Hi – I’m Sherri Renberg from the global liquid analysis marketing group, and I’d like to thank the many liquid analysis experts who have contributed to this blog series. We hope you will enjoy these useful answers to some of the most frequently asked questions we get from users about pH measurement.
While some of the questions are basic, that’s why they’re valuable. pH is a measurement where it never hurts to go back to the fundamentals. We’ll cover a few questions in this blog, and more in future.
Q) What is the shelf life of a pH sensor?
A) pH glass electrodes must remain hydrated which is why all manufacturers ship pH sensors with a cap saturated in a liquid solution. After being on the shelf for some time, the liquid solution inside the sensor cap can go dry, which is the primary reason sensors go bad on the shelf. It’s a good idea to re-saturate the pH sensor cap with a 4-buffer about every (6) months that the sensor remains on the shelf to extend the shelf life of the probe. The best way to determine if the sensor is functioning accurately is to see if it calibrates properly using the two-point calibration method.
Q) What is the proper way to install a pH sensor?
A) Most manufacturers insert an air bubble inside their glass electrodes to allow for temperature and pressure changes. Without this, pH sensors could crack with large temperature or pressure swings. If a sensor is mounted horizontally, the air bubble inside the sensor can move to the tip of the sensor, which can cause poor readings because it can impede the transfer of hydrogen ions. Therefore, pH sensors should be mounted at least 10 degrees above horizontal to ensure correct measurement. Sensors can also be installed vertically.
Q) I have a pH loop and I’m getting a “low slope” error message. What does this mean?
A) If you are getting a “low slope” error message, there are a few possible causes:
• The sensor may be coated or dirty. Try cleaning the sensor and repeating the calibration.
• The glass is dry and needs to be rehydrated before calibration. To rehydrate the sensor, soak it in pH 4 buffer solution overnight. Theoretically, a brand new sensor’s slope should be 59.16mV when the sensor is set to auto-temperature compensate to 25oC, however, a new sensor could potentially have a slope as low as 55mV/pH without causing any problems. Note that the calibration is only as good as the chemicals are fresh. Make sure there are no air bubbles on the glass and that the sensor is left in the solution long enough to stabilize the reading.
• The glass is old and may need replacing.
Q) What affects the accuracy of a pH calibration?
A) The first thing to consider when trying to get an accurate pH measurement is the proper calibration of your equipment. Make sure that you take the appropriate time to calibrate your pH meter or analyzer with a quality standard buffer solution.
Room temperature, buffer temperature, and sample temperature all impact the calibration process. Try to simulate the actual environment the sensor will be operating in for the best calibration results.
As the pH sensor depends on its glass tip to make readings, the cleanliness and the quality of the glass can also impact your accuracy. Time, heat, and harsh chemicals gradually eat away at the glass surface, changing its properties and degrading the quality of the reading.
Q) What buffer calibration errors can occur when calibrating my pH sensor?
A) Buffer solutions have a stated pH value at 25°C (77°F), but when that value is 7 pH or above, the actual pH of the buffer will change with temperature. The values of the buffer solution at temperatures other than 25°C (77°F) are usually listed on the bottle. The pH value at the calibration temperature should be used or else errors in the slope and zero values, calculated by the calibration, will result. An alternative is to use the “buffer recognition” feature on modern pH analyzers, which automatically corrects the buffer value used by the analyzer for the temperature.
Another type of calibration error can result from not allowing enough time for the buffer calibration to complete. If the pH sensor is not given enough time to fully respond to the buffer solution, it can cause errors, especially in the case of a warm pH sensor not being given enough time to cool down to the temperature of the buffer solution. Current pH analyzers have a “buffer stabilization” feature, which prevents the analyzer from accepting a buffer pH reading that has not reached a prescribed level of stabilization.
This is just a start of some of the great questions users have sent us. We’ll share some more in a future blog. What kind of questions do you have about pH measurement?
by Lee Ju Young, Senior Account Manager, South Korea, Emerson Automation Solutions
Most of us know that conductivity is an excellent way to detect the interface between a non-conductive liquid, such as a hydrocarbon, and a conductive aqueous solution. Even more impressive, however, is hearing how this vital analysis is saving time and money for real companies. Here’s a great example.
Hanwha Total Petrochemical is headquartered in Seoul, South Korea, but operates a large petrochemical complex, consisting of 13 separate plants, at Daesan, in South Korea’s Chungnam Province. The company manufactures building block chemicals that go into the making of a host of other chemicals needed for various consumer products. It starts with a naphtha cracker, yielding propylene and ethylene, which are the raw materials in the production of many of polymers, like naphtha.
