Teledyne 9110T instruction manual Internal Span GAS Generation

Model 9110T NOx Analyzer Getting Started

Figure 3-26: Rear Panel Layout with Internal Span Source (IZS) OPT 50G

INTERNAL SPAN GAS GENERATION

The primary component of the internal span option is a permeation tube containing liquid NO2. As zero air is passed over a permeable membrane on the end of the tube, molecules of NO2 slowly pass through the membrane mixing with the zero air.

The resulting concentration of the NO2 span gas is determined by three factors:

∙Size of the membrane: The larger the area of the membrane, the more permeation occurs.

∙Temperature of the NO2: Increasing the temperature of the permeation tube increases the pressure inside the tube and therefore increases the rate of permeation.

∙Flow rate of the zero air: If the previous two variables are constant, the permeation rate of the NO2 into the zero air stream will be constant. Therefore, a lower flow rate of zero air produces higher concentrations of NO2.

In the Model 9110T the permeation tube enclosure is heated to a constant 50° C (10° above the maximum operating temperature of the instrument) in order to keep the permeation rate constant. A thermistor measures the actual temperature and reports it to the CPU for control feedback.

The flow rate of zero air across the permeation tube is maintained at 50 ± 10 cm³/min by a critical flow orifice located in the analyzer’s exhaust manifold.

NITRIC ACID AND THE CHEMISTRY OF NO2 PERMEATION TUBES

The reaction of H2O with NO2 to form HNO3 (nitric acid) takes place whenever water and NO2 are present in the same gas mixture. In the 9110T this is mitigated as much as possible by passing the air supply for the span gas generator through a special dryer, however the permeable membrane of the NO2 tube will still allow H2O from the ambient environment to slowly collect in the tube at increasingly higher concentrations. Over time this results in the presence of HNO3 in the permeation tube which is exuded into the 9110T’s pneumatics along with NO2.

Teledyne Analytical Instruments 70