Hygrometer

Summary

A hygrometer is an instrument which measures the humidity of air or some other gas: that is, how much water vapor it contains. Humidity measurement instruments usually rely on measurements of some other quantities such as temperature, pressure, mass and mechanical or electrical changes in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Modern electronic devices use the temperature of condensation (called the dew point), or they sense changes in electrical capacitance or resistance to measure humidity differences. A crude hygrometer was invented by Leonardo da Vinci in 1480. Major leaps came forward during the 1600s; Francesco Folli invented a more practical version of the device, while Robert Hooke improved a number of meteorological devices including the hygrometer. A more modern version was created by Swiss polymath Johann Heinrich Lambert in 1755. Later, in the year 1783, Swiss physicist and Geologist Horace Bénédict de Saussure invented the first hygrometer using human hair to measure humidity. The maximum amount of water vapor that can be held in a given volume of air (saturation) varies greatly by temperature; cold air can hold less mass of water per unit volume than hot air. Temperature can change humidity. Prototype hygrometers were devised and developed during the Shang dynasty in Ancient China to study weather. The Chinese used a bar of charcoal and a lump of earth: its dry weight was taken, then compared with its damp weight after being exposed in the air. The differences in weight were used to tally the humidity level. Other techniques were applied using mass to measure humidity, such as when the air was dry, the bar of charcoal would be light, while when the air was humid, the bar of charcoal would be heavy. By hanging a lump of earth and a bar of charcoal on the two ends of a staff separately and adding a fixed lifting string on the middle point to make the staff horizontal in dry air, an ancient hygrometer was made.

Official source

https://en.wikipedia.org/wiki/Hygrometer

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Psychrometrics

Psychrometrics (or psychrometry, ; also called hygrometry) is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures. Although the principles of psychrometry apply to any physical system consisting of gas-vapor mixtures, the most common system of interest is the mixture of water vapor and air, because of its application in heating, ventilation, and air-conditioning and meteorology.

Hygrometer

Wet-bulb temperature

The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked (water at ambient temperature) cloth (a wet-bulb thermometer) over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature); at lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of evaporative cooling. The wet-bulb temperature is defined as the temperature of a parcel of air cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat supplied by the parcel.

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Sorption and transport in cementitious materials

Learn how to study and improve the durability of cementitious materials.

Construction material monitoring with "optical hair" hygrometers

Pascal Kronenberg

Moisture is an essential parameter in the behaviour of capillary-porous construction materials such as timber and concrete. It affects liquid and gas transport phenomena, chemical and biological degradation processes, mechanical properties and, in the case of concrete, hydration. From a scientific point of view, moisture monitoring is essential in order to improve the understanding of material behaviour. Moisture measurements may increase the predictive accuracy of established material behaviour models. In practice, the increased knowledge of material behaviour improves structural maintenance planning. The main objective of this work is to propose a measurement method for non-destructive, in-depth moisture monitoring of construction materials. In this context, an intrinsic point and an averaging fibre optic relative humidity sensor have been developed and tested. The sensors are based on optical fibres that are coated with a hygroscopically swelling transducer polymer. When wet, the swelling of the coating strains the fibre (analogy with the hair hygrometer). The induced strain is assessed with conventional fibre optic strain sensing techniques such as fibre Bragg gratings and Michelson interferometry. Theoretical and experimental studies lead to a detailed understanding of the influence of humidity and temperature on the steady and transient state sensor behaviour. The sensors have an accurate, linear, reversible and reproducible response to relative humidity between 5 and 95 %RH and between 13 and 60 °C, at least. The sensor response time is in the order of 20 minutes. However, when packaged, the sensor responds slower. The temperature cross-sensitivity of the fibre Bragg grating sensor may be compensated with an additional non-hygroscopic grating, while the Michelson interferometric sensor provides an auto-compensation of temperature effects. Tests in mortar and timber samples demonstrate that the sensors preserve their sensing ability when embedded. These tests have also clarified the multiplexing potential of fibre Bragg gratings for forming a multi-point RH sensor. Multi-point sensors are particularly useful for profile measurements.

EPFL2003

Construction material monitoring with "optical hair" hygrometers

Pascal Kronenberg

EPFL2003