Xi. addendum – Myron L Aquaswitch II User Manual

Page 49

XI. ADDENDUM

A. CONDUCTIVITY, TDS, RESISTIVITY and

TEMPERATURE RELATIONSHIPS

1 .

TEMPERATURE COMPENSATION

(Tempco) of Aqueous Solutions

Electrical conductivity indicates solution concentration and
ionization of the dissolved material. Since temperature greatly
affects ionization, conductivity measurements are temperature
dependent and are normally corrected to read what they would be
at 25°C.

a. Standardized to 25°C

Conductivity or Resistivity are accurately measured in the

AQUASWITCH II

by a method that ignores electrolysis,

electrode characteristics, etc., and uses a unique circuit to
perform temperature compensation. In simpler instruments,
conductivity values are usually assigned an average correction
similar to KCl solutions for correction to 25°C. The correction to an
equivalent KCl solution is a standard set by chemists. It
standardizes the measurements and allows calibration with
precise KCl solutions recognized for stability.

b. Tempco Variation

Most conductivity instruments use an approximation of the
temperature characteristics of solutions, perhaps even assuming
a constant value. The value for KCl is often quoted simply as
2%/°C. In fact, KCl tempco varies with concentration and
temperature in a non-linear fashion. Other solutions have more
variation still. The

AQUASWITCH II

uses corrections that

change with concentration and temperature instead of single
average values. See Chart 1.

The

AQUASWITCH

will provide the repeatability of data

needed for relative values for process control.

2 .

CONDUCTIVITY CONVERSION to TOTAL

D I S S O L V E D S O L I D S ( T D S )

a. How it’s Done

Once the effect of temperature is removed, the compensated
conductivity is a function of the concentration (TDS).
Temperature compensation of the conductivity of a solution is
performed automatically by the electronic circuit, using data
derived from chemical tables. Any dissolved salt at a known
temperature has a known ratio of conductivity to concentration.
Tables of conversion ratios referenced to 25°C have been
published by chemists for decades.

b. Solution Characteristics

Real world applications have to measure a wide range of materials
and mixtures of electrolyte solutions. To solve this problem,
industrial users commonly use the characteristics of a standard
material as a model for their solution, like the KCl favored by
chemists for its stability.

Users dealing with sea water, etc., commonly use NaCl as the
model for their concentration calculations. Users dealing with
freshwater work with mixtures including sulfates, carbonates and
chlorides, the three predominant components (anions) in
freshwater that Myron L Company calls “natural water”. These are
modeled in a mixture called “442™” which the Myron L Company
developed and markets for use as a calibration standard, as it
does standard KCl and NaCl solutions.

c. When does it make a lot of difference?

First, the accuracy of temperature compensation to 25°C
determines the accuracy of any TDS conversion. Assume we
have industrial process water to be pretreated by RO. Assume it
is 45°C and reads 1500 µS uncompensated.

1. If NaCl compensation is used, an instrument would report 1035

µS compensated, which corresponds to 510 ppm NaCl.

2. If 442 compensation is used, an instrument would report 1024

µS compensated, which corresponds to 713 ppm 442.

The difference in values is 40%.

In spite of such large error, some users will continue to take data
in the NaCl mode because their previous data gathering and
process monitoring was done with an older NaCl referenced
device.

Those who want true TDS readings that will correspond to
evaporated weight will select the correct Solution Type.

3 .

TEMPERATURE COMPENSATION

(Tempco) and TDS DERIVATION

When making conductivity/resistivity measurements, the Solution
Selection determines the characteristic assumed as the
instrument reports what a measured conductivity or resistivity
would be if it were at 25°C. The characteristic is represented by
the tempco, expressed in %/°C. If a solution of 100 µS at 25°C
increases to 122 µS at 35°C, then a 22% increase has happened
over this change of 10°C. The solution is said to have a tempco of
2.2 %/°C.

Another solution would have a different tempco because of its
ionization activity. And, that tempco may be a little different at a
different concentration or temperature.

NOTE: Resistivity TEMPCO’s change very drastically above
10M

Ω

Chart 1

1.500%

1.600%

1.700%

1.800%

1.900%

2.000%

2.100%

2.200%

2.300%

2.400%

2.500%

Temperature

0 5 10 15 20 25 30 35 40 45 50 55 60

KCl % / °C

% / °C

This manual is related to the following products:

Aquaswitch I