Multiparameter Sondes



6600 V2

6920 V2

600OMS V2

Legacy Sondes

All Legacy Sondes


CastAway CTD

Process Monitors

IQ SensorNet 2020 XT

IQ SensorNet 182

P 700 Orthophosphate

Aquaculture Systems

5200A Multiparameter

5400 MultiDO

5500 MultiDO Optical

AquaViewer App

Professional Series

Handheld Instruments

ProDSS Multiparameter

Pro Plus Multiparameter

ProODO Optical DO/OBOD

Pro30 Field Cond

Pro20 Lab/Field DO

Pro10 Field pH/ORP

Pro1020 Field DO/pH/ORP

Pro1030 Field pH/ORP/Cond

Pro2030 Field DO/Cond

Classic Handheld Series

556MPS Multiparameter

550A Dissolved Oxygen

63 pH/Conductivity

55 Dissolved Oxygen


900 Chlorine

910 COD

pHotoFlex Colorimeters

Laboratory Series


5000/5100 BOD/DO

3200 Conductivity

3100 Conductivity

MultiLab 4010-1

MultiLab 4010-2

MultiLab 4010-3

TruLab 1110

TruLab 1310 and 1310P

TruLab 1320 and 1320P


pH10A - pH Pen

pH100A - pH

ORP15A - ORP Pen

ODO200 - Optical DO

DO200A - DO

EC30A - Conductivity Pen

EC300A - Conductivity

9300/9500 Photometer

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EcoMapper AUV

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Underway Sampling System

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SonTek Acoustic Doppler Systems for water velocity

YSI Systems

DAA WaterLog

YSI Life Sciences


Blue-Green Algae

BOD (Biochemical Oxygen Demand)






Dissolved Oxygen



Oil / Hydrocarbon

ORP / Redox

Photosynthetic Active Radiation (PAR)





Specific Conductance


Total Dissolved Solids (TDS)


Colorimeter Parameters

Photometer Parameters





Drinking Water


Hydraulic Fracturing



Raw Water

Source Water

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Watersheds / TMDL

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In the beginning there were a few engineers and a lot of ideas.

The legacy of our global company reaches back to 1948 and a three-man partnership that was forged at Antioch College in Yellow Springs, Ohio, USA. This village, between Dayton and Columbus, still serves as the headquarters for what started as the "Yellow Springs Instrument Company" — now known as YSI Inc.

We invite you to browse our brief history and explore our timeline.


YSI's Corporate Office
A Brief History of YSI

Back in the late forties, two engineers, Hardy Trolander and John Benedict were joined by chemist David Jones and started looking for problems to solve through the application of good science. Shortly after the three men incorporated the company in 1948 and Hardy Trolander was appointed the first President, they were joined by David Case, another Antioch alumnus.

The four men worked with researchers at Fels Research Institute. Additionally, they bid on a job at the Wright-Patterson Air Force Base (formerly Wright Field) with the help of Fred Hooven, who became the fifth stockholder. Hooven had gained a reputation for his significant contributions to the Army Air Corps at Wright Field, among them being principal inventor of the automatic radio compass still in use today.

The Air Force accepted the men's concept for a 16k random access memory (RAM) with on-board crystal clock in response to its need for a programmable timing device with precision and resolution well beyond anything available at the time. This successful prototype led to Air Force contracts that produced cash flow to support development and commercialization of further ideas.

First instruments released and sales grow

After a few years as a struggling start-up, YSI's first commercial instrument was designed -- the Model 3A Dielectric Constant meter. Then followed interchangeable thermistor temperature probes, a notable industry first, and associated temperature instrumentation.

By 1960, YSI had several dozen employees and numerous instruments that had gained wide acceptance among scientists and researchers. A European distribution network was established early in this decade. The engineers continued to work hard to develop more innovative products.

Innovative sensor technology

The history of YSI would be incomplete without mention of Dr. Leland Clark, to whom not only YSI, but the world, owes a debt of gratitude. Dr. Clark was a researcher at Fels with whom Hardy Trolander and the others worked closely. He is best known for having developed the aptly-named Clark oxygen electrode that allows dissolved oxygen to be measured in liquids. Applications not only include environmental ones, but biomedical and industrial as well.

For example, his invention enabled physicians to perform open-heart surgery because, for the first time, immediate blood oxygen measurements could be taken real-time in the operating room rather than having a sample drawn which then had to be taken to a lab for analysis. Dr. Clark also developed the technology behind the glucose biosensor, used daily by millions of people with diabetes and other conditions.

In 1970, Dr. Clark demonstrated the enzyme-activated polarographic measurement of glucose in water and pointed out that glucose could be measured in whole blood with the presence of the glucose oxidase enzyme. Within a couple of years, YSI was able to market its pioneering Model 23 Glucose Analyzer with a polarographic electrode.

At this point in time, YSI was building a range of instruments based on precision sensor technology for temperature and biomedical applications.

Hardy Trolander believed strongly in the social benefits of a business enterprise -- one that gave its employees meaningful work; one that contributed to the economic health of communities and intellectual stimulation of industries. This legacy was continued upon Hardy's retirement in 1983, with the appointment of Malte vonMatthiessen as YSI's second President/CEO. Malte focused on expanding our niche markets, initiated quality improvement programs, and developed alliances with domestic and global partners. During this time, sales and distribution were set up in the Eastern hemisphere.

After several years with Malte at the helm, YSI began to sharpen its focus on product development and replaced the traditional hierarchical managerial structure with cross-functional and self-directed teams.

Innovation continues with upgraded R&D

Moving into the 1990s, the company adhered to a well-defined plan of providing quality products to specific global niche markets (environmental monitoring and testing, precision temperature measurement, and biotechnology) while continuing to reduce costs. YSI recognized its core competency in sensor technology and committed three million dollars for a new facility to house stepped-up sensor measurement technology teams.

Additionally, new product development and business processes incorporated environmentally responsible practices such as recycling, reduced-footprint packaging, and energy conservation.

During this decade, YSI introduced what became our most significant product line to date: the 6-Series environmental monitoring sondes. These instruments brought together YSI’s proven dissolved oxygen sensor with additional sensors into a family of products specific to monitoring water quality. Customers were able to leap out of the lab and into the field, where data was obtained more immediately and accurately.

Many changes have come about since the start of the new millenium. Rick Omlor became YSI's third President/CEO in 2002. In 2005, Rick and his leadership team made the strategic decision to focus the business in a select few areas rather than following the 60-year history of ever broadened markets and sensor development. This strategy, focused mainly on the water quality and water velocity applications in the natural resource market, has served the customer, company and shareholder very well.

The first decade of the twenty-first century brought several major natural disasters and a continually increasing scarcity of natural resources. With this as a backdrop the company is geared to provide products and data services that help to address these situations.

YSI will continue its legacy of invention and innovation as we move forward; and not just in the area of technology. We will continue to seek new creative solutions for remaining close to those who use our products and services, ways in which we operate our business, and methods for seeking and retaining talent. We leave behind our first 60 years with gratitude and admiration to those who established this great company and look forward to continuing to build on that firm foundation in the coming years.

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