pH/ORP Solutions

There are several advantages to using Memosens technology in pH (and other parameter) probes and sensors:

• Improved measurement performance
  Memosens technology uses digital communication to transmit data from the pH probe to the measurement instrument, eliminating the need for analog signal conditioning and reducing measurement errors caused by electrical noise or interference.

• Reduced maintenance requirements
  Memosens technology uses a digital sensor platform that enables the pH probe to store sensor-specific data, such as calibration data, in a non-volatile memory. This allows calibration of probes to be carried out remotely under clean and controlled conditions. The calibrated probe is taken to the field and once connected to an analyzer, automatically uploads its stored data to the analyzer electronics.

• Reduced downtime
  Because it is possible to calibrate probes remotely, the pH measurement system is only offline for as long as it takes to replace a sensor in the process and connect it to the analyzer. Using a retractable probe housing means that in many cases, the process can be running while this substitution is carried out

• Traceability
  Apart from calibration data, sensors with Memosens technology also store serial number, model number and date of manufacture. For some applications, they can also store the number of calibrations, number of sterilizations, date of commissioning and hours in operation. In a cGMP environment, this traceability provides additional certainty to calibration routines and records.

No. The costs in most cases are similar. The slightly higher price of a Memosens probe is offset by the omission of specialized analog input circuitry in the transmitter.

There are essentially four different types of reference interface available for industrial pH and ORP probes:

• PTFE Diaphragm
• Ceramic Diaphragm (sometimes ground glass)
• Open Channel
• REFEX Polymeric Interface

Diaphragms are porous pathways between the reference electrode in its electrolyte and the process. They serve to complete the electrical connection between the outside of the pH sensitive glass sensor and the connected analyzer electronics. Open channel and REFEX polymeric interfaces do the same thing but without a porous junction.

Porous PTFE diaphragms are highly resistant to chemical attack and can withstand high pressures and temperatures. In applications where coating and fouling are common, their large size makes them more tolerant to build up. However, as they have quite large pores, the reference electrolyte is diluted at a faster rate (relative to other diaphragm types) so salt reserves in the reference chamber (internal salt rings) are recommended to provide better stability when in operation. Poisoning is also a problem, so double junction arrangements and ion traps are highly recommended to reduce the effect of material getting into the electrode chamber.

Ceramic diaphragms are typically much smaller than PTFE diaphragms so electrolyte dilution rate, and therefore measurement drift, is much lower. Being smaller, ceramic diaphragms are however susceptible to occlusion and blockage from coatings, so are better suited to cleaner applications.

An open-channel reference in a pH probe with a set gel or solid electrolyte fill replaces a traditional porous junction with direct contact between the reference system and the process. Open channel probes are useful in applications where slurries and viscous materials might quickly clog porous junctions.

Refex reference interfaces are non-porous, making them highly suitable for aggressive and extreme applications as it is not possible to dilute or poison the reference electrolyte. Oily and other coatings do not impede measurement. They do have a higher reference impedance when compared to porous junction electrodes (<100kΩ vs. ~1kΩ), making them incompatible with some pH analyzers with built in diagnostic capabilities. Dual high impedance input analyzers are also highly recommended for the best performance.

For best performance, reliability and longevity, the pH/ORP sensor chosen should always use a reference technology that best suits the process conditions.

The use of flowing junction electrodes is quite common in UPW applications to prevent measurement drift due to electrolyte dilution. Electrodes with porous junctions allow the interchange of UPW with the electrolyte in the reference cell of a probe. As the electrolyte becomes diluted, the measurement will drift. Using a flowing junction electrode will ensure that the electrolyte salt concentration in the probe remains constant, ensuring a stable measurement. However, flowing junction electrodes require a lot of maintenance as the electrolyte in the probe reservoir must be replenished often. Lower maintenance alternatives are to use a differential pH probe, a probe with reserve salt rings or a REFEX non-porous probe.

