As shown in Figure 5.11.1, the control unit consists of the system's vacuum pump, valves, switches, thermometers, and the totalizing dry gas meter and is connected by a vacuum line with the last Greenburg-Smith impinger. The pump intake vacuum is monitored with a vacuum gauge just after the quick disconnect. A bypass valve parallel with the vacuum pump provides fine control and permits recirculation of gases at a low-sampling rate so that the pump motor is not overloaded.
Downstream from the pump and bypass valve are thermometers, a dry gas meter, and calibrated orifice and inclined or vertical manometers. The calibrated orifice and inclined manometer indicate the instantaneous sampling rate. The totalizing dry gas meter gives an integrated gas volume. The average of the two temperatures on each side of the dry gas meter gives the temperature at which the sample is collected. The addition of atmospheric pressure to orifice pressure gives meter pressure.
Precise measurements require that the thermometers are capable of measuring the temperature to within 5.4°F (3°C); the dry gas meter is inaccurate to within 2% of the volume; the barometer is inaccurate within 0.25 mmHg (torr) (0.035 kPa); and the manometer is inaccurate to within 0.25 mmHg (torr) (0.035 kPa).
The umbilical cord is an integrated multiconductor assembly containing both pneumatic and electrical conductors. It connects the two-module sampling unit to the control unit, as well as connecting the pitot tube stack velocity signals to the manometers or differential pressure gauges.
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