Practical Conductivity Application Tools

Welcome everyone and thank you for joining us today for Yokogawa’s Webinar on Basic Conductivity. This is the first session this year for this topic, but we will be repeating it on February 25th for those who find it useful and want to recommend it to others.

Our goal for this Webinar is to gain a practical understanding of the strengths,

Welcome everyone and thank you for joining us today for Yokogawa’s Webinar on Basic Conductivity. This is the first session this year for this topic, but we will be repeating it on February 25th for those who find it useful and want to recommend it to others.

Our goal for this Webinar is to gain a practical understanding of the strengths, and the limitations, associated with the measurement of the Conductivity of aqueous solutions.

We will explain what the differences are in measuring the conductance (conductivity), resistance (resistivity) or the percent concentration of the process in question. We will cover the type of equipment required (Contacting or Inductive) in order to obtain an accurate and reliable measurement, and discuss what the Cell (Probe) Constant or Cell Factor of a sensor are, and their importance in making the measurement. We will also take a look at some typical applications where conductivity is most often used

Throughout the presentation, we will touch on some of the typical problems encountered when measuring conductivity and a few troubleshooting tips that can be used to solve them.

Typically, Conductivity is preferred over other measurement parameters because it is less costly and maintenance intensive.

With that in mind, let’s start out with a basic definition. In an Aqueous Solution, Conductivity is defined as, (1) “the measure of the solutions ability to carry, or conduct an Electric Current.

An electric current will flow through certain solids and liquids when a voltage difference exists between two points. For solids, such as copper wiring and printed circuits, the current flow is achieved by electron charge transfer through the atomic structure. For liquids (2), the mechanism is different. The presence of ions or charged particles (represented by the colored dots in the diagram) are necessary as they are the “carriers” of the current flowing through the solution. If there are NO IONS present in the liquid, such as with ultrapure water, then NO CURRENT can flow and the solution is NOT conductive.

As an example, if you are taking a bath in ultrapure water and your spouse accidently drops a “plugged-in” blow-dryer into the bath. Nothing will happen, other than getting the blow-dryer wet. However, if the water is not ultrapure, if there are ions present . . . current would flow and your spouse would be reviewing the benefits from your life insurance policy.

The basis for measuring the Conductivity or “Conductiveness” of a liquid begins with the formula know as Ohm’s Law, shown in the diagram.  (3) This states that the amount of  Voltage (E) in a circuit is the result of the amount of Current (I) multiplied times the amount of Resistance (R) in the circuit. Since we are interested in how well the circuit conducts that current, which is determined by how much resistance to the current flow there is, we first rearrange the equation to find out the Resistance value by dividing the Voltage by the Current.  Once we have figured out the Resistance, it is an easy matter to determine the Conductivity, as it is simply the reciprocal , or inverse of the Resistance as shown in the 3rd equation where (G) represents Conductivity.

There are two basic approaches for measuring conductivity that are determined by the type of sensor used: there is (1) Contacting; and (2) Inductive (non-contacting), which is also referred to as Torodial or Electrodeless.

The Contacting method, as the name implies, uses a sensor with two or four metal or graphite electrodes which are actually in contact with the solution. It is represented by the two plates on either side of the solution in the diagram.

An Inductive sensor, on the other hand, has two toroidal coils (driver, receiver), like a transformer, that are encapsulated within an inert material such as PEEK or Teflon for example, to protect the coils from the process.  We will talk about both of these more later.

Here is a scale to give us a visual of the relationship between Conductivityand Resistivity .  Once again we see that Conductivity is simply the reciprocal or inverse of the Resistivity  or Resistance of the process.  If a loop is highly conductive, then it is low in resistance to current flow and vice versa.

The engineering unit used to represent resistance or Resistivity is the Ohm.
The engineering unit of measure for Conductivity is the Siemen. But you will also find the term “Mho” used. Mho is simply Ohm spelled backwards.  Mho and Siemen are treated as the same when you are making conductivity measurements.

From the examples shown on the scale, we can see that conductivity ranges from very low values (theoretically pure water at 0.057 microsiemens), to very high values (several hundred thousand microsiemens for acids or caustics) as the concentration of conductive ions increase.  Remember the amount of ions present in the liquid relates to the level of conductivity measured.

