SNOWPURE, LLC, 2005-2018
VERSION 3.5 (XL+EXL)
–FEBRUARY 2018
PAGE 27
Quality vs Temperature (re-optimization of operating conditions)
There is an optimum temperature for operation.
As temperature increases to 35
C, product quality will generally increase since ions are
more mobile and move more easily. Higher than this, quality will lessen due to
increases in ionic “leakage.” This is caused by lower adsorption of the ions to the
internal ion exchange resins. In addition, the actual ionic resistivity, uncompensated for
temperature, will increase and there is less accuracy in the reading (see section below).
A lower voltage is required at higher temperatures to move the ions into the
concentrate. As a guideline, reduce voltage by 10% for each 10
C above
25
C.
As temperature decreases toward 15
C, product quality may lessen. Some of this is due
to errors in resistivity measurement temperature compensation; some improvement is
due to the greater adsorption of the ions to the internal ion exchange resin. As
temperature decreases further, the activation of diffusion through the membrane will
become larger and quality will decrease.
At low temperatures, a higher voltage will be needed to continue to split water
effectively, and move sluggish ions faster. As a guideline, increase voltage
by 10% for each 10
C below 25
C.
Resistivity Measurement Correction with Temperature
Resistivity measurements change strongly with temperature, and are normally corrected
to a standard temperature (25
C). Impurity ions in water have a higher electrical
conductance at higher temperatures because the ions are more mobile. Similarly,
Ultrapure water has a lower electrical resistance as temperature is raised because water
dissociates into H
+
and OH
-
more.
The correction for temperature in the meters is large, and is subject to errors.
A high quality resistivity meter is recommended.
The correction for resistivity with temperature for ultrapure water is 5-7%/
C. (The
correction for conductivity with temperature for tap water and RO permeate is about
2%/
C.) In both cases the temperature correction is large and large errors may be
introduced. Accurate temperature compensation becomes more important as working
temperatures are different from 25
C.
5
5
Ultrapure Water, Jan/Feb 1996 (written by Thornton Associates, Inc.)