Dissolved Oxygen in Water

Has anyone any information of sensors for measuring Dissolved Oxygen in Water. There are complete Optical sensor units on the market but they are thousands of dollars per sensor and the equipment to interface and control are more expensive. Please any information is gratefully received.

Try the following: http://atlas-scientific.com/product_pages/sensors/do-sensor.html I never used that sensor, but the guy at atlas produce affordable and professional stuff.

The link is broken/404.

Edit: Nevermind. I have found the new link but also found out that Atlas Scientific DO sensors have problems while interfacing to arduino (https://forum.arduino.cc/index.php?topic=487535.0)

Would be going for this one instead