non - boil boiling point indicator includes a probe housing having inner and outer chambers and a heater located within the inner chamber at a free end of the probe housing . the probe housing has a free end provided with non - restrictive apertures . similar non - restrictive apertures are located between the inner and outer chambers and from the outer chamber to an outer environment surrounding the probe housing . the temperature of the test fluid is monitored by a temperature monitoring device . as the test fluid is rapidly heated , its temperature quickly rises then flattens out before boiling point . this change is used to extrapolate to the boiling point . localized steam or vapor bubbles are created next to the heater , which causes the test fluid above the heater to be propelled upwards and overflow through the unrestricted apertures at the top of the inner chamber into the outer chamber .