Question:
For Pure and Ultrapure Water applications such at used in Power Plants and  Computer Chip Manufacturers, some companies prefer to  measure the Resistivity of the process rather than the Conductivity or vice versa. What is the reason?

Answer:

Simply, Preference. Some prefer reading in very large resistance numbers 15-18 Megohm instead of very small conductivity numbers .05 microsiemen. However, the result is still the same. It is just two different ways to look at the same measurement.

Another key industrial use for Conductivity measurement, besides pure water, is for processes with much higher conductivity values. Here we are determining the chemical strength, or percent concentration of a solution. There are three factors we need to pay attention to in order to successfully make this type measurement. The first is, (1) that each chemical responds differently from the next with regard to its conductivity value versus percent concentration level. Often the difference is significant, but sometimes it is not.  These differences have to do with the size and mobility of the ion of each chemical. Thesecond factor to consider is, (2) that Conductivity is a non-specific measurement, and the third factor is (3) that temperature affects all conductivity measurements.

The first factor is illustrated on the graph where you can see the effect of the different sized ions of each chemical reflected in their different curves for Conductivity versus % Concentration. For the four chemicals shown they are all different. This points out how important it is that we set up our Analyzer with the correct curve information for the specific chemical we are measuring or our results will be useless. Many Analyzers have built-in tables for Conductivity versus % Concentration to select from and also the capability to enter your own table if desired.

You will also notice, that at a certain point the continued rise in the percent concentration results in a decrease in the conductivity value. (4) This is due to the increasing number of ions in the solution interfering with one another causing the mobility of the ions and therefore the conductance of the solution to decrease. This change in direction could be a problem should the measurement range include that part of the curve where the changes occurs, such as 18 – 50% Sulfuric Acid, or 10 – 30% Hydrochloric Acid.

As you can see, the problem is there will be one conductivity value that represents two different percent concentrations. At 18% and 50% Sulfuric Acid, the conductivity value is about 600,000 µS. So, just reading the conductivity will not tell you what percent concentration you are measuring. You also need know which side of the curve you are on.

Now, the simplest solution to this problem would be to make sure you set up the range such that it never crosses that peak point . However, there may be occasions where that’s not possible. What do you do then? Well, it becomes a little more difficult, but using two Analyzers; a secondary device (such as a Flow meter) to validate which side of the curve you are on,  and a way to switch the output from the first Analyzer to the second once the peak of the curve is reached the measurement could be made.

The second factor we need to consider is that conductivity  is a Non-Specific measurement. Sometimes the size of the ion of different chemicals, and the way they relate to conductivity are very similar. So, you have to know WHAT you’re measuring because a conductivity measurement only tells you the conductivity of the solution, not what the solution is.

This is very important as the graph shows. You can see that the actual conductivity value of various percentage concentrations of Sulfuric Acid (the red line) and Sodium Hydroxide (the blue line) are very nearly the same. For example a 4% concentration of Sulfuric Acid is approximately 179 mS and 4% Sodium Hydroxide is 185 mS. Just knowing the conductivity will not really allow me to know WHAT I’m measuring.

In cases like this, at lower concentration levels, a pH measurement may also be added as an indication of whether I am measuring an acid or a base.

Finally, the third factor we must consider is the temperature of the solution.

As the Slide reminds us, the Mobility of the ions in the process affect the conductivity values we will see. The higher the concentration of any solution  (up to a point – remember the curves we looked at earlier) the higher the  conductivity value.

Again, the type of ion also directly affects their mobility. A Hydrogen ion, for example, is smaller and more mobile and therefore more conductive than the larger Sodium ion. This is why their curves are different.

Temperature (1) also affects the conductivity measurement. As the temperature of any process increases it affects the mobility of the ions causing them to move faster resulting in a false Higher Conductivity value.

Because of this fact, all Conductivity sensors include a temperature electrode of some sort to compensate the process temperature back to a standard reference temperature value, usually (25ºC). Without this compensation, any temperature change would appear to us as a change in the conductivity value.

In some pharmaceutical applications it is desired to measure the un-compensated conductivity value as an alarm set point for impurity levels, but this value is not very practical for Process Control.

Alright , before we go on, lets take a quick review …

(1) Conductivity is defined as, “the measure of a solutions ability to carry, or conduct an Electric Current.

(2) For aqueous solutions, ions are necessary to carry the current.
No Ions – No Current

(3) An Increase in Ions reflects an increase in conductivity (up to the point they begin to interfere with one another). In addition, the size of the ions , which affects the mobility, and the temperature of the process affect the conductivity.

