TA you need the ovulation one, it measures smaller changes in temperature - when you're charting you are sometimes talking about really tiny temp shifts, like tenths or twentieths of a degree. So the ovulation or basal body temperature thermometer is the one to get, it should measure to two decimal places or if it's the old kind, it will only measure from about 35-38 degrees.

You can get them for less than $32 though - I paid about $18 for mine.