11:00 AM - 1:00 PM
[MTT45-P03] Development of a measurement technique of atmospheric pressure fluctuation for establishing a dynamic characteristic evaluation method for barometers
Keywords:Atmospheric pressure measurement, Laser interferometer, Calibration
Natural phenomena that cause disasters such as volcanic activity, tsunamis, and avalanches cause atmospheric pressure fluctuations with a frequency of less than 20 Hz, called infrasound. The infrasound is observed in various parts of Japan for early detection of the occurrence of the phenomenon and elucidation of the mechanism of the occurrence, and it is expected that the observation will contribute to disaster prevention. A highly responsive pressure gauge is used to observe the infrasound. To ensure the reliability of pressure measurement, the calibration and evaluation method of static pressure for the pressure gauges has already been established. However, the evaluation method for the dynamic characteristics of the pressure gauges used for the infrasound observations has not been established.
Since standard devices used for static pressure calibration are not suitable for dynamic measurements, we aim to develop a measurement technique that can be used as a reference for evaluation of dynamic characteristics. In this study, we attempted to measure the atmospheric pressure fluctuations with a laser displacement gauge based on the principle of Michelson interferometry. The laser interferometer is capable of high-speed measurement and suitable for dynamic measurements. The optical path length in a homogeneous medium is the product of the geometric length and the refractive index. The geometric length of laser path is constant in this study. According to the Lorentz–Lorenz equation, the change of the density of gas causes the change of the refractive index. Since the gas density depends on pressure, the change of pressure can be measured as a change in the optical path length by the laser displacement gauge when temperature is constant. By evaluating the relationship between the change of the optical path length and the change of the pressure in static condition, traceable measurements can be realized. Theoretically, the refractive index of air changes by about 0.27 ppm when the pressure increases by 100 Pa.
There are two pressure cells in a developed measurement system, one as a reference cell and the other as a measurement cell. The laser displacement gauge measures the difference of optical path length between the reference cell and the measurement cell. The pressure change in the measurement cell can be measured by keeping the pressure in the reference cell constant. The optical system is installed on a vibration isolated table to reduce noise. Since the optical path length is also affected by temperature, the environment is stabilized by covering the pressure cells with a heat insulator. To reduce the effects of temperature changes, the reference cell and the measurement cell are the same size. The length of the pressure cell is set to 1 m in order to measure a small change of a few Pa. The change in optical path can be expected to be about 2.7 nm when the pressure changes by 1 Pa.
The stability of the output value of the laser displacement gauge was evaluated when there was no pressure difference between the reference cell and the measurement cell. Sampling rate of the displacement gauge was 100 Hz. The variation in the output value was less than 3 nm in the measurement for 10 minutes. The output value of the displacement gauge was compared with the pressure change under static conditions. The relationship between the change in optical path and the change in pressure in this measurement system was about 2.657 nm/Pa. The displacement gauge has good linearity against the pressure changes between -300 Pa and 300 Pa. We will proceed with this study and contribute to the evaluation of dynamic characteristics of existing pressure gauges.
Since standard devices used for static pressure calibration are not suitable for dynamic measurements, we aim to develop a measurement technique that can be used as a reference for evaluation of dynamic characteristics. In this study, we attempted to measure the atmospheric pressure fluctuations with a laser displacement gauge based on the principle of Michelson interferometry. The laser interferometer is capable of high-speed measurement and suitable for dynamic measurements. The optical path length in a homogeneous medium is the product of the geometric length and the refractive index. The geometric length of laser path is constant in this study. According to the Lorentz–Lorenz equation, the change of the density of gas causes the change of the refractive index. Since the gas density depends on pressure, the change of pressure can be measured as a change in the optical path length by the laser displacement gauge when temperature is constant. By evaluating the relationship between the change of the optical path length and the change of the pressure in static condition, traceable measurements can be realized. Theoretically, the refractive index of air changes by about 0.27 ppm when the pressure increases by 100 Pa.
There are two pressure cells in a developed measurement system, one as a reference cell and the other as a measurement cell. The laser displacement gauge measures the difference of optical path length between the reference cell and the measurement cell. The pressure change in the measurement cell can be measured by keeping the pressure in the reference cell constant. The optical system is installed on a vibration isolated table to reduce noise. Since the optical path length is also affected by temperature, the environment is stabilized by covering the pressure cells with a heat insulator. To reduce the effects of temperature changes, the reference cell and the measurement cell are the same size. The length of the pressure cell is set to 1 m in order to measure a small change of a few Pa. The change in optical path can be expected to be about 2.7 nm when the pressure changes by 1 Pa.
The stability of the output value of the laser displacement gauge was evaluated when there was no pressure difference between the reference cell and the measurement cell. Sampling rate of the displacement gauge was 100 Hz. The variation in the output value was less than 3 nm in the measurement for 10 minutes. The output value of the displacement gauge was compared with the pressure change under static conditions. The relationship between the change in optical path and the change in pressure in this measurement system was about 2.657 nm/Pa. The displacement gauge has good linearity against the pressure changes between -300 Pa and 300 Pa. We will proceed with this study and contribute to the evaluation of dynamic characteristics of existing pressure gauges.