Salt rings act as a salt reserve within the reference cell and dissolve to replenish the reference electrolyte as it is diluted by the process stream over time. This extends the working life of the sensor and provides a visible indication of when the electrode is exhausted and should be replaced. Salt rings should always be included in sensors used in applications with low ionic conductivity such as boiler feed water pH measurement and WFI.

Yes. The only difference between Memosens sensors and analog sensors is that the Memosens has a small electronic chip built in to the sensor head. Memosens and analog sensors are alike in that the pH electrode bulb and reference junction need to be kept hydrated. When storing, place the sensor in a small amount of storage solution or fit a hydration cap with storage solution in it over the end of the sensor. REFEX sensors should also be kept hydrated in the same way.

NEVER STORE A pH ELECTRODE IN PURIFIED WATER as this will shorten the life of the sensor. The reference cell is filled with a salt solution. Placing the probe in purified water will cause the salt solution to become diluted through diffusion. Storage solution is formulated to maintain the reference cell salt concentration and also has chemicals to keep bacteria and fungus from growing in the solution. If storage solution is not available then use pH 4 buffer.

Like most pH sensors, REFEX electrodes have a standard cell with pH sensitive glass. They differ from other sensors in that they have a non-porous reference junction.

Basically, most pH/ORP sensors have a porous junction—ceramic or Teflon frit/diaphragm, wood stack, ‘solid’/immobilized gel, even paper—that creates an intimate connection between process fluid and the actual reference electrode. Because of this intimate connection, process fluids and the reference cell electrolyte can intermingle, causing undesirable effects such as drift and poisoning.

REFEX pH/ORP combination sensors have an impermeable barrier between the process liquid that you are measuring and the electrode’s reference half-cell and electrolyte—essentially, no liquid-liquid contact. The reference cell is sealed for life. The barrier surrounding the Ag/AgCl reference half-cell is a solid polymeric casing that has a large immobilized KCl loading, making it electrically conductive.

Glass electrodes are extremely sensitive to external damage and need careful handling or you could damage them permanently.

1. Do not "wipe" or rub the electrode.
   The glass used in electrodes is fragile and can be easily scratched. Rubbing an electrode can remove the microscopic “gel” layer that forms on the pH bulb and is sensitive to H2 ions, making the probe non-responsive until it is re-established. It is also possible to create an electrostatic charge, making probe readings unstable.
2. Swirl the electrode gently in the cleaning solution.
   It is important not to knock the probe on the sides of your cleaning solution container while cleaning it, as this can crack or break the bulb. The electrode does not need the cleaning solution to be pressure-washed against it to be effective. Let the solution soak into the impurities/contaminants and chemically remove them.
3. Gently rinse with deionized water.
   In between gentle swirls in the cleaning solution and after you have finished cleaning, lightly rinse the electrode with deionized water. It is important not to leave any cleaning solution on the electrode after cleaning, as this could affect the pH sensor performance. Always use deionized or distilled water. Using tap water can create an electrostatic charge that might make your readings inaccurate.

Once clean, store your electrode in storage solution, or if that isn’t available, a 4 pH buffer. NEVER STORE YOUR ELECTRODE IN WATER.

The most significant benefit is probe lifetime. Refex electrodes last much longer in areas where porous junction pH/ORP sensors are quickly destroyed.

For example, in the oil and gas industry, fluids are often reactive, volatile, and corrosive (and definitely not something you want to get on yourself!). Refex electrodes have been reported to last five times as long (or longer) in these extreme applications, compared to porous junction electrodes. Probes used in chemical production or bioreactor applications where it’s necessary to measure pH under pressure, porous junction reference electrodes may last only a few days. In this type of application, REFEX probes electrodes deliver excellent performance.

Although REFEX sensors are more expensive up front, in the long term, fewer replacement probes are needed and a lot less time is spent maintaining them. This equates to significant cost savings. Plus, with faster response to pH change and more accurate readings, you can count on faster corrections to chemical imbalances, helping improve process efficiency by preventing correction chemical overdosing (and reducing costs further).

