# User guide

change with concentration and temperature instead of single average

values.

C. An Example of 2 different solution selections and the

resulting compensation:

How much error results from treating natural water as if it were KCl at

15°C?

A tap water solution should be compensated as 442 with a tempco of

1.68 %/°C, where the KCl value used would be 1.90 %/°C.

Suppose a measurement at 15°C (or 59°F) is 900 microsiemens of true

uncompensated conductivity.

Using a 442 correction of 10 (degrees below 25) x 1.68% indicates the

solution is reading 16.8% low. For correction, dividing by (.832) yields

1082 microsiemens as a compensated reading.

A KCl correction of 10 (degrees below 25) x 1.9% indicates the solution

is reading 19% low. Dividing by (.81) yields 1111 microsiemens for a

compensated reading. The difference is 29 out of 1082 = 2.7%.

D. A Chart of Comparative Error

In the range of 1000 µS, the error using KCl on a solution that should be

compensated as NaCl or as 442, is shown in the graph below.

Users wanting to measure natural water based solutions to 1% would

have to alter the internal compensation to the more suitable preloaded

“442” values or stay close to 25°C. Some who have standardized to KCl

based compensation may want to stick with it, regardless of increasing

error as you get further from 25°C. The AR1 will provide the repeatability

and convertibility of data needed for relative values for process control.

E. Other Solutions

A salt solution like sea water or liquid fertilizer acts like NaCl. An internal

correction for NaCl can be selected for greatest accuracy with such

solutions. Many solutions are not at all similar to KCl, NaCl or 442. A sugar

solution, or a silicate, or a calcium salt at a high or low temperature may

require a value peculiar to the application to provide readings close to the

true compensated conductivity.

Clearly, the solution characteristics should be chosen to truly represent

the actual water under test for rated accuracy of ±1% of full scale. Many

industrial applications have always been relative measurements seeking a

number to indicate a certain setpoint or minimum concentration or trend.

The AR1 gives the user the capability to take data in “KCl conductivity

units” to compare to older published data, as in terms of NaCl or 442, or

as may be appropriate.

XIII. CONDUCTIVITY CONVERSION to TOTAL

DISSOLVED SOLIDS (TDS)

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 (ref. Temperature

Compensation, pg. 15).

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 internal processor, 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

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Chart 2

(2)%

(1)%

0%

1%

2%

3%

4%

5%

6%

7%

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

Temperature

NaCl error with KCl tempco

442 error with KCl tempco