STs Series ORP (redox) Sensor

Highlighted Features 

  • Replaceable electrode with platinum sensing element measuring -1000 to +1000 mV oxidation-reduction potential (ORP)
  • 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 monitoring applications 
  • Low power usage

Ranges available:

  • -1000 to +1000 mV
  • -1000 to +0 mV
  • -500 to +500 mV
  •  +0 to +1000 mV

For datalogging applications, we recommend pairing our devices with the HOBO RX3000.


The pHionics STs Series™ ORP is a low maintenance, dual-output sensor that pairs with a datalogger or RTU for real-time water quality monitoring.  A narrow diameter allows the STs Series to go where other sensors cannot 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.

Starting at $980 USD

All STs Series base prices include a 25 ft. (7.62 m) cable.  Custom cable lengths available.


Please see our STs Series ORP (redox) Price List for more information on available options and pricing. Our spare parts pricing is available here.

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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) 
  • Narrow diameter (0.75 inches or 1.9 cm) 
  • Removable electrode guard to reduce debris or algae buildup and protect from damage 
  • 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 and low power usage to avoid battery drain of datalogger or RTU
  • Isolated to prevent ground loops  
  • Signal conditioning to remove background noise  

Low Maintenance

The weak point of ORP sensors is their reference solution, which gets diluted by the sample over time.  STs Series ORP 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 junction, reduces the dilution rate to provide a more stable reference. 

Accurate Measurements

No noise. No nonsense.

Experience the difference with our proprietary noise reduction technology.


ORP (redox) Sensor

Output 4-20 mA
ORP (redox) Range -1000 to +1000 mV
Operating Temperature 0-70°C
Pressure Range 0-70 psi
Electrode Type

Platinum sensing element

Silver/silver chloride gel-filled reference

Transmitter Type 2-wire
Automatic Temperature Compensation None
Isolation 600 VDC
Power Supply Voltage 8-40 VDC
Warm-up Time 2 seconds
Loop Impedance (Max)

200 Ohms at 12 VDC

800 Ohms at 24 VDC

1600 Ohms at 40 VDC

Response time 95% < 20 seconds
Wetted Materials 316 Stainless Steel, Delrin®, Viton™, epoxy
Length 14.25 in (36.20 cm)
Width 0.750 in (1.9 cm) maximum
Weight (No cable) 0.50 lbs (0.227 kg)
Linearity ± 4 mV
Accuracy ± 4 mV*
Sensitivity ± 1 mV
Stability ± 2 mV
Repeatability ± 10 mV

*After proper calibration

Temperature Sensor

Output 4-20 mA
Temperature Range 0-50°C
Accuracy +/- 1°C
Linearity +/- 0.5°C
Power Supply Voltage 7-40 VDC


Cable Material Polyurethane
Type 4 conductor, Kevlar® reinforced, water blocked, shielded, twisted-pair wiring
Maximum Distance 3 miles
Resistance per 1000 ft (304.8 m) 26 Ohms
Replaceable? Yes

pHionics products comply fully with all applicable EU Directives to bear the CE Mark.

Delrin® and Kevlar® are registered trademarks of DuPont™.

Viton™ is a trademark of Chemours.

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