pH and Conductivity Best Practices

Presented by Chris English of Rosemount Analytical

Conductivity

• Theory

Conductivity measures the ability of a liquid to conduct measured in microsiemens (micromhos)

Resistivity is sometimes referred to as the inverse of conductivity (1/conductivity) and is measured in MegOhms

Conductivity

Theory

• Contacting sensors measure the conductivity of a liquid between two plates. Each plate has a determined surface area that determine the cell constant

• Toroidal sensors use a flow through design sometimes referred to as a “donut sensor”

Conductivity

Where is conductivity or resistivity used? Common applications include:

• Boiler feed water• Water and wastewater treatment plants• Special chemical batch applications• Condensate return• Pulp and Paper mills• WFI (water for injection)• Plant effluent monitoring

Conductivity

Typical Setup of Equipment• Analyzer (115 volts) or transmitter (24 volts)• Measuring cell (conductivity or inductive) referred to as

contacting or toroidal• Cell constant determines the range of the cell0.01 cell constant = 0-50 micromhos or 0-18.3 megohms0.1 cell constant = 0-500 micromhos1.0 cell constant = 0-5000 micromhos10.0 cell constant = 0-20000 micromhosToroidal cells = 200-2,000,000 micromhos

Conductivity

Setup

• Analyzer is installed either in the field or remotely

• Sensor is installed in the process typically as a flow through, insertion, retractable or submersion mounting

• Cable connects cell to analyzer or transmitter

• Cells are able to be installed up to several hundred feet from analyzer/transmitter (no preamplifier required)

• Cells usually last between 3-5 years with no consumable fill solutions or membranes

Conductivity

Calibration• Sensor “zero” provides the first calibration point

and is performed by having the cell out of solution and simply being exposed to air (no conductivity)

• Single point calibration is performed (sometimes referred to as a standardization) in a known solution (for example in a 1419 microsiemen solution)

Conductivity

Potential problems with conductivity sensors• Air bubbles in the line can cause errors in

readings• Coating of the electrodes and plugging of the

cell can cause a fault• Oils that are non-conductive will provide a zero

reading

pH

What is pH?• pH is the measure of the negative hydrogen

ion activity in a solution that determines the acidity (0-7 pH) or the alkalinity (7-14 pH) of a solution.

• The lower the pH value the more acidic and the higher the pH value the more alkaline

• Pure water is a value of 7 pH

pH

pH uses a negative logarithmic scale• The amount of acid it takes to take a solution from

a pH of 6 to 5 is 10 times that of taking it from a pH of 7 to 6

• For example: If you have a 55 gallon drum of pH 7 water it may take 1 cup of acid to make the pH of the drum 6. It would take 10 cups of that same acid to make the pH of the drum 5, 100 cups of acid to make the pH of the drum 4.

pH

Where is pH used. Common applications include:

• Plant effluent• Controlling a pH so a reaction can occur in

a process• Boiler feedwater• Water and wastewater treatment plants• Neutralization tanks

pH

Typical setup of equipment

• Analyzer (115 volts) or transmitter (24 volts)

• Measuring probe

• Preamplifier

• Interconnect cable

pH

How a pH sensor works• A pH sensor is comprised of a glass measuring

electrode, electrolyte fill solution, RTD for temperature compensation, reference junction

• The sensor essentially works as a battery. The fill solutions flows out of the sensors through the reference junction (a porous “cork”) into the process. This electrolyte causes the Hydrogen Ions to gather on the glass electrode. The glass electrode measures the Hydrogen Ion activity between the Hydrogen Ions in the process with the fill solution inside of the glass electrode

pH

The Hydrogen Ion activity in the process produces a low voltage signal that is carried through the silver anode inside of the glass electrode to the preamplifier or analyzer.

• A pH sensor is considered a “consumable” since the electrolye fill solution in the sensor will eventually deplete.

pH

• As the sensor ages and the electrolye depletes the low voltage output from the sensor will decrease over time.

• The theoretical optimum output from a pH sensor is 59.18 mV/pH, otherwise known as the slope.

pH

What is the slope and how is it determined?• The slope of a pH sensor can be determined

by calibration of the sensor using two buffer solutions.

