i started with a run of the mill cheapy and that worked alright - better when i started using Vaginally - not so tempermental with outside temp or if i slept with my mouth open but it wasn't back lit or have a memory so would have to get up to got to the toilet to check it and i would be fully awake afterwards so went and got a BBT one that has a memory and is back lit so i can stay snuggling in be - was only $29 or so....