Title: Use of a micro programmable logic controller for oxygen monitoring and control in multiple tanks of a recirculating aquaculture system Authors
|Wills, Paul -|
Submitted to: International Conference on Recirculating Aquaculture
Publication Type: Proceedings
Publication Acceptance Date: June 1, 2010
Publication Date: N/A
Technical Abstract: In intensive recirculating aquaculture systems the use of supplemental oxygen, specifically pure liquid oxygen, increases the mass of fish that can be supported and eliminates oxygen as a major limiting factor to a system’s carrying capacity. The use of pure oxygen in a recirculating aquaculture system creates supersaturated concentrations of dissolved oxygen and can reduce fish production costs by supporting greater fish densities and lowers water flow requirements thereby reducing pumping costs, tank size, and water treatment equipment. The components required to set up the PLC oxygen monitoring and control system can be divided into five categories: (1) the 4-20 mA galvanic dissolved oxygen probes (Sensorex DO 7241); (2) the relay output modules for solenoid control of the oxygen input to the tank diffusers; (3) the micro programmable controller (Automation Direct Mighty Micro DL06) for input/ouput and control functions; (4) the human machine interface (HMI) - a 15.2 cm diagonal operator touch screen panel with Ethernet (Automation Direct C-more touch panel); and (5) the DirectSOFT software programming for the PLC. The PLC is setup to monitor and control oxygen levels in thirty-two 3.05 m diameter (1.2 m deep) tanks using the DirectSOFT PLC ladder logic program. The C-more touch panel, programmed with an graphical object oriented language, displays and logs the dissolved oxygen data, allows user input for setting control levels and probe calibration, and communication interface by internet (http, ftp, and e-mail alerts). The control program actuates the tank solenoid based on the tank’s effluent dissolved oxygen concentration. The activated solenoid allows delivery of the supply oxygen to the ceramic ultra-fine bubble diffusers (30 cm in length) in the tank. When the effluent dissolved oxygen level reaches the high set point the solenoid is deactivated and the supply of oxygen is ended. Effluent oxygen levels are monitored every PLC program scan cycle and the solenoids have been programmed with a 10 second activation delay cycle to avoid on/off chatter of the solenoids. The control program maintains the percent saturated levels in the tanks between designated high and low set points. Oxygen to the in-tank emergency diffusers is also controlled by the program if tank dissolved oxygen levels fall below the low level set point or there is a lack of water flow to the tanks. If emergency oxygen is activated by the PLC the automated feeders are deactivated in the affected tank(s). In the four systems (16 tanks) where an oxygen injection cone is used for system level oxygen input a separate PLC controlled solenoid is used to regulate oxygen delivery. The PLC control system is a cost-effective means for maintaining in-tank dissolved oxygen levels within the managerial set points and can be simply constructed and programmed.