• When the slope falls to lower than 48 mV/pH the analyzer or transmitter will determine the sensor as not having enough output and should be replaced or rebuilt

pH

How does the calibration work?• For example: lets say your pH sensor has an

initial output of 60 mV/pH. When you put the sensor in a 4 buffer solution it will put out 240 mV. When you put it in a 10 pH buffer solution it will put out 600 mV. The analyzer draws a straight line between the two points and linearlizes the mV/pH or determines the slope of the sensor.

pH

Why are additional calibrations needed?• If the output from the sensor drops to say 55

mv/pH the analyzer or transmitter will not know this and will try to provide a pH reading based on the initial calibration. For example if the analyzer thinks that at 240 mV the pH is 4 but the slope has actually fallen to 55 mv/pH they analyzer will only read 220 mV and based on it’s scale will determine that the pH is about 3.8 instead of 4. This is known as “drift”.

pH

Performing additional calibrations will let the analyzer know what the new output from the sensor is and will allow it to provide the correct readings.

pH

What is standardization?• Standardization is a single point calibration. This

can be used to adjust a pH reading to match that of a lab instrument or to tell the analyzer what the actual pH value of the solution that it is in. This does not change the slope of the sensor and although it may provide a reading at a certain pH value it will not help the analyzer read correctly when the pH changes.

pH

Then why do a standardization?

• Standardizations are used to compensate for an offset in a process sometimes caused by a ground loop, change in flow rate or possible temperature effects.

pH

So what is the best way to calibrate a sensor, with what buffer solutions and how often?

• Going back to my example of the amount of acid or caustic needed to change pH the best buffer solutions to use are as far away from 7 as possible but still being stable. 4 and 10 pH buffer solutions are best. 10 buffer solutions are not always stable so you may find such manufacturers as Rosemount Analytical who offer a 9.18 solution.

pH

• A common problem people see with calibrations involve someone using a pH 4 and a pH 7 buffer solution. The sensor does fine in the 4 buffer and fine in the 7 buffer but the user still sees drift in the process. Since it does not take much to throw off a pH of 7 (based on the fact that it takes 1000 times less acid to change a pH 7 than it does a 4) it is easy to contaminate a pH 7 buffer to actually be say 6.9 or 7.1 pH. This can be caused by just a drop or two of the pH 4 buffer being on the sensor when it is placed in the pH 7 solution.

pH

Our recommendation is to place the sensor in a 4 pH buffer to perform the low calibration point and then place the sensor in a 10 pH buffer to perform the high calibration point. This will determine a good slope from the sensor. After the calibration is complete the sensor can be placed in a 7 pH buffer and have a standardization to make sure it is exact. Calibration determines the slope of the sensor and a standardization will move the slope up or down but not change the mV/pH reading.

pH

How often do I need to calibrate and how long should a sensor last?

• Every application is different. We initially recommend performing a calibration once a week. If all looks well try it in two weeks. If all still looks good then a normal maintenance schedule of at least once per month is recommended.

• pH sensors have varying life spans between a couple of months to well over a year. A water treatment plant may use a sensor for over a year where a hot caustic bath may go through sensors in a matter of weeks.

pH

How does temperature affect the life of a sensor?

• The glass electrode in a pH sensor also has a finite life. As the glass ages it becomes weaker and will not put out a good mV signal. Age, temperature and especially caustic will decrease the life of a sensor.

pH

A good rule of thumb for the life of a sensor in a hot process is as follows:

Lets say your sensor should last one year in ambient water (25 degrees C). For every 25 degrees C you go up in temperature the life of the sensor decreases in half. For example:

At 25 degrees C the sensor will last one yearAt 50 degrees C the sensor will last six monthsAt 75 degrees C the sensor will last 3 monthsAt 99 degrees C the sensor will last a few weeks

pH

Can a sensor handle boiling or freezing conditions?• The electrolye fill solution in a sensor will freeze

or boil as does water. With this in mind it will not function and the glass will most likely break in these applications. A small amount of antifreeze can sometimes be injected into some sensors to help extend their life in these applications and may be able to take you from 28 degrees F to about 215 or 220 degrees F.

pH

Lets say I have a pH sensor in a river that is frozen on the top, how do I calibrate it?

• If you remove the sensor from the water it will most likely freeze when it hits the air and then break. It is recommended to take a grab sample and measure it with a portable unit. Perform a standardization on the sensor and then calibrate it when the weather gets above freezing. If it will be freezing for some time you can perform a calculation of the analyzer reading to the portable unit and manually determine the slope. You can manually enter the slope in the analyzer.

pH

Possible problems with pH sensors

• Air bubbles can cause pH reading to vary

• If the glass gets coated with the process it can cause errors. Periodic cleaning may be required.

• The reference junction can become plugged. Periodic cleaning may be required.

pH

How do I determine what is the best pH sensor to use in an application?

Many suppliers will manufacture a number of different pH sensors for different applications. Some are HF acid resistant, some fouling resistant, some for high temperatures etc.

It is recommended that if you are uncertain to contact each respective manufacturer and provide as much information as possible about the process. Don’t necessarily try to recommend a sensor if you are not sure.

pH

Each manufacturer has years of experience with many applications. pH monitoring is sometimes considered a “black magic” and it is best to consult with experienced people in this field for recommendations. Usually with the completion of a simple application questionnaire most manufacturers can determine if they have a sensor to meet your needs.

pH and conductivity

Questions?