for a good reaction and because vapors are not as useful as a symptom of overheating the visual effect may be a change from one color to another from colorless to colored or vice versa from opaque to clear or vice versa or some other observable difference since some of the thermoparticulating compounds such as malonic acid and its derivatives produce acidic particulates indicators which change color in the presence of acid are often useful the examples given hereinafter illustrate various combinations of indicators and thermoparticulating compounds if the indicator is solid the particulate filter can usually be prepared by simply dissolving the indicator in a solvent depositing the solution on the filter material and evaporating the solvent some filter materials however must be activated generally this is accomplished by dipping the filter material in potassium silicate solution perfusing with carbon dioxide washing with ammonium chloride then clean water and heating at about 90 u00b0 c to provide an active silica gel coating on the filter material details are provided in u s pat no 3 689 224 the apparatus used for testing the indicator device is designed to simulate turbine generator conditions hydrogen 7 l min flow rate was passed over the samples which were contained in a stainless steel boat within a stainless steel tube 1 inch o d accurate temperature measurements were made by mounting the hot junction of a chromel alumel thermocouple within a small hole in the boat.