5.4 Precision Check
A precision check is a Level 2 calibration. This means that the instrument has a known concentration
of span gas (or zero air) running through it and an observation of the instrument’s concentration is
made with no adjustment. A precision check can be performed either manually or automatically. If
an instrument fails a precision check (based on your local applicable standards), perform a span
calibration (refer to Section 5.2) or zero calibration (refer to Section 5.1) where appropriate.
5.5 Converter Efficiency
The efficiency of the converter can affect NO
X
readings and therefore the converter efficiency must
be measured to ensure the efficiency is at an adequate level (96% or higher). It is also required to
correct the measurements to compensate for losses due to converter inefficiency.
To measure and correct converter efficiency the following steps should be followed:
1. Enter Main Menu Calibration Menu and set the Converter Efficiency field to 100%. Return to
Main Menu.
2. Supply the instrument with 90% required measurement range NO span gas.
3. Allow the instrument to stabilise then record the NO reading as [NO]
ORIG
and NO
2
reading as
[NO
2
]
ORIG
.
4. Turn on the O
3
generator in the Gas Phase Titration system and produce sufficient O
3
to generate
90% of required measurement range NO
2
.
Note: The NO
2
concentration must not exceed 90% of the original NO concentration from step 2.
5. Allow the instrument to stabilise then record the NO reading as [NO]
FINAL
and NO
2
as [NO
2
]
FINAL.
6. Calculate the converter efficiency from the following equation:
100
][][
][][
100
][
][
22
2
FINALORIG
ORIGFINAL
CONV
NONO
NONO
NO
NO
EFF
Equation 4 – Calculation for converter efficiency
7. If the converter efficiency is above 96%, enter the Main Menu Calibration Menu Converter
Efficiency and enter the number obtained in Equation 4.
8. If the converter efficiency is below 96% then the converter must be replaced.
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