Those can be pretty inaccurate, and usually just measure the temperature of the glass they are stuck to.
I would suggest that you pick up one of these from Walmart. They cost $12 and you only need a single unit for the average cage.
This thermometer/hygrometer has a probe for the hot side of the cage, and a second temperature sensor/humidity sensor behind its screen. To use it, simply place the probe on the hot side, and the unit on the cool side.
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