REFEX sensors can dramatically reduce plant downtime, and for any company focused on manufacturing, reducing downtime is absolutely essential to running an economically efficient operation.

This depends very much upon the application/process the probe is used in. Probes in clean, benign applications like swimming pools can last for several years, while those in aggressive applications, like chlor-alkali processes might be spent within a matter of days!

To get the maximum lifespan out of a pH probe in any application, the first step is to choose one that is suited to that process. The main point of failure in a pH probe is the reference junction. In general, for clean applications – those without solids or adherent material in the stream, a probe with a ceramic diaphragm is a cost-effective solution due to the small contact area with the process. For “dirty” applications, a probe with a PTFE diaphragm is a good idea. Material does not readily stick to PTFE and it has a large surface area. Open channel reference probes are quite suitable for slurries and viscous material applications. For processes with chemicals and substances likely to poison the electrode, a non-porous electrode is a good choice.

No matter the quality of the electrode, regular cleaning will extend the life of just about any probe. pH electrodes can be very sensitive and must be handled with care during cleaning. A good way to ensure adequate cleaning is carried out is to install the probe in a retractable housing with an automated cleaning system. Bear in mind, pH electrodes are not made to last forever, nor should they be expected to. No matter what your dedication is to regular cleaning and maintenance, eventually they will need to be replaced.

Selection and implementation of a probe suited to the process is a key decision up front. Thereafter, proper care and maintenance are essential to get the most out of your pH/ORP sensors and keeping your readings accurate. Frequent cleaning will ensure the longest possible service life for any electrode.

Fouling and coating are one of the major reasons pH/ORP probes require frequent maintenance and recalibration. The problem is with the porous liquid junction of the reference electrode. This porous junction, whether ceramic, PTFE, open, paper, or even wood, can over time become clogged by process medium, preventing proper [electrical] contact between the process and the reference electrolyte and affecting performance.

Clogging can become so significant that the electrode stops responding. Lime scale, manganese, and protein/fat build up in sewage and industrial effluents are all examples of material that can affect electrode performance through coating. Fine particle clogging—pigments and dye stuffs, precipitation between hydrogen sulfides and AgCl, etc.—is known to shorten electrode life considerably.

One solution to this problem has been to use a flowing junction electrode where the reference cell electrolyte is pressurized to create a positive KCl outflow through the junction, keeping it clear of contaminants. While this helps, it is maintenance and consumable intensive. Another solution is to use an automated cleaning system like the EXmatic 470. These systems periodically withdraw the probe from the process, rinse it off and reinsert it. These work well, but will not necessarily remove material already lodged within the probes’ connective diaphragm.

REFEX electrodes are very resistant to fouling and buildup. Being non-porous, there is nothing to block, and provided any coating on the electrode is conductive, will continue to operate as if clean. Note that eventually, it will be necessary to clean off heavy buildup from the electrode; it is difficult to accurately measure the process with the electrode buried in its own ‘micro-environment’ of buildup.

Cleaning electrodes is a labor-intensive process, and in continuous process applications, access for servicing becomes a challenge.

For manual cleaning of probes in process installations, devices such as the Exner EXtract-M are available to allow probes to be withdrawn from the process while the process line is live. Safety interlocks in these retractable housings ensure that the process is properly isolated while the probe is removed for cleaning and servicing. For additional safety, these devices are fitted with cleaning ports to allow potentially hazardous process fluids to be rinsed from electrodes before they are removed and handled.

A further step is to use an automated retraction system with a cleaning controller, such as the Exner EXtract and EXmatic products. These systems will retract a pH (or other parameter) probe from a process and execute a pre-programmed cleaning cycle on the electrode without the need for anyone to be present. Maintenance on such a system is ensuring a local supply of the required cleaning solution(s) remains stocked. Automated systems allow pH probes to be cleaned far more frequently than manual cleaning alone and this translates into much longer electrode life.