STs Series Dissolved Oxygen FAQ

Commonly asked questions and answers regarding STs Series Dissolved Oxygen maintenance and features.

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 should the electrode be stored?

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.

Do you provide a warranty?

Yes, our products come with a 2-year warranty, excluding the electrode. These products are designed to last far longer if properly cared for and we will do what we can to help if any problems occur in or out of warranty.  Please don’t hesitate to reach out!

Read our full warranty here.

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.  Please visit our article on voltage drop for more information and example calculations.

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.

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.

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.

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.

How can the STs Series be installed/mounted?

The STs Series can be mounted in-line or immersed with the cmp-k compression fitting in a 3/4″ pipe, or freely suspended by the Kevlar™ cable.  Please see the pHionics STs Series Device Manual or contact us at for more information.

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