Humidity sensing

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Our NIR (near infrared) LEDs and InGaAs or Silicon photodiodes can be used for humidity measurements. Humidity is the presence of water vapour in air, or other gas, and the instrument used to measure humidity is called a hygrometer. Gas and liquid analysis are traditional optical applications in which Hamamatsu has extensive historical experience.

There are many techniques to measure humidity and of the optical methods there are two practical methods for use in cars. The main method is absorption spectroscopy, which measures the infrared light absorbed by water vapour. The other is optical fibre measurement, which looks at the change in reflected or transmitted light, using an optical grating or a hygroscopic coating.

Humidity and moisture sensing has a variety of possible uses in vehicles today and in the future. Air conditioning systems often control humidity, and consume energy to cool the air and remove water vapour. Therefore measuring the humidity can optimize the ambient conditions for the car occupants as well as minimize the energy used in doing so. Typically there is ~1% water in air at room temperature and if there is higher humidity then warm temperatures feel warmer, and if there is lower humidity then people can suffer from dry throats and other conditions.

Other examples of moisture detection, in liquid form rather than a gas, include measuring water content in fuel or other parts of the car, which can be used for safety, environmental and efficiency improvements, or whenever there is a need to prevent condensation, corrosion or warping on any part of the car. Measuring moisture or humidity outside of the vehicle or on the road surface underneath the car can give valuable information for safety or performance purposes.

NIR LEDs like the L10660 series are specifically designed for moisture detection measurements at 1450 nm, a strong absorption line in water molecules. Depending on the chosen operating wavelength then InGaAs photodiodes or Silicon photodiodes can be used as the detector in the humidity sensing system.

It should be noted that the same technique, absorption spectroscopy, can be used to detect not only humidity but also many other substances in gasses or liquids, such as dangerous gases in the cabin, or fuel quality measurements.

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