Sensors dew-point

Three types of instruments have received wide acceptance in dew point measurements tllthe saturated salt dew point sensor 121 the condensation-type hygrometer and t.lt the aluminum oxide sensor. Many other instruments are used in specialized applications, including pressure tatio dev ices, dewcups. and fog chambers. The luttei ate manually operated... 

Saturated Salt Dew Point Sensors. The saturated sail (lithium chloride) dew point sensor is widely used because of its inherent simplicity, ruggedness, and low cost. Both the United Slates and Canadian government weather services use this type of sensor lor must official groundbused humidity measurements. However, some of these are being converted to the more accurate condensation hygrometers... 

The principle of the saturated sail dew point sensor is based on the relationship that the vapor pressure of water is reduced in the presence of a salt. When water vapor in the air condenses on a soluble salt, it forms a saturated layer on the surface of the salt. This saturated layer has a lower... 

The temperature of a surface reaches the dew point when the first molecular layer of condensation appears on it. Dew point sensors of this type can be grouped according to the method used to detect the appearance of this condensate into conductivity and optical (mirror) types. 

Figure 31.13 Experimental rotating jet spouted bed with central and peripheral air jets 1—air distributor, 2—screen, 3—distributor cover, 4—ball bearings, 5—v-belt, 6—pulley, 7—drive, 8—motor controller, 9—glass vessel, 10—bir blower, 11— electric heater, 12—PID controller, 13—Pitot tube, 14—gate valve, 15—pressure transducers, 16—pressure taps, 17—thermocouples, 18—multiplexer, 19—IR hygrometer, 20—dew-point sensor, 21—dew-point hygrometer, 22—humidity meter, 23—computer, 24—terminal panel, 25—filter. (From Jumah, 1995.)...

There are many types of devices such as mirror-chilled hygrometers, psychrometers, optical sensors, and solid-state sensors which can be used for humidity control (Nitta 1981 Rittersma 2002). Mirror-based dew point sensors have better accuracy, but these devices are more costly to make and use (Chen and Lu 2005). Solid-state humidity sensors can be fabricated at low cost and used for moisture control more conveniently. Solid-state humidity sensors are broadly classified into three types gravimetric (QCM and SAW), resistive, and capacitive, according to the output form of the sensor... 

Properly used, a saturated salt sensor is accurate to 1 C between dew point temperatures of -12 and +38°C. Outside Ihtfse limits, small errors may occur ns a result of the multiple hydration characteristics of lithium chloride, which may produc d ambiguous results at 4I C. 12 C. 

Limitations of saturaled sail sensors include (11 relatively slow response lime and (2) a lower limit to the mcasurcmenl range imposed by ihe nature of lithium chloride. The sensor cannot be used to measure dew points when the vapor pressure of waier is below- the saturation vapor pressure of lithium chloride, which occurs at about I 1% RH. In certain gases, ambient temperatures can he reduced, increasing the RH to above I 1% but the extra effort needed to cool the gas usually warrants selection of a different type of sensor. Fortunately, a large number of scienlilie and induslrial measurements fall above this limitation and are readily handled by the sensor. 

Advantages of the aluminum oxide sensor are 11) small size and suitability for in.tint use (2t it can be used very economically in multiple sensor arrangements (3) suitability lor very low dew point levels without file need lor senstn cooling (j as required in condensation-type sensors—(typically, dew points down to — IOO°C can be measured without serious difficulty) (4t the unit covers a wide span. 

The aluminum oxide sensor is also used for moisture measurements in liquids (hydrocarbons). Because ot us low power usage, it is suitable lor use in explosion prool installations. These sensors are frequently used in petrochemical applications where low dew points are to he monitored on line and where the reduced accuracies and other limitations are acceptable. The advantages of the sensor must be weighted against the fact that... 

A2.1 Factors to Consider When Estimating Water Vapor Pressure A2.2 Dew-Point Method for the Determination of Water Activity A2.3 Measurement of Water Activity Using Isopiestic Method A2.4 Direct Manometric Determination of Vapor Pressure A2.5 Measurement of Water Activity by Electronic Sensors... 

Figure A2.2.2 A representation of a modern dew-point chamber consisting of (1) mirror, (2) optical sensor, (3) infrared thermometer, (4) internal chamber fan, and (5) sample cup with sample.

In a modern dew-point instrument, a sample is equilibrated within the headspace of a sealed chamber containing a mirror, an optical sensor, an internal fan, and an infrared thermometer (Figure A2.2.2). At equilibrium, the relative humidity of the air in the chamber is the same as the water activity of the sample. A thermoelectric (Peltier) cooler precisely controls the mirror temperature. An optical reflectance sensor detects the exact point at which condensation first appears a beam of infrared light is directed onto the mirror and reflected back to a photodetector, which detects the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror accurately measures the dew-point temperature. The internal fan is for air circulation to reduce vapor equilibrium time and to control the boundary layer conductance of the mirror surface (Campbell and Lewis, 1998). Additionally, an infrared thermometer measures the sample surface temperature. Both the dew-point and sample temperatures are then used to determine the water activity. The range of a commercially available dew-point meter is 0.030 to 1.000 aw, with a resolution of 0.001 aw and accuracy of 0.003 aw. Measurement time is typically less than 5 min. The performance of the instrument should be routinely verified as described in the Support Protocol. 

Dew-point measurement is a primary method based on fundamental thermodynamics principles and as such does not require calibration. However, the instrument performance needs to be verified using salt standards and distilled water before sampling (see Support Protocol). To obtain accurate and reproducible water activity results with a dew-point instrument, temperature, sensor cleanliness, and sample preparation must be considered. Equipment should be used and maintained in accordance with the manufacturer s instruction manual and with good laboratory practice. If there are any concerns, the manufacturer of the instrument should be consulted. Guidelines common to dew-point instruments for proper water activity determinations are described in this protocol. The manufacturer s instructions should be referred to for specifics. 

Reliable laboratory instrumentation is required to guarantee the safety of food products and enforce government regulations. Two main types of commercial instruments are available. One uses chilled mirror dew-point technology, while the other measures relative humidity with sensors that detect changes in electrical resistance or capacitance. Each has advantages and disadvantages. The methods vary in accuracy, repeatability, speed of measurement, stability in calibration, linearity, and convenience of use. 

To obtain accurate and reproducible water activity results with a dew-point instrument, the temperature, sensor cleanliness, and sample preparation must be considered. Temperature influences water activity measurements in two ways one is its effect on the water activity in the sample and the second is the effect on the dew-point measurement. 

The operating ranges of different humidity sensors. Most humidity/dew point detectors can make measurements at higher values of humidity and temperature, but are limited at low temperatures and at low concentrations. Most are also limited to a maximum operatng temperature of about 200°F (95°C). 

Dew point is detected by either lithium chloride or, if high precision is required, by the cooled mirror-type sensors, discussed in Section 3.2.13. For relative humidity measurements, the elongation and contraction of hygroscopic elements are used. 

The frequency and amplitude of an oscillating quartz crystal are influenced by even a minimal mass load. A sensitive dew sensor was developed by using a Peltier-cooled quartz plate [54]. This sensor is capable of measuring relative humidity with an accuracy better than l.St o of the measured relative humidity in automatic dew-point hygrometry. Since the frequency but not the amplitude of specially cut quartz crystals depends strongly on the temperature, the dew-point temperature can be determined simultaneously with the same sensor element. Compared with optical dew-point hydrometers, this quartz sensor combines dewpoint detection and temperature measurement in a single element. 

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