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T400 Ozone Analyzer Operator’s Manual EPA Protocol Calibration
213
A well-designed properly built photometer is a precision instrument, and once it is
operating adequately, it is likely to continue to do so for some time, particularly if the
photometer is stationary and is used intermittently under ideal laboratory conditions. If
the photometer is commercially manufactured, it should include an operation/instruction
manual. Study the manual thoroughly and follow its recommendations carefully and
completely.
10.3.4. LINEARITY TEST
Because the required photometric measurement is a ratio, a simple linearity check of the
photometer is a good indication of accuracy. Linearity of commercially made
photometers may be demonstrated by the manufacturer. The linearity test is conducted
by first generating and assaying an ozone concentration near the upper range limit (80%
of full scale is recommended) of the reporting range in use.
Other data points can be created by adding zero air (F
d
) to the flow of originally
generated concentration (F
o
) and pass the mixture through a mixing device to ensure a
homogeneous concentration at the Inlet to the analyzer being calibrated.
The First step of performing this linearity test is to determine the dilution ratio of the
various test points according to the following formula:
Equation 10-1
)FF(
F
R
do
o
+
=
For this test, the flow rates F
o
and F
d
must be accurately measured within ±2% of the
true value. To help ensure accurate flow measurements, the two flowmeters should be
of the same general type and one should be standardized against the other. The dilution
ratio R is calculated as the flow of the original concentration (F
o
) divided by the total
flow (F
o
+ F
d
).
With stable, high resolution flowmeters and with careful technique, R should be accurate
to within 1%.
06870C DCN6332