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Aqua TROLL

Aqua TROLL 200 Measurement Methodology
Page 1 of 5
Aqua TROLL
®
200 Measurement Methodology
Duane McKee, Principal Engineer
January 2007
Introduction ...................................................... 1

Actual Conductivity .......................................... 1

Temperature .................................................... 2

Specific Conductivity........................................ 2

Salinity ............................................................. 3

Total Dissolved Solids ..................................... 3

Resistivity......................................................... 3

Density of Water .............................................. 3

Pressure .......................................................... 4

Depth ............................................................... 4

Level (Surface Elevation)................................. 4

Level (Depth to Water)..................................... 4

Automatic Density Correction .......................... 5


Introduction
The Aqua TROLL
®
200 water quality instrument
measures and records conductivity, level, and
temperature. It is also capable of calculating
and recording useful derived parameters such
as salinity and water density.
This technical note provides an overview of the
instruments measurement techniques and the
equations used to calculate its derived
parameters.
Actual Conductivity
The Aqua TROLL 200s primary conductivity
measurement is its actual conductivity
parameter (the term actual conductivity is
used here to distinguish the parameter from
specific conductivity since the general term
conductivity is often used to refer to both
types of conductivity interchangeably).
Actual conductivity is a measure of the ability of
an aqueous solution to carry an electric current.
In water, an electric current is carried by ions
since electrons do not pass through water by
themselves. Thus actual conductivity is
dependent on the concentration of ions in the
solution. The measurement responds to all ionic
content in the solution and cannot distinguish
particular conductive substances in the
presence of others. Actual conductivity is
temperature dependent. As water becomes less
viscous at high temperatures, ions move more
easily.
The Aqua TROLL 200 measures actual
conductivity by applying a controlled excitation
voltage across electrodes in the solution and
measuring the resulting electric current that
flows between them. An internal factory
calibration is then applied to convert the
measured electric current into the actual
conductivity of the solution.
In order to prevent altering the solution by major
ionic movement or electrochemical reactions at
the electrodes, an alternating current is used.
By using an alternating current, the polarity of
the applied excitation voltage changes
frequently enough that ions do not significantly
move between, or react with, the electrodes.
The instrument controls both the magnitude of
the applied voltage and its frequency to
optimize performance over its operating range.
The conductivity cell utilizes six titanium
electrodes, three on each side of the cell. The
two outer electrodes on each side are drive
electrodes. The center electrodes on each side
are sense electrodes. The drive electrodes
apply the excitation voltage symmetrically about
the sense electrodes. Because the drive
Technical Note Aqua TROLL 200 Measurement Methodology
Page 2 of 5
electrodes carry current, they are subject to
fouling and must be compensated to maintain
an accurately controlled drive voltage. The
sense electrodes are used to make high
impedance voltage measurements. Since the
high impedance measurements draw negligible
current, the sense electrodes are much less
susceptible to fouling. A control loop within the
Aqua TROLL 200 continually adjusts the
voltage applied to the drive electrodes to keep
the voltage measured at the sense electrodes,
and thus the voltage applied across the cell,
constant. To prevent interference from other
electrical sources that may be present in the
solution, both the drive and the sense
electrodes are electrically isolated from the
instruments external power and communication
signals.
Factory calibration of the actual conductivity
parameter consists of making measurements in
several precision conductivity solutions spread
over the operating range of the instrument. The
factory calibration takes into account the
physical geometry of the instruments
conductivity cell and normalizes the cell
constant to a value of one across its entire
operating range. The Aqua TROLL 200 is
capable of meeting its published accuracy
specifications without requiring additional
calibration by the user.
Provision is made for the user to calibrate the
actual conductivity parameter by adjusting the
cell constant. The instrument should only
require a user calibration if its measurement cell
has undergone physical change (deposits on
the cell walls or on the electrodes that cannot
be removed, physical damage to the cell walls
or electrodes, etc.). The user may also need to
calibrate the instrument to conform to a
standard operating procedure. The user
calibration takes the following form:
AC = K
× AC
f
+ K
0

Where:
AC = reported actual conductivity
K = cell constant (default = 1)
K
0
= cell offset (default = 0)
AC
f
= factory calibrated actual
conductivity
When K and K
0
are set or restored to their
default values, the factory calibrated actual
conductivity is reported.
A simple one-point calibration is performed by
measuring a precision conductivity standard
using the default factory calibration, then
calculating a new cell constant as follows.
K = Standard Value / Measured Value
The cell offset K
0
is only used when two
standards are measured to determine a straight
line calibration, or three or more standards are
measured to determine a best-fit straight line. K
is then the slope of the line and K
0
is the offset.
The internal unit of measure for actual
conductivity is microsiemens per centimeter
(µS/cm). All derived parameter equations use
this internal unit of measure. Changing the
reported units of the parameter does not affect
internal calculations.
Temperature
The Aqua TROLL 200 measures the solution
temperature using a precision thermistor
encapsulated in a titanium button. The
temperature parameter is factory calibrated by
taking several temperature measurements over
the operating range of the instrument and curve
fitting these to measurements taken at the
same time by a precision reference
thermometer. The temperature parameter will
meet its published accuracy specifications
without requiring additional calibration by the
user. No provision is made for user calibration
of this parameter.
The internal unit of measure for temperature is
degrees Celsius (°C). All derived parameter
equations use this internal unit of measure.
Changing the reported units of temperature
does not affect internal calculations.
Specific Conductivity
Specific conductivity is a means of expressing
what the actual conductivity of a solution would
be at a standard reference temperature. It can
be a useful way of observing changes in the
conductivity of a solution independent of
changes in temperature.
The Aqua TROLL 200 derives specific
conductivity from its actual conductivity and
temperature measurements using the following
general equation. Aqua TROLL 200 Measurement Methodology
Page 3 of 5




When the instruments factory default
coefficients for the general equation are used,
specific conductivity is calculated per Standard
Methods 2510B as shown below.




By leaving coefficient
0
= 1 and coefficients
1

through
7
= 0, can be set to temperature
compensate different types of solutions and T
ref

can be set to different reference temperatures.
By setting = 0 and T
ref
= 0, the equation
becomes a general polynomial that can
accommodate more sophisticated temperature
compensation schemes.
Salinity
The Aqua TROLL 200 derives salinity from
actual conductivity and temperature using
Standard Methods 2520A with low range
extensions. Results are reported in Practical
Salinity Units (PSU) and are suited to
applications ranging from 0 to 42 PSU.

S = a
0
+a
1
R
t1/2
+a
2
R