A chilled mirror hygrometer sensor originally designed to work with the Swiss radiosonde has been adapted for use with the VIZ Manufacturing Co. MK-2 radiosonde. Comparisons between the chilled mirror sensor, the carbon hygristor, and capacitive sensor types flown simultaneously on the same balloon platform are discussed. Although all of the profiles indicate moisture layers there are, sometimes, major differences in the relative humidity values. Although the chilled mirror technique is not new, this is the first known application of its use on a low cost radiosonde. Paired chilled mirror instruments flown together indicate that its repeatability is very good. It is believed that the chilled mirror sensor is accurate enough to serve as a reference standard for evaluating operational radiosonde relative humidity sensors and for validating remote measurements