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A.  A leak that admits gas containing NO. 

B.  Contaminated reaction cell. Remove and clean cell. 

C.  Wet (i.e. undried ambient air) air into the ozone generator. Check the PermaPure drier and 

associated plumbing for leaks and correct operation. 

D.  Zero gas that isn't really zero. Make sure you're not trying to zero the machine with 

sample air or span gas. 

E.  AutoZero filter is corrupted with high readings. Fix the cause of the high readings, then 

input zero gas and wait for 15 minutes for the filter to clear. 

Common causes are: 

1.  Leak in pneumatic system. 

2.  Calibration gas tanks concentration in error. 

A.  Tanks certification test in error. 

B.  Even if the NO concentration is correct, different tanks may have variable concentrations 

of interfering gasses. CO

2

 is often blended in the NO tanks, and is a known interferent 

with chemiluminescent NO

X

 analyzers. If the CO

2

 concentration is markedly different 

between tanks, the CO

2

 quenching effect on NO will produce non-linearities. This effect 

may also be present with gas standards that blend SO

2

/N

2

/NO. The problem arises 

because the gas manufacturer uses FT-IR to do the analysis. FT-IR does not have the same 
interaction between components as do chemiluminescent analyzers. 

3.  Variation in sample inlet pressure due to sample manifold inlet flow changes. 

4.  High zero background - The PMT TEST function should be near 0 mV with zero gas. 

Readings above 25 mV indicate a light leak, contaminated reaction cell, bad zero gas, or wet 
air coming into the ozone generator. If the reading is not less than 20 mV the instrument will 
not zero or span properly. Check Section 9.2.9. 

5.  Calibration device in error, re-check flowrates and concentrations. This is especially true at 

low concentrations. Mass Flow calibrators often become non-linear if the flow is < 1% of the 
full scale flow.  

6.  Contamination in sample delivery system: 

A.  Dirt in sample lines or reaction cell. 

B.  Contaminated cal gas source (NO

2

 in NO cal gas is common). 

C.  Dilution air contains sample or span gas.