The pHionics STs Series™ pH sensor provides accurate pH and temperature measurements to a datalogger or RTU for continuous water quality monitoring. A large, double junction reference cell provides accurate, stable pH measurements for long periods of time. In addition, isolation, differential amplification, and shielding eliminate background noise without draining the battery of the datalogger or RTU.

The STs Series goes where other sensors cannot with a 0.75″ (19mm) diameter while a durable, chemical-resistant housing ensures many years in-use.

All parts, including the cable, are easily replaceable without tools using pHionics patented pHiConn™ System, saving frustration and money.

These features make our pH sensor well-suited for applications such as groundwater monitoring, wastewater treatment, aquaculture, pollution control, and many others.

Highlighted Features

• Accurate double-junction electrode measuring 0 to 14 pH
• Large reference half-cell to reduce calibration frequency
• Secondary 0-50°C temperature sensor
• 2-wire, 4-20 mA analog outputs compatible with wide range of dataloggers or remote telemetry units (RTU)
• Narrow diameter (0.75 inches or 1.9 cm) for groundwater or in-line mounting applications
• Low power usage

Ranges available:

• 0-14 pH

Low Maintenance

The weak point of pH sensors is their reference solution, which gets diluted by the sample over time.  STs Series pH sensors mitigate this problem by incorporating a reference half cell with a large reservoir.  The high volume, combined with a low-flow reference gel and double-junction, reduces the dilution rate to provide a more stable reference.

Full Features List

• Combination double-junction electrode with large reference reservoir
• Secondary 0-50°C temperature sensor
• Chemical-resistant Delrin®, PVDF, and 316 Stainless Steel housing (Titanium optional)
• Metal housing acts as the solution ground, removing background noise caused by stray electrical currents
• Narrow diameter (0.75 inches or 1.9 cm)
• Removable electrode guard to protect from damage while allowing access to electrode for cleaning
• Fully submersible up to 70 psi
• Shielded, Kevlar®-reinforced, water-blocked polyurethane cable
• 4-20 mA signal capable of transmission up to 3 miles (with proper wire gauge)
• Automatic Temperature Compensation (2% per degree Celsius)
• Reversible Input Protection and Operation
• 2-second warm-up time to avoid battery drain of datalogger or RTU
• Low power usage for long-term water quality monitoring
• Isolated to prevent ground loops
• Signal conditioning to remove background noise

Accurate Measurements

Application List

• Groundwater Monitoring
• Agricultural Runoff
• Mining Pollution
• Industrial Effluent Monitoring
• Wastewater Treatment
• Ballast Monitoring
• Environmental Remediation
• Aquaculture

*After proper calibration

What is the proper way to store an ORP electrode?

Please see our STs Series Storage guide >>

How long does the electrode last?

Typical electrode life in an innocuous application is about three years.  The nature of the solution being measured (flow rate, pH range, ion concentration etc.) all affect longevity so the lifespan can range from 6 months to 5 years.  Electrode life must be determined empirically for each application through consistent calibration.

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.

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 pH?

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 the 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 three 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 pH electrodes are silver/silver chloride with a Teflon double junction.  They are replaceable with a gel electrolyte instead of refillable electrolyte solution to reduce overall maintenance time.

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 allows for swapping of devices during calibration and cleaning so the measurement or process is not significantly disrupted.  At a minimum, a spare electrode is recommended in the event of damage–or the need for eventual replacement–when waiting for a replacement to be shipped could be detrimental to your application.

How often should the STs Series pH 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 pH Electrode Care Guide for cleaning and troubleshooting procedures.

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 for more information.