Dew Point temperature (wet-bulb temp) along with the air temperature (dry-bulb temp) is measured by a psychrometer. I'm not aware of any inexpensive devices that contain a psychrometer to display an accurate wet-bulb temp.
An accurate Relative Humidity is derived from those wet/dry bulb reading. Many mechanical (analog) and electronic (digital) instruments only give a gross RH approximation. If that RH value is calculated back to a wet-bulb temp from those displays, the margin of error would be +/- 5 degrees (or more). That error makes a dew point temp display grossly inaccurate.
Oregon Scientific sells a nice line of home weather instruments.
THIS is the one I have in my trailer.
