Georg Fischer 2819-2823, 2850 User Manual
CSA is cross sectional area.
*
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Technical Reference Section: Conductivity/
Resistivity
Principle of operation
AC
+
Half Cells
Voltage
+
z
+
+
+
+
+
+
+
+
+
+
+
+
+
+
1cm
1cm
+
+
2
-1
Graphs
221
Most conductivity electrodes consist
of two measuring half-cells. The
geometry of the half-cells can be tailored to provide highly accurate measurements over a specific conductivity range. Cell constants
help to
describe electrode geometry for the
purpose of selecting the appropriate
x
electrode for a given application. A
cell constant is defined as the length
y
between the two half-cells divided by
the area of the cells.
Conductivity Cell Constant = =
Length
CSA*
z
xy
As an example, When x = y = z = 1cm the cell constant becomes = 1cm
Solutions of very low conductivity (high resistivity) such as ultra-pure water are
best measured with half-cells that are very close together (i.e., cell constant =
-1
0.01cm
). Highly conductive solutions should be measured with half-cells that
are farther apart and that have relatively little cross sectional area between
-1
them (i.e., cell constant = 20.0cm
Temperature Compensation
The conductivity of a solution is
highly dependent upon temperature.
Therefore, conductivity measurements are almost always converted
to an equivalent conductivity at the
common reference temperature of
25°C (77°F). This is accomplished by
means of temperature compensation algorithms in the instruments,
which require temperature as well as
conductivity measurement input. To
simplify and facilitate this requirement all Signet conductivity electrodes contain high-quality temperature sensing elements intelligently
positioned for quick and accurate
response.
).
siderably with the ionic composition
of the solution and can range from
less than 1% to more than 3% per °C.
This is true of regional ground water
sources as well as for other solutions
such as brackish water, acids and
bases. Signet instruments allow the
entry of custom linear compensation
coefficients for these applications.
See the instruction manual of any
Signet conductivity instrument for
details.
The conductivity or resistivity of pure
water is not a linear function with
respect to temperature. In fact, the
latest Signet conductivity instruments utilize a sophisticated polynomial to compensate for the peculiar
effects. For seamless measurement
Temperature effects on conductivity
are more or less linear for normal
water-based solutions, hovering
around 2% per °C. However, the
actual linear relationship varies con-
accuracy all current Signet conduc-
tivity instruments switch automati-
cally between linear and pure-water
compensation as certain measure-
ment thresholds are crossed.
Temperature Compensation Exception
One exception to the requirement for temperature compensation has been established by USP (United States Pharmacopeia), which prescribes limits of acceptability for ultra-pure water quality based upon non-compensated measurements.
This methodology is used to eliminate measurement variances that may result
from differences in the pure-water temperature compensation algorithms used by
different manufacturers of conductivity measurement equipment. A more thorough treatment of the USP standard and instrument functionality can be found in
the instruction manuals of the following Signet conductivity instruments: Model
8900 Multi-Channel, Multi-Parameter Controller (Appendix D), model 8860 Dual
Channel Conductivity/Resistivity Controller.
Choosing
Products
Instrument
Multi-Pa-
rameter
Flow
pH/ORP
Conductivity/
Resistivity
Pressure, Level
Temperature,
Products
Other
Installation
Wiring &
Reference
Technical
Temperature/
Pressure
of Terms
Glossary
Reference
Part No.
Index
p
p
Relay Information
The two most common methods of controlling a process are “on/off” and “proportional” control. In on/off control, relay setpoints are defined as either high
or low limits on the process variable. When the measurement value reaches a
limit the relay is energized, typically for the purpose of opening a valve or starting a pump to introduce a chemical reagent to the process. This should cause
the measurement value to change in the direction of the setpoint as shown in
these on/off control diagrams:
High limit on/off relay control
= HI setpoint
= Hysteresis
= Relay energized
= Relay de-energized
pH
Notice the relay will not de-energize
until the setpoint is exceeded by the
hysteresis value. This is a programmable value and is primarily used to
prevent ”relay chatter”, which occurs
if a relay is set to energize and deenergize at the same value. Because
of hysteresis, and because reagent
delivery is fairly constant while the
relay is energized, a condition known
as “overshoot” is inherent to the on/
off control method. Overshoot refers
to the introduction of more chemical
reagent than is absolutely necessary
for achieving a desired adjustment to
the process value, and can be expensive over time.
Proportional control is a popular
alternative to the on/off control
method. This method typically makes
use of variable-rate metering pumps
to reduce overshoot and improve
precision. Establishing a proportional control scenario requires the
selection of setpoint(s), deviation
Low limit on/off control
pH
= LO setpoint
= Hysteresis
= Relay energized
= Relay de-energized
range(s) and maximum pulse rates.
The example shown here illustrates
how two relays in ”pulse mode” can
be used to proportionally control pH
within a desired range, or to a single
setpoint. This is called “Dual Proportional Control”. Of course, a single
relay in proportional pulse mode can
be used to establish a high or low
limit and will also reduce overshoot.
Metering pumps are idle at and between setpoints. When a setpoint is
exceeded, the pump begins delivering
reagent at a rate proportional to the
difference between the measurement
value and the setpoint. The larger
the difference, the faster the delivery. The programmed deviation value
defines how quickly the maximum
pulse rate is reached. Depending on
the input requirements of the metering pump, proportional control can
also be accomplished with scaleable
4 to 20 mA outputs instead of pulsing
relays or open collectors.
Dual proportional pulse relay control
Maximum Pulse Rate
0
pH
Deviation=
5.30 pH
LO
5.30
H
HI
7.50
H
Deviation=
3.50 pH
11.00
pH
222
14
pH
pH
Maximum Pulse Rate
Deviation=
5.20 pH
0
2.10
pH
LO and HI
7.30 pH
Deviation=
4.90 pH
12.20
pH
14
pH
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