The pHionics STs Series™ Dissolved Oxygen sensor measures dissolved oxygen (0-20 mg/L or percent saturation) and temperature (0-50°C) in a single narrow probe. Two noise-resistant 4-20 mA outputs connect to most dataloggers, PLCs and RTUs.

The latest STs Series uses a user-serviceable galvanic electrode. You no longer replace the whole electrode when it wears or gets damaged. Instead, you change the membrane and electrolyte solution yourself on the bench using the Membrane Replacement Kit included with the sensor. Servicing is more regular (typically every 1-2 months, depending on the site), but it needs no special tools and costs far less than a new electrode. A damaged membrane becomes a quick swap rather than a replacement order and downtime.

The large-surface-area anode keeps oxygen exchange across the membrane consistent and reduces fouling. The chemical-resistant PVDF and 316 stainless steel housing is built for long-term, unattended deployment in wastewater, aquaculture and environmental monitoring.

STs Series Dissolved Oxygen Sensor Features List

• Galvanic electrode with a large surface area anode to increase stability and prevent clogging
• User-replaceable membrane and electrolyte for low-cost field servicing
• Secondary 0-50°C temperature sensor
• Chemical-resistant PVDF and 316 Stainless Steel housing (Titanium optional) for long-term monitoring
• Narrow diameter (1.9 cm or 0.75 inches)
• Removable electrode guard to reduce debris buildup and protect from damage
• Fully submersible up to 483 kPa (70 psi)
• Shielded, Kevlar®-reinforced, water-blocked polyurethane cable
• 4-20 mA signal capable of transmission up to 4.8 km (3 miles) with proper wire gauge
• Patented pHiConn™ connections to easily replace any part without special tools
• Automatic Temperature Compensation (2% per degree Celsius)
• Reversible Input Protection and Operation
• Low power usage for long-term water quality monitoring
• Isolated to prevent ground loops
• Signal conditioning to remove background noise

Accurate Measurements

Application List

• Industrial Effluent Monitoring
• Wastewater Treatment
• Environmental Monitoring
• Aquaculture

*After proper calibration

How long does a new electrode take to burn off oxygen in the cell?

New dissolved oxygen electrodes have trapped oxygen from the manufacturing process. This trapped oxygen must be burned off before accurate measurement can be taken. While many dissolved oxygen electrodes require multiple days, our electrodes burn off trapped oxygen within 15 minutes.

How long are STs Series devices expected to last?

While the electrode will require replacement every two years on average, the housing, seals, and cables are intended to last 10-20 years with proper care. To increase longevity of the seals, the Viton O-rings should be relubricated with a silicon grease during each disassembly, or at least inspected for any debris or damage. The harshness of the solution (high/low pH, extreme salinity, high pressure, etc.) also affects longevity. All parts are replaceable to save money and limit waste in the event that any piece gets damaged.

How should the electrode be stored?

Please see the STs Series Storage guide.

What type of electrode is provided?

Our dissolved oxygen electrode is a galvanic cell containing propionic acid. It has a larger surface area than other electrodes, allowing for extremely fast response time and a longer lifespan.

How long will the electrode last?

The standard life is 24-30 months while engaged with the sensor, powered or not. To extend the life, disengage the electrode from the transmitter at least one full turn, or completely remove the electrode and store it. Always protect the electrode with the provided rubber boot or similar. Unlike many dissolved oxygen electrodes, ours do not have any internal resistors or connections so they do not passively consume the electrolytes until attached to the sensor.

What spare parts are recommended?

For critical measurements, a duplicate STs Series sensor is always recommended so a replacement for every part is available at any time. A duplicate simplifies troubleshooting by always having a functional STs Series available. It also for swapping of devices during calibration and cleaning, so measurement is not significantly disrupted. At a minimum, a spare electrode is recommended in the event of damage or eventual replacement.

How often should the STs Dissolved Oxygen Series be calibrated and/or cleaned?

The frequency of calibration must be determined empirically as rate of drift and debris accumulation differs for each application. Calculating the offset during calibration provides information on whether the electrode is still viable. See our article on Proper Sensor Calibration for assistance in the process. Check out our Dissolved Oxygen Electrode Care Guide for cleaning and troubleshooting procedures.

What voltage is required for the STs Series Dissolved Oxygen?

An 8VDC power supply is sufficient with standard lengths of cable. Voltage drop must be accounted for over long lengths of cable (50+ ft.) to ensure enough power is supplied to any water quality sensor. To calculate it, first double the length of cable to get the account for the full length of the circuit, then multiply that by resistance per foot/meter to get total resistance. pHionics uses 24 AWG tinned-copper wire which has a resistance of 0.026 ohms per foot (0.084 ohms per meter). Multiply total resistance by the maximum expected current (20 mA, or 0.02 A, is safest) to obtain expected voltage drop. Verify that the voltage drop plus required voltage for pHionics sensors (8 VDC) is less than the power supply voltage.

Is the output automatically temperature-compensated (ATC)?

The output is adjusted by 2% per °C relative to 25°C. A thermistor within the housing is used which is why the ATC is most accurate when the device is fully submersed. ATC functions regardless of whether the temperature output is in use.