The naphtha is kept in storage tanks before use. During storage, water accumulates and sinks to the bottom of the tank. Because water interferes with the cracking process, it must be periodically removed. Conductivity is ideal for monitoring the drain. The water has a conductivity between 650 and 1000 uS/cm, and the naphtha has essentially no conductivity. As the water drains, the conductivity is high. When the water/naphtha interface is present, the naphtha in the interface, being non-conductive, causes the conductivity to drop. When naphtha alone is present, the conductivity is practically zero. Thus, by stopping the drain at the first sign of a conductivity drop, the operators are ensured that only water has been drained with minimum loss of naphtha.
Prior to Hanwha Total Petrochemical’s decision to use the conductivity analyzer, draining the tank of water was manual, requiring substantial human intervention. One person was positioned at the control valve at the tank outlet to watch the water drain. This person used a visual check to make sure that only water drained out. If naphtha was observed, the person called to the DCS to close the valve in order to minimize the loss of naphtha.
The simple addition of a Rosemount 1066 conductivity analyzer and sensors has significantly reduced the personnel hour demands on the plant’s staff, and even more significantly, has dramatically reduced leakage of costly naphtha from the tank. In addition, naphtha in wastewater increases the load on wastewater treatment and makes it more difficult to comply with environmental regulations – possibly leading to fines.
Conductivity analysis is one of the most used liquid measurements – for good reason. A simple addition of instrumentation can significantly improve the process efficiency, quality, and reliability.
The global dairy industry is growing, and one of the dairy producers’ biggest challenges is to maintain high product quality, while also increasing efficiency and minimizing waste and downtime. Recently Philip Edwards wrote an article on this topic for What’s New in Food Technology and Manufacturing titled, “Conductivity Measurement: A Hidden Key to Dairy Industry Success.” Below is an excerpt from this piece:
To ensure this consistent product quality, the equipment used in the manufacturing of dairy products is not only made from the highest grade of material, but also needs to be cleaned and maintained in such a way as to minimize any possible contamination when changing from one product to another or from one batch to another. This process is called CIP (clean-in-place).
To remain competitive, it is important to minimize production downtime without compromising on the safety and quality of the end product. In the CIP process, conductivity measurement is used to determine how effectively equipment has been cleaned and flushed. Conductivity in CIP picks up the change in the electric conductivity of a sample stream to indicate when a flushing process has started and ended. On a rinse cycle, for example, low conductivity indicates that all chemicals in the process stream have been flushed out and it’s ready for the next batch of product.
An Interesting Case History
A major global dairy company with plants around the world was experiencing challenges with its liquid analytical systems, particularly as related to CIP. CIP systems thoroughly clean wetted components such as tanks, vessels, fermenters, process lines and inline sensors. The CIP process controlled the flow of pre-rinse, wash and post-rinse cycles, which include caustic rinse, acid rinse and water rinse cycles.
Conductivity sensors are a critical component in the design of CIP systems. The various cleaning solutions have more conductivity than the water used for flushing and final rinse. Since many systems are a ‘re-used design,’ the sensor can monitor the strength of cleaning solutions as chemicals get used up through successive cleaning cycles. Conductivity measurements can indicate the need for replenishment.
Any sensors that have to withstand CIP and sterilization must be able to function under very harsh conditions — not a simple requirement for a sensitive analytical sensor. The dairy company was experiencing up to a 50% failure rate on sensors each year, at an approximate cost of $1,200 per sensor. Much worse, however, was the cost of plant downtime — up to $100,000 per hour. The significant failure rate called the reliability of every sensor into question after a short usage period. As a preventive measure, every conductivity sensor was replaced at the end of the season, which required another CIP cycle to be performed, adding even more costs and delay to production. It was preferable, however, to the possible dumping of milk product that would have to occur in the event of a sensor failure during processing.
To learn more about the unique solution the dairy producer implemented to overcome these challenges, click HERE to read the full story.
By Rob Clemons, Sales Manager, PCE Pacific, Inc.
Hi. I’m Rob Clemons, Sales Manager for the Instrument and Automation Division of PCE Pacific, Inc. I’ve got a great customer story to share with you. Chances are, most of you don’t own cruise lines, but as you can imagine, cruise lines have very stringent water quality rules – both for potable water and for water used in recreational facilities like hot tubs. The Center for Disease Control (CDC) places strict regulations requiring the continuous measurement of both halogens, such as chlorine, and pH. So we’re very honored to have been selected by the unique cruise line, Un-Cruise Adventures, as their water quality measurement company. This selection might have implications for your application as well.
