Hygrometers mechanical

Moisture content was determined by drying them under vacuum to constant weight at 60°C (AOAC, 1980). Water activity was measured at 25°C by using a Decagon Cx-3 (Aqualab) hygrometer. Mechanical properties of dried samples with different water content were analyzed at 30°C by using a puncture test with a Universal Texture Analyser (TA.XT2, Stable Micro Systems). A cylindrical 2.0 mm diameter pimch was used at a penetration rate of 1.5 mm/sec until total sample penetration. Determinations were made in five different samples with the same water content. 

Mechanical Hygrometers Materials such as human hair, wood fiber, and plastics have been used to measure humidity. These methods rely on a change in dimension with humidity. 

The optical chilled-mirror dew point technique is a fundamental measurement, because the saturation temperature determines the saturation partial pressure of the water vapor. A temperature element is then placed in thermal contact with the mirror, and the mirror temperature is utilized directly as the dew point or saturation temperature. The cooling of the mirror surface in the past was done by acetone and dry ice, liquid C02, mechanical refrigeration, and later by thermoelectric heat pumps. The thermoelectrically cooled, optically observed dew point hygrometer probe is illustrated in Figure 3.26. 

These methods use mechanical probes or hygrometers that respond to the variations in the relative humidity of air that are caused by variations in dielectric resistance or capacity. These probes directly measure the relative humidity of air, and the most commonly used is lithium chloride. In general, these methods require frequent calibration and should only be used within a narrow range. 

Mechanical hygrometer is based on animal or human hair whose length increases when the relative humidity increases and contracts when it decreases. The change in length actuates a lever, which turns the pointer on a dial. This is not a very accurate device however. 

The main types of instruments measuring humidity in gases are various types of hygrometers (gravimetric, mechanical, condensation, infrared absorbance detector, electric sensor, thermal conductivity, Al203/sili-con, P2O5) and psychrometers. 

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