It's best to have multiple means of measuring when you use tubs. For the smaller tubs, I use ExoTerra, Zilla or Coralife probed thermometer and hygrometers. Just place the probes in the tub and the units outside with their suction cups and you're good to go.

units outside tub


probes inside tub



For tanks and larger tubs, I place the Acurite unit in the center to read ambient temp/humidity with the probe under the warm hide (wire held down by the hide or by a suction cup under the bedding). To read cool side, I have an Acurite single unit or an ExoTerra/Zilla/Coralife with just the probe in the tank/tub.

Like this:


It works great and lets me get multiple readings. I also have a Temp gun that I use to double check readings and make sure batteries don't need replacing.