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Teledyne API – Model T700 Dynamic Dilution Calibrator Overview of Operating Modes and Basic Operation
109
4.2.3. GENERATE GPT: PERFORMING A GAS PHASE TITRATION
CALIBRATION
4.2.3.1. GPT Theory
The principle of GPT is based on the rapid gas phase reaction between NO and O
3
,
which produces quantities of NO
2
as shown by the following equation:
Equation 4-4
)(
223
lighthONOONO
It has been empirically determined that under controlled circumstances the NO-O
3
reaction is very efficient (<1% residual O
3
), therefore the concentration of NO
2
resulting
from the mixing of NO and O
3
can be precisely predicted and controlled as long as the
following conditions are met:
The amount of O
3
used in the mixture is known.
The amount of NO used in the mixture is AT LEAST 10% greater than the amount
O
3
in the mixture.
The volume of the mixing chamber is known.
The NO and O
3
flow rates (from which the time the two gases are in the mixing
chamber) are low enough to give a residence time of the reactants in the mixing
chamber of >2.75 ppm min.
Given the above conditions, the amount of NO
2
being output by the T700 will be equal
to (at a 1:1 ratio) to the amount of O
3
added.
Since the O
3
flow rate of the T700’s O
3
generator is a set fixed value (typically about
0.105 LPM) and the GPT chamber’s volume is known, once the TOTAL GAS FLOW
requirements, the source concentration of NO, and the target concentration for the O
3
generator are entered into the calibrator’s software. The T700 adjusts the NO flow rate
and diluent (zero air) flow rate to create the appropriate NO
2
concentration at the output.
4.2.3.2. Choosing an Input Concentration for the NO
It is important to ensure that there is enough NO in the GPT chamber to use up all of the
O
3
. Excess O
3
will react with the resulting NO
2
to produce NO
3
. Since NO
3
is
undetectable by most NO
x
analyzers, this will result in false low readings.
The EPA requires that the NO content of a GPT mixture be at least 10% higher than the
O
3
content. Since there is no negative effect to having too much NO in the GPT
chamber, Teledyne API recommends that the NO concentration be chosen to be some
value higher (as much as twice as high) as the highest intended target NO
2
value and
kept constant.
As long as the flow rate is also kept constant three of the four conditions listed in
Section 4.2.3.1 above are therefore constant and the NO
2
output can be easily and
reliably varied by simply changing the O
3
concentration.
EXAMPLE:
Calibration values of NO
2
from 200 ppb to 450 ppb will be needed.
The NO gas input concentration should be no lower than 495 ppb and can be
as high as 900 ppb.
06873B DCN6388