44 70th ANNIVERSARY In-situ measurements with the Model 52 Model 52 FIELD D.O. METER The first true field- ready dissolved oxygen instrument takes YSI customers out of the lab. 6-Series Sondes sontek YSI purchases respected leader in flow and discharge measurement technology, SonTek, expanding into hydrology. 1990 1996 2001 1992 70 YEARS of INNOVATION early sonde prototype An early prototype that would eventually lead to the successful 6-Series & EXO sonde platforms. 6-Series YSI introduced what became our most significant product line to date: the 6-Series environmental monitoring sondes. Customers leap into the field.