By Dave Joseph, Emerson
Conductivity measurement is one of the most ubiquitous in industry so the issue of how to correctly calibrate a toroidal conductivity sensor comes up for a lot of plant personnel. Because the purpose of the measurement is to get information about the total concentration of ions in solution (i.e., the conductivity), the effect of sensor dimensions and windings must be accounted for. The correction factor is the cell constant. Conductivity is equal to the measured conductance multiplied by the cell constant.
There are two basic ways to calibrate a toroidal sensor: against a standard solution, or against a referee meter and sensor. A referee meter and sensor is an instrument that has been previously calibrated and is known to be accurate and reliable. The referee instrument can be used to perform either an in-process or a grab sample calibration. In-process calibration involves connecting the process and referee sensors in series and measuring the conductivity of the process liquid simultaneously. Grab sample calibration involves taking a sample of the process liquid and measuring its conductivity in the laboratory or shop using the referee instrument. No matter which calibration method is used, the analyzer automatically calculates the cell constant once the known conductivity is entered.
The cell constant is also influenced by the nearness of the vessel walls to the sensor, the so-called wall effect. Conductive and non-conductive walls have opposite effects. Metal walls increase the amount of induced current, which leads to an apparent increase in conductance and a corresponding decrease in cell constant. Plastic walls have the opposite effect.
Calibration against a standard solution requires removing the sensor from the process piping. It is practical only if wall effects are absent or the sensor can be calibrated in a container identical to the process piping. The latter requirement ensures that wall effects during calibration, which are incorporated into the cell constant, will be exactly the same as the wall effects when the sensor is in service.
Calibration against a referee – in-process – involves connecting the process and referee sensors in series and allowing the process liquid to flow through both. The process sensor is calibrated by adjusting the process analyzer reading to match the conductivity measured by the referee instrument.
Calibration against a referee – grab sample – is useful when calibration against a standard is impractical or when in-process calibration is not feasible because the sample is hot, corrosive, or dirty, making handling the waste stream from the referee sensor difficult.
If you click HERE, you’ll find a very useful white paper that walks you through the issues related to calibration of a toroidal conductivity sensor and explores each method in some depth. I hope this is useful in simplifying your conductivity measurements.
Please let me know if I can answer any questions. Thanks for stopping by.
Hi. I’m Marc Mason, business development manager, and I’m happy to be your analytic expert today. You know the old saying, “You have to spend money to make money”? Well, in the water industry we’re finding that many water plants have to spend money to save money. Recently, Tom Johnson, water industry business development manager at Emerson, wrote an article for Water & Wastes Digest that talks about advanced technologies like radar leveling, reagent-free liquid analysis, ultrasonic control, wireless measurement devices, advanced predictive diagnostics, and SCADA control systems, and how case histories are showing the cost savings that water treatment plants can garner from investing in emerging advanced analytical, diagnostics and measurement technologies, as well as the control systems that manage those technologies. The case history described in the article demonstrates this premise pretty clearly –
Taylorsville-Bennion Improvement District serves 70,000 people in approximately 14 square miles in the center of the Salt Lake Valley, Utah. The district has approximately 16,700 connections and 229 miles of water lines. For many years, it tried to keep its old chlorine and fluoride sensors and analyzers running by constantly rebuilding, recalibrating and replacing parts. While this seemed like the cost-effective thing to do, it was proving too much for the district’s small staff – a situation familiar to many managers. The units were laborious to rebuild and required replacement of two to three probes per year; plus, they used expensive membranes that were difficult to replace and often broke during installation. The district estimates that the cost to operate the old sensors and analyzers was approximately $9,000 per year at its three locations. The units required daily attention and annual rebuilds, adding labor costs to the equation.
When the district decided to replace the old sensors and analyzers with the latest technology, its situation changed drastically. The new systems were built to last three years, versus one year, and were known to be effective as long as 15 years. The new technologies were reagent-free, reducing costs and maintenance, and needed far less frequent calibration. Bottom line: the district now replaces the membranes and electrolyte of the chlorine systems for $150 per year, compared to more than $6,000 in maintenance costs for the old systems. While the new equipment was costlier to purchase, the dramatically lower cost of ownership is rapidly offsetting that differential – a situation that can apply to many technologies.
