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Model 551 pH/ORP Sensor

551 pH/ORP Sensor

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Axial Ion Path® Reference
  • Patented design increases sensor life, accuracy and reliability
  • High resistance to poison: Reduced calibration offset error
  • Large surface area reference junction eliminates plugging issues
  • Eliminates error due to fluctuating pressure
  • No exotic gel or polymer electrolyte which may be incompatible with the process

In some in-line applications sensor removal for routine cleaning or calibration becomes difficult due to conduit or cabling. The 551 Quick Change Sensor offers a unique method to extract the sensor through a “Nut Lock” Adapter system. The Nut Lock Adapter threads directly into 1 inch NPT process connections. Sensor length options up to 12 inches allows the sensor to fit through flanges and stand-off piping. The 551 sensor also includes a 3/4 inch rear connection for use in submersible applications.

Pressure/Temperature Ratings


Quick Change Nut Lock Adapter Style


Threaded plastic or
metal body with hand nut

 Threaded metal body
with metal hex nut
 Plastic body with metal hex nut
(flanged or threaded mounting)
 Kynar (white)          150 PSIG @ 158°F (70°C)    
 40 PSIG @ 266°F (130°C)
 300 PSIG @ 176°F (80°C)    
 40 PSIG @ 266°F (130°C)
 150 PSIG @ 73°F (25°C)
 25 PSIG @ 266°F (130°C) 
 Peek (tan)  150 PSIG @ 158°F (70°C)
 40 PSIG @ 266°F (130°C)
 300 PSIG @ 176°F (80°C)
 40 PSIG @ 266°F (130°C)
 150 PSIG @ 73°F (25°C)
 25 PSIG @ 266°F (130°C)

Insertion Depth

The depth that pH / ORP sensor protrudes into the pipeline can greatly affect the measurement. Applications where the sensor tip is recessed can lead to coating and slow response. In high particulate applications abrasion of the electrode can be a concern.

A typical installation goal is to get the sensor tip at least 1/4" (6mm) into the stream. At this depth coating issues lessen and response improves due to flow velocity. 546, 551 and 547 sensors offer a variety of insertion depths. Here are some guidelines.

551 pH/ORP Insertion Depths

551 pH/ORP Slipstream Installation

Installation Methods

In-Line Applications (Threaded)
In-Line Flange Applications