This unit was designed as stationary low cost local display on tanks and cages. The MD unit has its own 9v PP3 batteries so it can act as a stand alone unit,  or used in conjunction with oxygen probes on our multichannel AquaTraul systems.  The unit was designed as a stationary unit with our standard dissolved oxygen probe. However the unit has proved to be incredibly robust, low cost and simple to use, and because of the internal battery,  it is mobile. Our clients have now started to use them as portable dissolved oxygen meters.  While they were not designed for this purpose,  they do seem to be a practical alternative to the more expensive purpose designed instruments.