Oxyguard Dissolved Oxygen probe

While the cost of the Dissolved Oxygen Module PM3
is very reasonable, the down side is that the DO probe itself is relatively
expensive.

The DO Probe certified and sold by Neptune Systems
is a lab grade device and costs about 5x the cost of the PM3 module
itself. 

That’s the bad news. The good news is that the
probe has replaceable membranes so you don’t have to throw it away when it
wears out 

like other probes. Instead, every three months you
simply put a new membrane in the probe