Some typical fields of applications are industrial drying, chemical and pharmaceutical industry, production of plastics, flue gas measurement in power plants, agriculture, food processing, heating, ventilation and air conditioning, paper production selleck bio and coloring of textiles.One option to measure relative humidity is by means of acoustic techniques. From kinetic gas theory it follows that the speed of sound in air depends on the composition and condition of the air [2]. The most important parameters that determine the speed of sound in air are: temperature, relative humidity, CO2 concentration and to a lesser extent absolute pressure. By simultaneous measurement of speed of sound and air temperature, relative humidity can be calculated for given pressure and Inhibitors,Modulators,Libraries CO2 concentration.

Acoustic sensors are non-intrusive, in contrast to other conventional humidity Inhibitors,Modulators,Libraries sensors like wet and dry bulb sensors or capacitive sensors. This ensures no pressure loss for in-line applications, high life expectancy and insensitivity to contamination. Another advantage over conventional sensors is the high temperature range. Most Inhibitors,Modulators,Libraries important advantage is the high sensitivity and the increase of sensitivity with increasing temperature.Recently, many studies for humidity sensors have been published, particularly acoustic sensors. Inhibitors,Modulators,Libraries However, most of these recent developments in acoustic humidity sensors are based on Surface Acoustic Wave (SAW) sensors, which works with another principle, see the work of Wu et al. [3], for example. A comprehensive review on magnetoelastic sensors which can be applied for humidity measurements is given by Grimes et al.

[4]. The above mentioned conventional and SAW techniques for humidity measurement are local techniques, i.e., with a measuring volume in the order of one cubic mm. The acoustic technique of the present paper, on the other hand, yields a chordal beam average of humidity in the desired portion of the duct. Another Cilengitide type of acoustic sensor was developed by Zipser et al. [5], which has a different layout and is not in-line. Tsai et al. uses an ultrasonic sensor for temperature measurement with a correction for humidity [6].In the present study, the design and tests of a high accuracy in-line acoustic relative humidity sensor for flowing air-steam mixtures in a duct flow are presented. This includes theory, construction, calibration, considerations on dynamic response and results.

2.?TheoryThe speed of sound in a gas for Gemcitabine supplier which the second virial coefficient, B, is given, can be calculated from [2,7]. In the equation below, T is in K:c02=��RTM(1+2pBRT)(1)For each constituent of a gas mixture, �� and B must be known to calculate the speed of sound in the gas mixture. By measuring the speed of sound at constant temperature, T, and pressure, p, determined from measurements of air, the composition of air at constant T and p uniquely depends on the speed of sound.