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%.
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
30
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 Ultrameter 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 “User” 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%. Many industrial
applications have always been relative measurements seeking a number
31
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
