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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 cell
0.01 cell constant = 0-50 micromhos or 0-18.3 megohms
0.1 cell constant = 0-500 micromhos
1.0 cell constant = 0-5000 micromhos
10.0 cell constant = 0-20000 micromhos
Toroidal 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 year
At 50 degrees C the sensor will last six months
At 75 degrees C the sensor will last 3 months
At 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?