Twolunger

There are possibly 3 different temperatures when you use an under tank heat source. There is the temp you set with a thermostat, or heat controlling device. There is the actual temp under the substrate, usually on the inside glass bottom of the tank or vivarium, and then there's the temp on top of the substrate. I set my thermostat and then test the temps with thermometers with probes. I check the temp at the heat source, then place one thermometer probe under the substrate, and one on top of the substrate. Some people prefer to use infrared temp guns, and their prices have come down so they are quite affordable. I use thermometers with probes because mine have alarms that let me know if my heat controller malfunctions.