What you have is probably measuring the air temps. You need something to measure the bottom surface, since the UTH will only affect the air temp by a couple degrees. Nothing substantial.
A thermostat really is a must. A lot of people come here with burned snakes due to unregulated UTHs that didn't seem that hot to the touch. When your snake lays on top of a +100º for an extended period of time, they will start to burn. A thermostat ensures that your heat source only heats up to the temp you want, ensuring the safety of your animal. If you want to live life on the edge, so be it. But I would never put any of my animals in with an unregulated heat source.