Why A Little Knowledge About Sensors Can Be A Dangerous Thing

By Mark Spencer, President, Water Analytics

Every day our technical support people answer questions on the selection of sensors for pH, ORP, conductivity, dissolved oxygen and turbidity. No issue generates more confusion than cell constants for conductivity sensors. The vast majority of returns we process are for conductivity sensors that were ordered with the wrong cell constant. It turns out that cell constants are something that we all read but that most of us don’t really understand. Pick the wrong cell constant for a probe and your analyzer will happily give you numbers to 3 significant digits. The only problem is those numbers are wrong.

I am forced to confess that, for many months, I was confused about what a cell constant is. So I thought I’d try to bring clarity to a subject that can be misconstrued in so many ways.

Before we define a cell constant let’s back up and explain why we need one at all.