Assessments - 3, GPA: 3.7
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d. Calibration of Automatic Pressure Compensation Oxygen readout is automatically corrected to within ±1% of fullscale for barometric pressure variations within ±3% of the target value and is corrected to within ±2% of fullscale for variations within ±5% of the target value. The target value may be set anywhere within the range of -2.7 to 3.3 psig ±3 psig ( 18.6 to 22.8 kPa ±21 kPa). The factory setting is 0 psig (0 kPa). This setting is suitable for applications where (a) the analyzer exhaust port is vented directly to the atmosphere, and (b) the installation site is at or near sea level. If conditions are otherwise, the pressure compensation circuit must be recalibrated by the following procedure: 3-8 Operation Rosemount Analytical Inc. A Division of Emerson Process Management Instruction Manual 245364-V May 2002 Model 755A F1 Compound Range Pressure Gauge or Mercury Manometer Pressure Gauge Vacuum PumpPR1 Sample InSample In 755A 250 cc/minV1 V310 psig V2 V4 Figure 3-3. Calibration by Pressure Decrease Setup Standard Procedure: Calibration by Pressure Decrease 1. Verify that the Pressure Compensation Option is selected, i.e., Jumper inserted between E1 and E3 on the Control Board. 2. Connect a compound range pressure gauge (or mercury manometer), pressure gauge, pressure regulator, flowmeter, needle valves and vacuum pump as shown in Figure 3-3 above. 3. Open V2, set PR1 to approximately 10 psig (69 kPa) and adjust V1 for a flow rate of 250 cc/min. into the sample inlet line. Compound range pressure gauge (or mercury manometer) should indicate close to atmospheric pressure (0.00). 4. Set TEST switch to PRESS CAL 1 and adjust CAL 1 potentiometer R98 (see Table 3-2, page 3-6, and Figure 3-2, page 3-7) for a reading of 00.00±5 counts on the display. 5. Set TEST switch to NORM and note display reading. 6. Start vacuum pump. 7. Close V2 and adjust V3 and V4 to decrease the pressure on the sample exhaust by about 3%. (12.00 inches of water; -0.4 psi; -23 mm of mercury; -3 kPa). 8. Adjust CAL 2 potentiometer R99 (see Table 3-2, page 3-6, and Figure 3-2, page 3-7) until reading on display is the same noted in step 5. 9. Repeat steps 3 through 7 as many times as necessary to ensure repeatability of compensation calibration. NOTE Although not mandatory, it isdesirable, in order to obtain maximum accuracy, that the oxygenconcentration of the gas used duringcalibration be close to that of the process stream which is to beanalyzed. Rosemount Analytical Inc. A Division of Emerson Process Management Operation 3-9 Instruction Manual 245364-V May 2002 Model 755A 3-5 COMPENSATION FOR COMPOSITION OF BACKGROUND GAS Any gas having a compensation other than 100% oxygen contains background gas. The background gas comprises all non- oxygen constituents. Although instrument response to most gases other than oxygen is comparatively slight, it is not in all cases negligible. Contribution of these components to instrument response cab be computed for each individual case. If the downscale and upscale standard gases contain the same background gas as the sample, the routine standardization procedure automatically compensates for the background components; therefore, they introduce no error. If the background gas in the sample is different from that in the downscale and/or upscale standard gas(es), however, background effects must be taken into consideration to ensure correct readout. During adjustment of the ZERO and SPAN controls (on the analyzer door), the instrument is set to indicate, not the true oxygen content of the downscale and upscale standard gases, but slightly different values calculated to provide correct readout during subsequent analysis of the sample gas. The calculations are explained in Section 3-5c below. a. Oxygen Equivalent Values of Gases For computation of background corrections, the analyzer response to each component of the sample must be shown. Table 3-3 (page 3-11) lists the percentage oxygen equivalent values for many common gases. The percentage oxygen equivalent of a gas is the instrument response to the given gas compared to the response to oxygen, assuming that both gases are supplied at the same pressure. In equation form: %O2 Equivalent of Gas = Analyzer Response to Gas X 100 Analyzer Response to O2 To select a random example from Table 3-3 (page 3-11), if analyzer response to oxygen is +100%, the response to xenon would be -1.34%. b. Oxygen Equivalents of Gas Mixtures The oxygen equivalent of a gas mixture is the sum of the contribution of the individual gas components. Example: At lower range limit, i.e., 0% oxygen, composition of sample is 80% CO2, 20% N2. From Table 3-3(page 3-11), the % oxygen equivalents are CO2. -0.623 and N2, -0.358. % oxygen equivalent of the mixture = 0.8 x (-0.623) + 0.2 x (-0.358) = (0.4984) + (-0.0716) = -0.570 c. Computing Adjusted Settings forZero and Span Controls During instrument calibration, Adjusted Values may be required in setting the ZERO and SPAN control to c...