Differential pH/ORP Sensors

The cation differential pH and ORP sensors were designed for difficult applications where conventional sensors are ineffective. These include measurements such as brine solutions to applications as diverse as electrolysis processes and cheese manufacturing.

The problems experienced in these applications most often relate to the reference cell and are the result of either:

  • high diffusion potentials at the reference junction
  • high temperatures (often encountered in applications such as in chlorinated brine processes)
  • and high ground-loop currents.

Which Sensor is right for you?

There are three known different types of differential pH measurements.

Glass-in-Glass Design

The first is a Glass-in-Glass design. This design uses a standard glass electrode as the measuring electrode which generates a potential proportional to the process pH. The second glass electrode serves as the reference electrode and consists of an internal measurement electrode immersed in a stable buffer solution. The internal electrode makes electrochemical contact with the process via a salt bridge chamber (double junction) and generates a standard reference potential. Both glass electrodes have a common potential developed at the third electrode-the solution ground electrode.

This design eliminates the stability problems experienced with conventional references due to process poisoning of the reference element. However, since it utilizes a liquid junction interface with the process, this reference design will still suffer from plugging and coating problems.

Dual Enamel Design

The second is the Dual Enamel design. This is an ion specific technology. The electrode is a 316SS rod coated with enamel. The top enamel functions as the pH measuring site by reacting to Hydrogen Ions. A positive or negative change in the activity of the Hydrogen Ions in the measured fluid results in corresponding change output of the pH enamel. The bottom enamel band measures Sodium Ions and provides the reference voltage. The sodium enamel is a measurement band sensitive to all sodium ions. Increases or reductions in the concentration of sodium (in any formulation) have a corresponding effect on the output of the sodium glass enamel.

A disadvantage to this technology is that the reference voltage of the sodium ion electrode is not definite and predictable as is true in traditional reference electrodes. The reference voltage is determined by and is dependent on the background concentration of sodium ions in the process solution. The stability of the reference voltage is enhanced with higher concentrations of sodium ion. Low conductivity solutions, below 5 mS, lack sufficient sodium and will result in unstable measurements.

Cation-Sensitive Reference Electrode

The third design is a cation-sensitive reference electrode. It is actually a measuring electrode which measures or responds to the concentrations of salts present in the solution. Therefore, as salt concentrations change, so will the reference voltage it generates. Therefore it is process dependent and applications specific.

However, the sensor has NO junction. There is NO path from the process to the internal element; so NO poisoning can occur. Also since there is NO junction, there are NO plugging or coating problems to worry about and there is NO electrolyte depletion problem because there is NO electrolyte.

So when applied correctly differential pH is an exciting new technology and provides a giant step in solving difficult or nearly impossible process pH measurements. 




Measurement Theory Cation Reference Sodium Reference
pH Range 2-14 3-11
ORP Range -1500 to 1500 mV NA
Temperature Range 0 to 105°C (14 to 221°F)

0-140°C (32 to 284°F)

*Reference specificaiton sheet for corrosion resistant curve*

Pressure Range

0 to 10 bar (0 to 145 PSIG) @ 25°C

0 to 5 bar (0 to 72 PSIG) @ 105°C

-1 to 15 bar ( -14.5 to 214 psi)



The FU20-FTS is the newest development in pH sensor technology available from Yokogawa. This sensor combines the measuring technology of our 12 mm differential sensor and the ruggedness of the appreciated wide body FU20 design in one product.

As is common in the market, Yokogawa has used silver/silver chloride reference cells in its products. In a wide range of applications, this solution has proven very effective and remains a cost-effective solution.

The lifetime of the conventional sensors is highly dependent on regularly maintaining pH equipment. Frequent cleaning is required to eliminate reference poisoning. 70-80% of industrial users will fully benefit from using differential sensor technology in their high temperature and pressure applications.


  • No junction
  • No open connection from the process to the inside of the sensor
  • No possibility of poisoning reference element
  • No use of diaphragm hence no issues of plugging or coating of the junction diaphragm
  • No outflow of electrolyte so no depletion issues

The model PH18 differential pH sensor is unique and offers the possibility or maintenance free operation for the correct application.

Construction of the enamel coated probe is based on a rigid steel rod. A blue base enamel is over-laid with two yellow bands of sensitive enamel. A pH sensitive enamel and sodium ion enamel are combined with a rhodium liquid earth electrode and a temperature sensor.

