Apparatus for sensing overheated electrical insulation through the detection of thermoparticulates in a dynamoelectric machine's cooling gas system is disclosed. The cooling gas as it circulates will entrain thermoparticulates produced in the system when there is overheating. This gaseous carrier is then withdrawn and conducted to an ionization chamber where it is ionized. Downstream from the ionization chamber there is placed a first ion collector for collecting ions with relatively small radii. A second ion collector which is biased for collecting ions with relatively large radii is also placed downstream from the ionization chamber. By comparison of ion current from the first ion collector with ion current from the second ion collector it is possible to detect the presence of thermoparticulates in the cooling gas relatively independent of gas pressure, flow rate through the monitor, and effectiveness of the radiation source.