http://www.paracademy.com/events/abst_june01.htm
This study was conducted in '97 and submitted in '01. There are many more but I chose this one as it more closely resembles my own experience and challenges the results of most other studies claiming only 1-5% undetected esophageal intubation. Further searching did reveal that most studies including recent ones achieved results this study challenged.
My search results show end tidal co2 detectors including colorimetric as far back as '92 aboard EMS with one OR study as far back as '88-
ccrn