The differential probe measuring principle combines the normal potential generated by the pH enamel with the potential from a sodium membrane. In suitable applications the level of sodium ions creates a stable reference voltage. Hence the measurement can be made without a conventional reference electrode, and drawbacks associated with a liquid junction.

The sensitive membranes are directly bonded to a metallic substrate, which eliminates the need for conventional internal reference elements. This unique construction makes the PH18 independent of the drift which can be caused as reference elements age or become contaminated. When used in a system which is cleaned and/or sterilized with hot water or steam, the membranes produce a signal which has long term stability unrivaled by conventional systems.

The use of this sensor is highly application specific. Your local Yokogawa sales office will be please to advise on the suitability of your application, on receipt of the completed application data sheet. Any and all information received by Yokogawa will be treated in the strictest confidence.

  • Sterilizable pH sensor
  • All enamel construction
  • No reference electrode or junction
  • Easy installation and commissioning
  • No routine maintenance
  • Flexible installation options
  • 140 ºC, 15 bar (284 ºF, 214 psi)



Continuous technology improvement is ongoing in the pulp & paper industry to obtain the best possible performance. Problems at the wet end (stock preparation) can rarely be corrected downstream. That is why monitoring and controlling pH in pulp stock is critical to the paper making process. Essentially, at every stage in the manufacture of paper, correct pH values play a vital role.


Problems at the wet end of a paper machine can rarely be corrected downstream. That is why monitoring and controlling pH in pulp stock is critical to the paper-making process. Essentially, at every stage in the manufacture of paper, correct pH values play a vital role. Variations in the pH value at the headbox have a negative effect on the quality of the paper produced. Proper monitoring ensures minimizing plugging and fouling problems that occur with pH measurements in paper mills. 


The lifetime of a pH sensor has a significant impact on the overall annual costs of a pH measuring loop. Optimizing four key factors will decrease these costs and optimize process control and overall plant efficiency.


Current trend for increasing mercury awareness throughout the public sector has caused the government to take action. Recently, the Environmental Protection Agency (EPA) has focused their efforts on controlling mercury levels produced in various coal fired power plants. Based on information from several case studies, the EPA developed the Mercury and Air Toxics Standards to cut back mercury emissions. The most popular technology utilized by coal plants to meet the new standards is a scrubber which cleans the off gas from the combustion process. ORP sensors can further monitor the effluent from these scrubbers to ensure optimal mercury emission levels are achieved. By closely monitoring the mercury concentrations in the effluent, plant managers will be able to easily confirm their plants are meeting the EPA's standards.

Media Publication
Why is pH Calibration important? How to clean pH glass? Which pH buffers should I use?  How fresh should pH buffer solution be?  How long between calibrations?
Edition 1

Why is pH Calibration important? How to clean pH glass? Which pH buffers should I use?  How fresh should pH buffer solution be?  How long between calibrations?

The reference potential difference between 1 molar and 3.3 molar is approximately 30 mV. The sensitivity of a pH sensor at reference temperature is 59mV/pH. So if you combine a measuring cell filled with 3.3mol KCl with a reference cell with 1mol KCl...
That is possible but not guaranteed. The 225 mm sensor body is longer, so it has more electrolyte. Often the reference electrode fails because the electrolyte is depleted as it diffuses through the reference junction into the process. With ...
Most pH sensors with a glass membrane cannot be mounted upside down. To absorb the thermal expansion of the internal buffer solution there is always a sizeable air bubble inside the sensor. When the sensor is mounted upside down the reference element...
Yokogawa offers the PH18 sensor. This sensor is nonglass and suitable for regular CIP and SIP cycles.
An Error 2 means that the calculated asymmetry potential is outside the acceptable range. The Asymmetry Potential (AS), also referred to as the millivolt offset, is an indication of the condition of the reference electrode. Theoretically when the ele...
The Error 3 is related to the slope value on a pH loop, indicating that the slope value is outside of the allowable range. The Slope (SL), also referred to as the efficiency of the electrode, is an indication of the condition of the pH electrode. The...
Air bubbles are normal in pH sensors and in some reference sensors. They are necessary because the pH sensor is a sealed device with no vent. Without the air bubbles the expansion and contraction of the liquid with temperature changes could cause the...




This video aims to answer the following: How does a traditional pH reference work and what are it's common problems? How does a sodium pH reference work and what are it's benefits? This webinar was presented February 12th, 2013.

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