Water activity meter

The Basic Protocol describes the determination of water activity of a product using a chilled mirror dew-point water activity meter. Dew point is a primary measurement of vapor pressure that has been in use for decades (Harris, 1995). Dew-point instruments are accurate, fast, simple to use, and precise (Richard and Labuza, 1990 Snavely et al., 1990 Roa and Tapia de Daza, 1991). In a dew-point instrument, water activity is measured by equilibrating the liquid-phase water in the food sample with the vapor-phase water in the headspace, and then measuring the vapor pressure of the headspace. The basic principle involved in dew-point determinations of vapor pressure in air is that air may be cooled without change in water content until it saturates. The dew-point temperature is the temperature at which the air reaches saturation. It is determined in practice by measuring... 

The Aqualab CX2 water activity meter (Decagon Devices Inc., Pullman, WA) detects water condensation on a chilled mirror (dew point temperature). The instrument is sensitive to water activity units of <0.001. Readings take 5 min or less and are accurate to 0.003 water activity units. 

Water activity a ) determination a was measured at 25°C with ar Aqualab TE8255 water activity meter (Decagon Devices, Inc. Washington, USA). 

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. 

Roa, V. and Tapia de Daza, M.S. 1991. Evaluation of water activity measurements with a dew point electronic humidity meter. Lehensmittel-Wissen-schaft Technol. 24 208-213. 

Species in solution containing the PO4 species of the orthophosphates, using the ferric salts FeCl3 and Fe2(S04)3 as precipitants Milligrams per liter of snspended sohds in raw water Cubic meters of water treated Water of hydration of alnm Activity coefficient of the calcium ion Activity coefhcient of the hydrogen ion Activity coefhcient of the hydrogen phosphate ion Activity coefhcient of the dihydrogen phosphate ion Activity coefhcient of the phosphate ion... 

For the monitoring of the released liquid effluents the following methods are used. Samples of effluents are collected by simple dipping devices and analyzed before release. In the case of monitoring streams in the neighbourhood of installations, automatic samplers collecting samples over a 24-hour period are used. Samples are analyzed and records must be kept of results. The water effluent meter monitors water or coolants and may be connected to a rate meter, recorder or alarm system. On site and off-site environmental monitoring at and near nuclear power plants, nuclear reactors and other fuel cycle activities are shown in Tables 7.5 and 7.6. 

Favetto, G., Resnik, S., Chirife, J. Ferro Fontan, C. (1983). Statistical evaluation of water activity measurements obtained with the Vaisala Humicap humidity meter. Journal of Food Science, 48, 534-538. 

Many municipal water sources are chlorinated and contain sufficiently high levels of chlorine so as to be toxic to aquatic life. Chlorine can be removed by passing the water through activated charcoal filters or through the use of sodium thiosulfate metered into the incoming water. Municipal water is usually not used in aquaculture operations that utilize large quantities of water, either continuously or periodically, because of the initial high cost of the water and the cost of pretreatment to remove chlorine. 

Although manufactured gas was too expensive for space heating, the small quantities necessary for domestic cooking and water heating provided a desirable market, and several local gas companies actively pursued it. Often a second meter was installed for the heating and cooking service, which was charged at a lower price. Manufacturers of coal stoves responded... 

Similar to the pH meter, gas meters employ specific ion electrodes. The electrodes generate a potential proportional to the activity of a specific ion in solution. The calibration is achieved in standard solution and results read in mV or concentration in mg/L or ppm on the meter. The water can be adapted to monitor the concentration of carbon dioxide, hydrogen sulfide, ammonia, chloride, calcium, potassium and sodium to name a few. 

Cherry and Crandall in 1932 (86) used olive oil as substrate with gum acacia as the emufsTfier. This method has served as the basis for a number of modifications that increased the stability of the emulsion, decreased incubation time and gave better precision. When a serum sample is incubated with a stabilized olive oil emulsion, lipase acts at the interface of substrate and water to hydrolyze olive oil into fatty acid plus diglycerides, and to a small extent to monoglycerides and glycerol. The bile salt sodium deoxycholate activates the reaction. These methods measure the liberated fatty acids by titration with a standardized NaOH solution. An indicator such as phenolphatalein, thymolphthalein or methyl red or a pH meter are used to detect the end point. 

Where Z is the water depth in meters, is the activity of in seawater (= 2750... 

The ACTIV-OX chlorine dioxide system evaluated in this trial overcomes many of the problems associated with chlorine dioxide for the small water user. A chlorine dioxide precursor solution and a dilute acid solution are mixed in a 1 1 ratio immediately prior to injection into the water to be treated. The dose rate of chlorine dioxide is controlled by water meter signal to two proportioning pumps. The mixing of the two chemicals immediately produces a chlorine dioxide solution which is diluted to the required strength by injection into the water to be treated (Fig 3). 

Hydrants should be considered as a backup water supply source to monitors and fixed fire suppression systems. Hydrants should be located on the ring main at intervals to suitably direct water on the fire hazard with a fire hose. Hydrants monitors and hose reels should be placed a minimum of 15 meters (50 ft.) from the hazard they protect for onshore facilities. Hydrants in process areas should be located so that any portion of a process unit can be reached from at least two opposite directions with the use of 76 meters (250 ft), hose lines if the approach is made from the upwind side of the fire. Offshore hydrants are located at the main accessways at the edge of the platform for each module. Normal access into a location should not be impeded by the placement of monitors or hydrants. This is especially important for heavy crane access during maintenance and turnaround activities. 

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