There are many other examples of cost savings quoted in the article. Click HERE to read it.
How about you? Have you invested in what seemed a costly technology, only to discover it saved money? We’d love to hear your story.
By J. Patrick Tiongson, Product Manager, Emerson
At first glance, calibrating conductivity sensors may seem straightforward; however, many times, this is not the case. I’d like to share with you some of the various methods for calibrating contacting conductivity sensors and outline some of the potential issues that can accompany such procedures.
There are three main methods for going about calibrating contacting conductivity sensors: 1) with a standard solution, 2) directly in process against a calibrated referee instrument and sensor, and 3) by grab sample analysis. Understanding the fundamentals of each method, and the issues associated with each, can help make the decision on how best to calibrate a sensor.
Calibration with a standard solution consists of adjusting the transmitter reading to match the value of a solution of known conductivity at a specified temperature. This method is best when dealing with process conductivities greater than 100 µS/cm. Use of standard solutions less than 100 µS/cm can be problematic as you run into the issue of contaminating the standard with atmospheric carbon dioxide, thereby changing the actual conductivity value of the standard. For maximum accuracy, a calibrated thermometer should be used to measure the temperature of the standard solution.
If dealing with low conductivity applications (less than 100 µS/cm), in-process calibration against a referee instrument is the preferred method. This method requires adjusting the transmitter reading to match the conductivity value read by a referee sensor and transmitter. The referee instrument should be installed close to the sensor being calibrated to ensure you are getting a representative sample. Best practices for attaining a representative sample across both the process and referee sensors include using short tubing runs between sensors and increasing sample flow. Though usually not as accurate as calibrating against a standard solution, this method eliminates the need to have to remove your sensor from process, and removes any risk associated with contamination from atmospheric carbon dioxide.
The least preferred method of calibrating a contacting conductivity sensor is by analyzing a grab sample. This method entails collecting a sample from process and measuring its conductivity in a lab or shop using a referee instrument. Calibration via grab sample analysis is only ever recommended if calibration with a standard solution is not possible or if there are issues with installing a referee instrument directly in process. The collected grab sample is subject to contamination from atmospheric carbon dioxide and is also subject to changes in temperature when being transported. In other words, there are many potential sources for error in the final calibration results.
More information regarding calibrating conductivity sensors can be found HERE.
By Jill Jermain
Our Customer Service Tech, Paul Lim, has received customer calls regarding a common issue that happens during the shipping and storing of pH sensors and thought we should share this information. It’s a standard industry practice to ship pH sensors with a protective potassium chloride-filled rubber boot, but sometimes crystals form on the rubber boot. This causes some customers to assume that the sensor is defective and order an immediate replacement, but this is not the case.
The potassium chloride helps keep the sensor hydrated and ready for immediate use, while the rubber boot prevents damage, such as cracking or scratching, to the glass bulb during shipment. Crystals can sometimes form on the rubber boot of new, unopened pH sensors. However, what most customers don’t realize is that the potassium chloride solution in the rubber boot tends to creep out and crystallize over time. This is a completely natural phenomenon. White crystals of potassium chloride form due to the evaporation of the solution, and will appear around the cap and tape as shown in the photographs. In order to remove these crystals, one should simply rinse off the outside of the sensor before removing the tape.
It’s also essential to check whether the potassium chloride solution completely evaporated out of the rubber boot. If this occurs, then the sensor should be soaked in either pH 4 or 7 buffer or in tap water for several hours to restore response. One should note that it’s never acceptable to use deionized water for soaking as it will damage the special glass membrane and shorten the operating life of the sensor. After conditioning is completed, the pH sensor is now ready for calibration and prepared to measure the process liquid.
In summary, the potassium chloride salt crystallization is not destructive to the electrode, nor does it hinder the electrode’s performance if treated properly. Customers can save valuable time and money by avoiding the risk of delaying a critical process measurement due to the misperception that the electrode is not functional because of the potassium chloride crystallization present on the sensor.