The heating indicator seems to come on and off. Why isn’t this consistently on when the mat is plugged in?

The mat’s electrical heating system does not work all the time; it shuts off when the desired temperature is reached. When the mat cools down or crystals lose heat (if you suddenly stand up from the mat), the heating system will resume. It may be ON only several minutes per hour if the mat is on the lowest level. If you want to compare the EMF level of different mats, be sure that all of them are in heating mode when you measure this.