Background & Challenge
A marine aquaculture operation in Korea needed closed-loop dissolved oxygen (DO) control for its breeding ponds. The requirement was specific: hold DO at a defined target — the customer used 1 mg/L as their example — and use that threshold to drive a nitrogen (N₂) solenoid valve, opening the valve whenever DO rose above the setpoint and closing it the moment DO fell back to the target.
The enquiry raised four engineering questions that decide whether such a loop can be built with off-the-shelf equipment: does the controller support independently adjustable ON and OFF setpoints; how many ponds one controller can supervise; whether every pond needs its own probe and relay; and whether an additional external relay is required.
Why the KWS-630
Saltwater is hard on membrane-based DO probes: chloride attacks the electrolyte and biofilm builds up on the membrane, so the sensor drifts and needs frequent servicing. The KWS-630 optical (fluorescence) DO sensor measures without consuming oxygen and without a membrane, which makes it far more stable in seawater and reduces maintenance to periodic cleaning and calibration.
Solution & Installation
A single-threshold controller switches the output on and off at the same DO value. Near that value, the reading drifts by a few hundredths of a milligram per litre, so the solenoid valve chatters — opening and closing repeatedly within minutes — which shortens valve life and destabilises the pond. Independent ON and OFF setpoints leave a deliberate dead band: the valve opens above the upper value and only closes again once DO has dropped to the lower value, so it actuates once per cycle instead of dozens of times per hour.
The number of ponds one controller can supervise is governed by its input and relay channel count. Every pond that must be controlled independently needs its own DO probe and its own switching channel; a relay module is added where the controller output current is below the coil current of the N₂ solenoid valve. Each configuration is built from the same blocks:
| Item | Per pond | Notes |
|---|---|---|
| Optical (fluorescence) DO probe | 1 | Luminescent DO measurement; no membrane, no electrolyte refill |
| Controller channel | 1 | Independent ON/OFF setpoints, one loop per channel |
| Relay module | as required | Only where the solenoid coil exceeds the controller output rating |
| Cable | 1 run | Probe-to-controller and controller-to-valve runs |
| Software/configuration | system | Setpoint entry, logging and alarm setup |
| Calibration service | per probe | Zero and span verification before hand-over |
| Freight, customs, tax, warranty, lead time | system | Quoted to the delivery country |
Results
The customer requested three configurations — 2, 4 and 8 ponds — so that the same control philosophy could be compared at three scales before committing to the largest one. Each option was built on one independent ON/OFF channel and one optical probe per pond, with the full scope (probe, controller, relay module, cable, software, calibration, and delivery terms) so the options could be compared like for like.
Related Products
- KWS-630 fluorescence dissolved oxygen sensor
- KWS601 self-cleaning DO sensor
- KWS-650C online dissolved oxygen sensor
- KMPW series universal controller
- Dissolved Oxygen Sensor (category)
Frequently Asked Questions
Q: Can one DO controller run several ponds at different setpoints?
A: Yes, provided the controller has one independent measurement input and one switching channel per pond. Each channel keeps its own ON and OFF setpoints, so pond A can be held at a different target from pond B. The exact number of channels is confirmed against the controller datasheet for the chosen model.
Q: Is a separate probe needed for every pond?
A: Yes. Dissolved oxygen is local to a body of water, so a probe in one pond says nothing about the pond next to it. Every independently controlled pond needs its own probe.
Q: Do I need an external relay as well?
A: Only when the solenoid valve coil draws more current than the controller output can switch. In that case, a relay module is inserted between the controller and the valve; for small valves, the controller output drives the valve directly.
Q: Why use optical rather than electrochemical DO probes in seawater?
A: Optical probes have no membrane and no electrolyte to be attacked by chloride, and they do not consume oxygen at the measurement surface. That gives better stability and much longer service intervals in marine and brackish water.
Q: What should be specified to get an accurate quotation?
A: The number of independently controlled ponds, the target DO band for each, the gas valve type and coil current, the cable run lengths, and the delivery destination. With those five items, a 2-, 4-, or 8-pond package can be quoted without ambiguity.
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