It is stated in various sources that the HHO flame temperature is130 degrees C in open air but a source is never cited. I wonder if this value was not obtained by using an infrared thermometer to measure the flame temperature.  An infrared thermometer operates on the assumption that the heat source is a black body,  that is, the spectral signature of the emission is described by Planck's law. However, this is not true of any flame. The emission signature of a flame typically has peaks at 0.185 mm,  0.26 mm, 0.4 mm,  0.7 mm,  1.1 mm,  and 4.4 mm according to Wikipedia. So an attempt to measure flame temperature using an infrared thermometer is certain to give erroneous results. 

I vaguely recall seeing a note stating that an infrared thermometer was used to perform this measurement and it even gave a model number. If nobody can recall where this is found,  maybe someone could simply replicate such a measurement.