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> Rosemount analytical 499ATRDO dissolved oxygen sensor
Rosemount analytical 499ATRDO dissolved oxygen sensor
The Model 499A TrDO sensor is intended for the determination of trace (ppb) levels of dissolved oxygen in steam power plants and high purity cooling water.
The 499A TrDO is a membrane-covered amperometric sensor. The sensor consists of a gas-permeable Teflon membrane stretched tightly over a gold cathode. A silver anode and an electrolyte solution complete the internal circuit. During operation, oxygen diffuses from the sample through the membrane to the cathode. A polarizing voltage applied to the cathode converts all the oxygen entering the sensor to hydroxide ions. The reaction produces a current between the cathode and the anode, which the analyzer measures. The current is directly proportional to the rate at which oxygen reaches the cathode, which is ultimately proportional to the concentration of oxygen in the sample.
Because the rate of diffusion of oxygen through the membrane depends on temperature, sensor response must be corrected for changes in membrane permeability caused by temperature. A Pt100 RTD in the sensor measures the temperature, and the analyzer automatically performs the correction. The analyzer also uses the temperature measurement during automatic air calibration.
Calibration is easy. Simply expose the sensor to water-saturated air and press a button. The analyzer measures the barometric pressure using an on-board pressure sensor and calculates the equilibrium solubility of atmospheric oxygen at the prevailing temperature and pressure.
Maintenance is fast and easy. Replacing the membrane requires no special tools or fixtures. Simply place a few drops of electrolyte solution in the membrane assembly, place it on the cathode, and screw the retainer in place. To replenish the electrolyte solution, unscrew the fill plug, add the reagent from a squeeze bottle, and replace the plug.
Pressure changes have little influence on sensor response. A flexible bladder in the sensor prevents distortion of the membrane by keeping the pressure inside the sensor equal to the sample pressure.
Comedown time is rapid. An out-of-the-box 499A TrDO sensor is accurate at the 1 ppb level within five hours after being placed in service. Unlike competing sensors that use a separate oxygen-scavenging electrode, the 499A TrDO relies on a self-depleting design to reduce internal oxygen and achieve fast comedown. The result is a more robust, reliable sensor.
The 499A TrDO sensor is available with a Variopol (VP) watertight connector. Wire the interconnecting cable to the analyzer and run the cable to the sensor. The sensor plugs into the cable receptacle. To replace the sensor, simply disconnect the Variopol fitting and plug in a new sensor.
