All relevant product information can be found on the Analox Sensor Technology website, take a look at the O2NE+ and Safe-Ox+ product pages.  

The O2NE+ and Safe-Ox+ use a type of sensor that measures the partial pressure of oxygen in the atmosphere where the instrument is located. These sensors are not cross sensitive to helium so will measure correctly in a helium atmosphere. This means that the sensor can be safely used to detect oxygen displacement by helium gas leaks.