To measure ambient temperature and relative humidity (RH), mount the HC2S3-L probe inside a 41003-5 or 41003-5A 10-Plate Radiation Shield. The 27731 hex plug is shipped with the HC2S3-L so that it can be used with older 10-plate radiation shield models. The 6-plate radiation shield (pn 41303-5A) cannot be used with the HC2S3-L.