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9.1.1.1. ZERO AIR 

Zero air is similar in chemical composition to the Earth’s atmosphere but scrubbed of all 
components that might affect the analyzer’s readings. For SO

2

 measuring devices, zero 

air should be similar in composition to the sample gas but devoid of SO

2

 and large 

amounts of hydrocarbons, nitrogen oxide (NO) and with a water vapor dew point 
≤ -15°

 

C. 

Devices such as the API Model 701 zero air generator that condition ambient air by 
drying and removal of pollutants are available. We recommend this type of device for 
generating zero air. 

9.1.1.2. SPAN GAS 

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 500 ppb, the span gas should have an SO

2

 concentration of about 

400 ppb. 

Span gases should be certified to a specific accuracy to ensure accurate calibration of the 
analyzer. Typical gas accuracy for SO

2

 gases is 1 or 2 %.  

  If using a secondary dilution source with zero air through a calibrator, then use a 

bottle of SO

2

 balanced nitrogen. 

  If calibrator and zero air source are not available, then use a bottle of SO

2

 balanced 

air. 

Teledyne API offers an IZS option operating with permeation devices. The accuracy of 
these devices is about ±5%. Whereas this may be sufficient for quick, daily calibration 
checks, we strongly recommend using certified SO

2

 span gases for accurate calibration.  

9.1.1.3. CALIBRATION GAS STANDARDS AND 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 acquiring 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. 

1693a 
1694a 
1661a 

Sulfur dioxide in N

2 

Sulfur dioxide in N

2 

Sulfur dioxide in N

2

 

50 ppm 

100 ppm 
500 ppm 

06807C DCN6650