E C O P H Y S I C S Gas Phase Titration
CLD 780 TR / July 2000 151
A3.2 Apparatus
(as per US EPA recommendations)
Fig. A3, a schematic of a typical GPT apparatus, shows the suggested
configuration of the components listed below. All connections between
components in the calibration system downstream from the O
3
generator
should be of glass, PTFE, or other nonreactive material.
a) Air Flow Controllers
Devices capable of maintaining constant air flows within
±
2 % of the
required flow rate.
b) NO Flow Controller
A device capable of maintaining constant NO flows within
±
2 % of the
required flow rate. Component parts in contact with the NO should be
of a nonreactive material.
c) Air Flowmeters
Calibrated flowmeters capable of measuring and monitoring air flow-
rates with an accuracy of
±
2 % of the measured flow rate.
d) NO Flowmeter
A calibrated flowmeter capable of measuring and monitoring NO
flowrates with an accuracy of
±
2 % of the measured flow rate. (Rota-
meters have been reported to operate unreliably when measu-
ring low NO flows and are not recommended.)
e) Pressure Regulator for Standard NO Cylinder
This regulator must have a nonreactive diaphragm and internal parts
and a suitable delivery pressure.
f) Ozone Generator
The generator must be capable of generating sufficient and stable le-
vels of O
3
for reaction with NO to generate NO
2
concentrations in the
range required. Ozone generators of the electric discharge type may
produce NO and NO
2
and are not recommended.
g) Reaction Chamber
A chamber constructed of glass, PTFE, or other nonreactive material
for the quantitative reaction of O
3
with excess NO. The chamber
should be of sufficient volume (V
RC
) such that the residence time (t
R
)
meets the requirements specified in Section A3.4. For practical rea-
sons, t
R
should be less than 2 minutes.
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