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%