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Calibration Procedures Teledyne API - Model 200EH/EM Operation Manual
156
7.1.3. SPAN CALIBRATION GAS STANDARDS & TRACEABILITY
NOTE
We strongly recommend that span calibration is carried out with NO span gas, although
it is possible to use NO
2
or a gas phase titration (GPT) system.
Quick span checks may be done with either NO, NO
2
or a mixture of NO and NO
2
as is
used in GPT.
Span gas is specifically mixed to match the chemical composition of the gas being measured at about 80% of
the desired full measurement range. For example, if the measurement range is 120 ppm, the span gas should
have an NO concentration of about 96 ppm.
Span gases should be certified to a specific accuracy to ensure accurate calibration of the analyzer. Typical gas
accuracy for NO
X
gases is 1 or 2%. NO standards should be mixed in nitrogen (to prevent oxidation of NO to
NO
2
over time), whereas NO
2
standards should be mixed in air (to keep it oxidized).
For oxygen measurements, we recommend s reference gas of 21% O
2
in N
2
. the user can either utilize the NO
X
standards (if mixed in air). For quick checks. ambient air can be used at an assumed concentration of 20.8%.
Generally, O
2
concentration in dry, ambient air varies by less than 1%.
7.1.3.1. Traceability
All equipment used to produce calibration gases should be verified against standards of the National Institute for
Standards and Technology (NIST). To ensure NIST traceability, we recommend to acquire cylinders of working
gas that are certified to be traceable to NIST standard reference materials (SRM). These are available from a
variety of commercial sources.
Table 7-1: NIST-SRM's Available for Traceability of NO
x
Calibration Gases
NIST-SRM
4
TYPE
NOMINAL
CONCENTRATION
2627a
2628a
2629a
Nitric Oxide (NO) in N
2
Nitric Oxide (NO) in N
2
Nitric Oxide (NO) in N
2
5 ppm
10 ppm
20 ppm
1683b
1684b
1685b
1686b
1687b
Nitric Oxide (NO) in N
2
Nitric Oxide (NO) in N
2
Nitric Oxide (NO) in N
2
Nitric Oxide (NO) in N
2
Nitric Oxide (NO) in N
2
50 ppm
100 ppm
250 ppm
5000 ppm
1000 ppm
2630
2631a
2635
2636a
Nitric Oxide (NO) in N
2
Nitric Oxide (NO) in N
2
Nitric Oxide (NO) in N
2
Nitric Oxide (NO) in N
2
1500 ppm
3000 ppm
800 ppm
2000 ppm
2656
2660a
Oxides of Nitrogen (NO
x
) in Air
Oxides of Nitrogen (NO
x
) in Air
2500 ppm
100 ppm
2659a Oxygen in Nitrogen (O
2
) 21 mol %
04521C (DCN5731)