Teledyne T200, Руководство по эксплуатации, Страница: 63 / 425

Teledyne API
– T200 NO/NO
2
/NO
X
Analyzer Operation Manual Getting Started
61
Figure 3-26: Rear Panel Layout with Internal Span Source (IZS) OPT 50G
I
NTERNAL
S
PAN
G
AS
G
ENERATION
The primary component of the internal span option is a permeation tube containing
liquid NO
2.
As zero air is passed over a permeable membrane on the end of the tube,
molecules of NO
2
slowly pass through the membrane mixing with the zero air.
The resulting concentration of the NO
2
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 NO
2
(increasing the temperature of the permeation tube
increases the pressure inside the tube, thereby increasing the rate of permeation)

flow rate of the zero air (if the previous two variables are constant, the permeation
rate of the NO
2
into the zero air stream will be constant; therefore, a lower flow rate
of zero air produces higher concentrations of NO
2
)
In the Model T200 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.
06858E DCN7057