- Measurement range, membrane controller
- 0 to 200 percent saturation and 0 to 20.00 mg/L or ppm, with a temperature channel from 0.0 to 65.0 degrees Celsius
- Measurement range, optical controller
- 0 to 20.00 mg/L or ppm, with the saturation range carried as a reserved parameter; electrode temperature 0 to 60 degrees Celsius
- Accuracy
- plus or minus 1.5 percent of full scale on the membrane controller; plus or minus 0.5 mg/L on the optical controller; temperature to plus or minus 0.5 degrees Celsius
- Compensation
- Temperature automatically by a sensing element in the electrode or manually; salinity entered manually; pressure entered manually
- Outputs
- Analogue current output selectable between 4-20 mA, 0-10 mA and 0-20 mA, with a stated maximum load resistance
- Alarm relays
- Two relays, configurable for concentration, for temperature or off, with the contact rating stated separately for the two controllers
- Communication
- RS485 serial port on the Modbus RTU protocol, with a selectable baud rate
- Display
- Backlit 3.5 inch 128 x 64 dot matrix liquid crystal panel showing both channels, with the interface switchable between English and Chinese
- Electrode protection
- IP68 on the optical electrode, which is sealed and needs no electrolyte and no membrane replacement
- Supply and protection
- AC or DC 100 to 240 V; controller housing stated at IP65 with a flame-retardant rating
Oxygen dissolved in water is the least visible figure in a treatment plant and one of the few that decides the biology underneath it. Too little, and the aerated basin stops working; too much, and energy is being spent on air that the process cannot use. Either way the consequence appears hours after the cause, which is why the measurement has to be continuous rather than sampled.
An online dissolved oxygen analyzer is therefore a control instrument rather than a laboratory one, and it is judged on whether the reading holds while nobody is watching it.
Product overview
The range is built around two sensing routes sharing one controller format. A membrane controller works with a stainless-steel electrode containing an electrolyte behind a gas-permeable membrane, and reports dissolved oxygen as a concentration or as a percentage of saturation. An optical controller works with a sealed electrode containing a luminescent layer, and reports concentration.
Both are panel instruments of the same footprint, both carry a two-line backlit display, an analogue output, two alarm relays and a serial port, and both apply the same three compensation terms. What separates them is the electrode, and the electrode is what decides the maintenance the installation will carry for the rest of its life.
How the reading is produced
A membrane electrode is an electrochemical cell. Oxygen passes through the membrane and is reduced at the cathode, and the current produced is proportional to the rate at which oxygen arrives, which is why this route depends on a minimum flow past the membrane and on the membrane itself staying clean and intact. The electrolyte and the membrane are consumables, and the material states a calibration every two months.
The optical electrode measures the same quantity without consuming it. A dye layer is excited by a light source and emits at a longer wavelength, and dissolved oxygen shortens the duration of that emission. The instrument reads the lifetime, which does not depend on how much water moves past the sensor, so the electrode tolerates still water and is sealed to IP68. The consumable here is the sensing cap.
Both routes then go through the same conversion. The sensor responds to a partial pressure, and the controller turns that into a concentration using the temperature, the salinity and the pressure of the water. All three are set at the panel, the temperature automatically from an element in the electrode and the other two by hand.
How to select the right analyzer
Begin with the electrode, because everything else follows from it. Where the water is still, or where the site cannot guarantee that a membrane and an electrolyte will be replaced on schedule, the optical route is the one that will still be reading correctly a year later. Where the water moves and the maintenance is real, the membrane electrode is the less expensive instrument doing the same job.
Then confirm the compensations. A duty in an estuary, a brine tank or a pressurised main cannot use the defaults, and the values have to be known before the range and the resolution are settled rather than after. After that the questions are electrical: how many relays the installation needs and what they switch, whether the reading has to reach a plant network, and whether the record has to exist at all.
Measurement limitations and selection considerations
The analyzer reports what the electrode senses, and the electrode is exposed to everything in the water. Turbidity, algae, oil and biological growth all affect a membrane electrode before they affect the reading in any visible way, so a cleaning interval belongs in the commissioning plan alongside the instrument specification.
Two figures that a buyer would normally use to compare the two routes are absent. The optical electrode has no stated response time and no stated calibration interval, against the under-thirty-second response and two-month interval given for the membrane electrode. Until those are confirmed, the choice between the routes rests on the maintenance argument rather than on a like-for-like comparison of performance.
Every value below is taken from source documentation. Where a parameter is not stated in the available material it is marked as information required rather than estimated.
