Target Basal Thermometer

Has anyone had any experience with the target basal thermometer? On several occasions I’ve taken my temp and I will be the exact same as the previous day. This morning it registered as the exact same as yesterday’s temp (97.20•F). So I just restarted it as soon as it beeped and that time my temp was much higher (99.01•F). I know that you warm up as you wake up but that was literally about a 1 minute difference. I’m look at different thermometers now, can anyone suggest one?