Simply put, we must be able to accurately measure the ions in the solution; know when they are beginning to interfere with one another, and eliminate the affects that temperature would have on the process, if we are to get a usable measurement.

How Do We Do That?

Let’s start with the basic equipment required for a typical conductivity measurement loop. It includes:

An (1) Analyzer or Transmitter to convert the sensor signal to 4-20mA or other output signal usable for control or monitoring.

(2) We also need an Interconnecting Cable to mate the sensor to the electronics.

(3) A Sensor which is typically either a Contacting or an Inductive style. We will discuss the differences between these types in more detail as we go on.

And finally, (4) we need some way to get the sensor on-line, such as a Holder like the flow through version shown in the slide or some other type of process connection.

Pretty simple and straight forward.

As we have mentioned a couple of times previously, the two common Sensor designs currently used for Conductivity measurement are Contacting and Inductive. Also called Toroidal or Electrodeless.

The key differences to consider when deciding which type sensor to select are maintenance and accuracy.

The Contacting style sensor get’s its name from the fact the actual measuring electrodes are in direct contact with the process. As a rule of thumb, contacting sensors are most reliable and require the least maintenance when they are used in LOW conductivity applications, such as pure water applications in Power Plants and computer chip manufacturing. Don’t misunderstand, contacting sensors can and are used for applications with high conductivity values, but problems of material compatibility, coating and corrosion increase the maintenance they require and their expected life in these applications.

Inductive sensors do NOT have any measuring element that comes in direct contact with the process, and therefore do not suffer from coating or corrosion problems. However, they are typically less accurate at lower conductivity values than contacting sensor are.
Let’s take a closer look at these two types of sensors. First Contacting.

Since we’re interested in the measuring conductivity of a solution, the exact volume between the electrodes becomes important. It must be well defined and repeatable to be of use – otherwise we could not tell if the changes in current flow were due to the concentration change of the process or just random positioning of the measuring electrodes. This volume between the electrodes of the sensor is defined by what is called the “probe or cell constant” and determines what the effective measurement range of the sensor will be. The basic probe constant of 1.0 is defined as two 1 cm2electrodes separated by a distance of 1 cm. As shown in the slide.

The cell constant is calculated by dividing the distance (length) between the two measuring plates by the area of the plates.

Remember, we are measuring the current between these plates.  We want to keep that current at a level where the analyzer can measure it most accurately.

If we increase the area of the plates, and shorten the distance between them, more ions will be able to travel to the plates and make contact.  This is why we use small cell constants (0.01) in low conductivity solutions that have a low concentration of conductive ions.  It ensures that the current between the cells is at a level high enough that we can measure it accurately by the analyzer.  The raw conductivity value is then multiplied by the cell constant which is why we see the unit microsiemen/cm.

In solutions with high concentrations of conductive ions, we want to do the opposite:  Decrease the area of the plates and increase the distance between them.  This ensures that the current is low enough that we can measure it accurately.  This is why we choose cell constants of 10.0  or higher for highly conductive solutions.

The key is knowing your measurement range and picking the correct cell constant to match.

What happens if the wrong cell constant is selected, or the process conductivity actually  increases for some reason? In either instance, something we call Polarization will occur rendering the measurement useless.

(1) In the slide, you can see in the first example titled Normal Operation, a solution with the correct cell constant where the ions are free to travel from one plate to the other effectively carrying the current.

The second example (2), titled Polarization, shows the same cell constant being used in a highly conductive solution. 

When the voltage alternates (switches polarity), the ions cannot freely move to the other plate because the ion density is too high.  This results in less ions contacting the correct  plate which will result in a false low reading. The electrodes effectively are polarized.

How do we prevent this?

• Always select the correct sensor cell constant.
• Increase maintenance to clean electrodes.
• Use Inductive Sensors.

Let’s look at the Inductive style sensor.

Some of the obvious benefits over Contacting style sensors are that No actual contact with the measuring elements and the process occurs. Therefore, inductive sensors are not as affected by coating or corrosion and will not polarize like contacting types.

However, to be fair, should the hole in the “doughnut” become plugged, it will stop measuring correctly.

