Name: Differential pH and ORP Sensors
Brand: Yokogawa
SKU: differential-phorp-sensors

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The cation differential pH and ORP sensors are 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.

What Sensor Design Is Best for My Measurement?

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

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

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. Consequently, 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.

	FU20-FTS	PH18
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)

What Is the FU20-FTS Sensor?

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.

Features

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

What Is the PH18 Differential pH Sensor?

The 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)

Application Note

Accurate pH Measurement in Limestone Scrubber

Application Note

pH in Strong Chlorate (Electrolytic Cooler)

Application Note

pH Monitoring of Pulp Stock

Application Note

Monitoring pH at the Headbox

Application Note

Sulfur Dioxide Scrubber: pH & Conductivity Control

eBook

pH Sensor Lifetime

White Paper

Monitoring Mercury Control in Coal Fired Power Plants using ORP

White Paper

Understanding Differential pH Measurement and its Benefits

4.8 MB

Media Publication

Pick the Optimal pH Sensor

What are pH Measurement and Calibration Best Practices?

(Thursday Tip 8)

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

What happens if we combine a measuring electrode with 3.3 mol KCl electrolyte with a reference electrode with 1 mol KCl in a pH sensor or vice versa?

(KB-000-0262)

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...

Can I improve the life of pH sensor when I choose 225 mm electrode in place of 120 mm electrode?

(KB-000-0311)

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 ...

Can we mount pH sensors upside down?

(KB-000-0312)

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...

Does Yokogawa offer a nonglass pH electrode?

(KB-000-0313)

Yokogawa offers the PH18 sensor. This sensor is nonglass and suitable for regular CIP and SIP cycles. ...

I am seeing an Err-2 on my pH instrument. What does it mean?

(KB-000-0223)

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...

I am getting an Err-3 on my pH instrument. What does it mean?

(KB-000-0224)

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...

Is it possible to remove air bubbles from inside a pH probe?

(KB-000-0239)

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...