The analyzer that proved to be ideal for the Un-Cruise Adventures application is the 56 Advanced Dual Input Analyzer configured for FCL (free chlorine) measurement. The 56 is a benchmark in ease of use and met the stringent requirements of the CDC.
The CDC regulations regarding measurement of halogens states:
A HALOGEN analyzer-chart recorder must be installed at a distant point in the POTABLE WATER distribution system where a significant water flow exists and represents the entire distribution system. In cases where multiple distribution loops exist and no pipes connect the loops, there must be an analyzer and chart recorder for each loop. Potable Water; 48 220.127.116.11.2 Data Logger Electronic data loggers with certified data security features may be used in lieu of chart recorders.
Needless to say, the choice of an electronic data logger is far superior in terms of precision, ease-of-use, and reduction in maintenance time. The CDC specifications go on to say:
18.104.22.168 Operation 22.214.171.124.1 Maintenance. A manual comparison test must be conducted daily to verify calibration. Calibration must be made whenever the manual test value is > 0.2 ppm higher or lower than the analyzer reading. 126.96.36.199.2 Calibration (06) The daily manual comparison test or calibration must be recorded either on the recorder chart or in a log. 188.8.131.52.3 Accuracy (05) The free residual HALOGEN measured by the HALOGEN analyzer must be ± 0.2 MG/L (ppm) of the free residual HALOGEN measured by the manual test.
The built-in data logger of the 56 again saves time, and enhances accuracy.
Un-Cruise Adventures is using the 56 equipment to provide CDC-mandated chlorine and pH control of three public hot tubs as well as for ensuring CDC compliance with onboard potable water systems on two passenger vessels. Since pH measurement is part of the free chlorine measurement on the 56, the system saved both time and money. To accommodate Un-Cruise Adventure’s space requirement, the back-panel of the assembly was trimmed down on one side and the 56 analyzer re-installed in order to fit inside a weather-proof enclosure they supplied. Our team is available to customize a system for you. Are there any water quality monitoring issues you have?
Dan Emigh, Port Engineer of Un-Cruise Adventures, stated, “I chose to use the Rosemount analytical equipment from Emerson for this important function based on my past experiences when installing this panel on our first vessel back in 2000. I had fantastic support from the rep and the equipment worked precisely as expected. When it became necessary to add the same type of equipment to the second vessel in 2014 I started by contacting Emerson and again received immediate and full support from the technical staff. As a direct result of the fantastic customer and technical service I have always received from everyone, I have no reason to consider anyone else for our water treatment needs. One phone call to Emerson and questions are answered, issues are resolved, and parts are on their way to our distant ships. Thanks for all the past and present support.”
And we didn’t even have to twist Dan’s arm to say that. Remember, the next time you’re sailing with Un-Cruise Adventures, the quality of the onboard water is in good hands.
By Pete Anson, Senior Product Manager, Liquid Analytical Instrumentation
Averting regulatory violations and fines. Avoiding system disruptions. Maintaining effective monitoring in complex applications. How do you tackle these kinds of age-old plant monitoring problems? New technologies in advanced analyzers offer innovative, but simple ways to address these challenges.
For example, a unique capability of the Rosemount Analytical 56 four-wire analyzer enables continuous flow measurement and totalized volume including flow rates, in addition to pH, ORP, ISE, conductivity, and other typical liquid measurements like dissolved oxygen and turbidity. The 56 can accept pulse flow type inputs and receive 4-20mA input from any flow meter to display and output flow values to a host DCS or a PLC via raw current outputs or HART® or Profibus® communications. An analyzer that measures dual-input flow allows reporting values like flow ratio, percent recovery, percent flow ratio, flow difference and totalized difference. In addition, the 56 can power and receive the 4-20mA signal from any 24vdc 2-wire transmitter. Only one AC or DC power source is needed to power the 56 analyzer, which then powers up to two loop-powered transmitters for the flow inputs and the analytical inputs.
Some examples of applications for these kinds of advanced analyzers include:
The Smart Wireless THUM Adapter is a device that can retrofit on any existing two or four-wire HART device, and it enables wireless transmission of measurement and diagnostic information that was previously not available. It’s an easy way to gain access to the field intelligence already in the plant. Key benefits include:
What are your ideas to utilize flow input with a four-wire 56 analyzer? Does the possibility of releasing stranded diagnostics with wireless sound like something worth investigating? We would like to hear your questions.