5 , near 293 K using 60 s measurement time can be achieved, which is to our knowledge the best reported resolution.

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Section: Introductionmentioning

confidence: 99%

Spectroscopic Measurement of Air Temperature

Hieta

Merimaa

2010

Int J Thermophys

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“…Nevertheless we have planned as a future work to measure u A in a controlled environment to decrease its uncertainty, as in [10], to reach lower levels of uncertainty in T.…”

Section: The Acoustic Methodsmentioning

confidence: 99%

“…Korpelainen and co-authors at MIKES used an acoustic thermometer for the correction of the index of refraction in a laboratory for dimensional metrology using pulses of 50 kHz emitted by acoustic piezo transducers. The claimed uncertainty is 25 mK in a distance range limited to 12 m and temperature in the 19.5 °C -21.0 °C interval [10]. Underwood and co-authors at NPL realized a device based on the integration of an acoustic thermometer and a tuneable diode laser absorption spectrometer able to carry out fast measurements of relative humidity and temperature with an uncertainty of less than 0.1°C [11].…”

Section: Introductionmentioning

confidence: 99%

An acoustic thermometer for air refractive index estimation in long distance interferometric measurements

2017

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We present a method to measure the temperature along the path of an optical interferometer based on the propagation of acoustic waves. It exploits the high sensitivity of the speed of sound to air temperature. In particular, it takes advantage of a technique where the generation of the acoustic waves is synchronous with the amplitude modulation of a laser source. A photodetector converts the laser light to an electronic signal used as reference, while the incoming acoustic waves are focused on a microphone and generate the measuring signal. In this condition, the phase difference between the two signals substantially depends on the temperature of the air volume interposed between the sources and the receivers. The comparison with the traditional temperature sensors highlighted the limit of the latter in case of fast temperature variations and the advantage of a measurement integrated along the optical path instead of a sampling measurement. The capability of the acoustic method to compensate the interferometric distance measurements due to air temperature variations has been demonstrated to the level of 0.1°C corresponding to 10 -7 on the refractive index of air. We applied the method indoor for distances up to 27 m, outdoor at 78 m and finally tested the acoustic thermometer over a distance of 182 m.

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“…Thus, in order to measure the real time temperature distribution along the interferometer path, a method based on the propagation of acoustic waves has been developed, exploiting the fact that the acoustic waves are about 2 thousands times more sensitive to air temperature variation than the electromagnetic waves. The principle has already been used by Korpelainen [11] for distances of about 5 m. The set-up and the measuring method together with the comparison with the local temperature measurements are described in this paper. The effectiveness of the acoustic method to correct fast temperature changes effects on interferometric measurements is finally demonstrated.…”

Section: Aim Of the Researchmentioning

confidence: 99%

Air temperature measurements based on the speed of sound to compensate long distance interferometric measurements

Astrua

Pisani

Zucco

2014

EPJ Web of Conferences

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Abstract.A method to measure the real time temperature distribution along an interferometer path based on the propagation of acoustic waves is presented. It exploits the high sensitivity of the speed of sound in air to the air temperature. In particular, it takes advantage of a special set-up where the generation of the acoustic waves is synchronous with the amplitude modulation of a laser source. A photodetector converts the laser light to an electronic signal considered as reference, while the incoming acoustic waves are focused on a microphone and generate a second signal. In this condition, the phase difference between the two signals substantially depends on the temperature of the air volume interposed between the sources and the receivers. The comparison with the traditional temperature sensors highlighted the limit of the latter in case of fast temperature variations and the advantage of a measurement integrated along the optical path instead of a sampling measurement. The capability of the acoustic method to compensate the interferometric distance measurements due to air temperature variations has been demonstrated for distances up to 27 m.

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Acoustic method for determination of the effective temperature and refractive index of air in accurate length interferometry

Cited by 21 publications

References 20 publications

Spectroscopic Measurement of Air Temperature

Spectroscopic Measurement of Air Temperature

An acoustic thermometer for air refractive index estimation in long distance interferometric measurements

Air temperature measurements based on the speed of sound to compensate long distance interferometric measurements

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