Inductive sensors typically have a wider measurement range than contacting, but as we mentioned earlier, they are less accurate at low conductivity values. As a rule of thumb, below 50  microsiemens. Also, attention must be given to the material of construction. Inductive sensors are available in a variety of plastic materials, with PEEK probably being the most common. However, in high percent concentrations of some chemicals, H2SO4 for example, PEEK will not survive. A Teflon version would be a better choice.
How do they work? Well, their operation is based on how a transformer works. (1) When voltage is applied to a coiled wire, a current flows through it and creates an electromagnetic field. This is what is done at the Drive Coil shown in the diagram.

Conversely, an electromagnetic field will create (induce) a voltage in a coil that is exposed to it. This is what happens at the Pickup or Receiver coil. 

(2) The strength of the electromagnetic field and consequently the strength of the Induced voltage at the receiver coil is dependant on the conductivity of the liquid passing through these two coils. (3) So, by measuring the voltage received at the Pick-up coil against the voltage supplied to the driver coil we are able to determine the conductivity of the solution passing between them.

While Inductive sensors have a great many advantages over the Contacting style, they do have some limitations we need to pay attention to. The Inductive Sensor has a Cell Factor which is somewhat similar to the Cell Constant for the Contacting Sensor.

Typically, the Inductive sensor Cell Factor can cover a much wider measurement range than is possible with a contacting style, but you must be sure that this value is entered properly in the Analyzer as it acts like a cell constant and directly affects the accuracy of the reading. Because the Inductive sensor works by creating electromagnetic lines of flux, these can be interfered with if the “doughnut” portion of the sensor, where the coils are located, is installed too close to the pipe or tank wall. This interference will change the effective value of the Cell Factor as shown in the graph.

There are two ways to address this issue. Obviously, the first would be to make sure there is sufficient distance between the sensor and the process equipment walls. This is usually about 1” all around, but check with the manufacturer to be sure. The second, which is also a kind of benefit, is that you can do a calibration with the sensor installed and the affects of the location will be automatically corrected and the Cell Factor adjusted by the Analyzer. It is not always easy to calibrate in place, but it is a solution for applications where there is not other choice for the sensor location.

OK, let’s have another Quick Review.

To make is simpler for everyone, here is a brief guide I use when considering either one of these style sensors. Basically, for low conductivity 5,000 microsiemens or less, I would use the contacting style. Above 5,000 microsiemens I would choose Inductive. Of course, there is a great deal of latitude in making this choice since contacting sensors have been used successfully for many, many years in high conductivity applications. This guideline is based purely on what would best minimize maintenance requirements and offer the longest potential sensor life.
Why do we choose to measure Conductivity rather than some other parameter such as pH or maybe Dissolved Oxygen?

Primarily, because it is less expensive and requires less maintenance and calibration than other parameters like pH.Conductivity can be used in a variety of applications from pure water to percent concentration measurements.
Here is a list of some of the areas where Conductivity is used. pH is used in many  of these applications as well. Often these two parameters are used together for measurement and control, such as in Cooling Tower or Boiler Feedwater and Boiler Blowdown applications.

Once again, the two most common uses for conductivity measurements are (1) to determine and /or control the relative purity of the process water, and (2) measure the percent concentration of a specific chemical.

We have skirted around the issue, but what kind of maintenance is actually required for Conductivity loops?
You will need to initially calibrate the measurement loop and may need to recalibrate periodically (sometimes only annually) depending on the application.

This is a simple one point calibration using solutions that can either be purchased commercially or made up in the lab, if the plant so desires.

Then it is a matter of avoiding polarization (if it is a contacting sensor) or plugging if it is an Inductive version, you will get a continuous reliable measurement.
A closer look at some of the applications mentioned earlier.

This Graph shows the significant advantage of using conductivity versus a manual or timed method for Boiler Blowdown control off Total Dissolved Solids.
You can see, as indicated by the SC labels in the diagram, the quantity and variety of application sites for conductivity in Pure Water applications in a Power Plant is significant.

Due to the chemical content, coating and potential fouling issues in a Pulp and Paper Plant, Inductive conductivity is ideally suited for the different applications we see there.
Selecting the proper equipment for a Conductivity application is not difficult. You need some basic information as listed on the slide.

• Measurement Range
• Chemical Compatibility
• Installation Requirements

Once you have this information,  your vendor can easily work with you to get the right products for the application.

We would like to Thank You for taking time to attend today’s Webinar. If you have any questions…