I have seen two numbers relating response of Cl 2 in a ClO 2 detector. One is 3.1 to 1 and the other is 10 to 4, both in favor of ClO 2 . What is the relative response of the Interscan ClO 2 detector and can Cl 2 be used to spot calibrate it?

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As you may know, surrogate gas calibration for chlorine dioxide is recommended because it is extremely difficult to generate a stable source of ClO₂, especially for practical field use. Indeed, the use of chlorine as a surrogate for chlorine dioxide calibration is one of the very few instances whereby Interscan advocates such a method.

Pioneering work by NCASI, the The National Council for Air and Stream Improvement, had pegged the response ratio such that approximately 3 ppm of chlorine is required to cause a 1 ppm reading on an Interscan chlorine dioxide analyzer.

Subsequent efforts have refined the number a bit. We now advise our customers that:

2.8 ppm of chlorine will show a 1 ppm reading on a chlorine dioxide analyzer, and this ratio is stated with an accuracy of ±10%