Model 755A
Instruction Manual
c.Normal Operation at Positive Gauge Pressures
Pressure at Sample Inlet (All Instruments) - Normally, the sample is supplied to the analyzer inlet at a positive gauge pressure in the range of 0 to 10 psig (0 to 69 kPa).
CAUTION
HIGH PRESSURE SURGES
High pressure surges during admission of sample or standard gases can damage the detector.
Sample Exhaust - The analyzer exhaust is vented directly to the atmosphere through an exhaust line with inner diameter sufficiently large as not to cause any back pressure. Internal circuitry automatically corrects the oxygen readout to within ±1% of fullscale for atmospheric pressure variations within ±3% of target value and within ±2% of fullscale for barometric pressure variations within ±5% of target value. The target value may be set anywhere within range of
d.Operation at Negative Gauge Pressures
Operation at negative gauge pressures is not recommended but may be used in certain special applications. A suction pump is connected to the analyzer exhaust port to draw sample into the inlet and through the analyzer. Such operation necessitates special precautions to ensure accurate readout. There is the basic consideration of supplying the standard gases to the analyzer at the same pressure that will be used for the sample during subsequent operation. In addition, any leakage will result in decreased readout accuracy as compared with operation at atmospheric pressure.
The minimum permissible operating pressure is
Operation below this limit may damage the detector and will void the warranty.
e.Sample Flow Rate
Operating limits for sample flow rate are the following: Minimum 50 cc/min.; maximum 500 cc/min. A flow rate of less than 50 cc/min. is too slow to sweep out the detector and associated flow system efficiently resulting in a slow system response. Too rapid a flow will cause a back pressure that will affect the reading. The optimum flow rate is between 200 and 300 cc/min.
Deviation from the set flow should be held to within ±1% or ±2 cc/min, whichever is smaller. If so, zero and span drift will be within the limits given on the specifications page, provided that operating pressure remains constant.
Bypass Flow - Preferably the analyzer should be installed near the sample source to minimize transport time. Otherwise the time lag may be appreciable. For example, assume that sample is supplied to the analyzer via a 100 foot (30.5 m) length of 1/4 inch (6.35 mm) O.D. thin walled tubing. With a flow rate of 100 cc/min., sample transport time is approximately 6 minutes.
Sample transport time may be reduced by piping a greater flow than is required to the analyzer, then routing only the appropriate portion of the total flow through the analyzer. The unused portion of the sample may be returned to the stream or discarded.
Rosemount Analytical Inc. A Division of Emerson Process Management | Installation |