Page 7 - Spring 2024

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These same techniques can also be applied to other
If you wanted a light to switch at 15 µS, you’d need to
O-ring leakage issues in your EDM systems.
specify one with a switch point of 66.7 KOhms. But to
the best of my knowledge, nobody makes a light with
that switch point. Besides, using 15 µS bottle output
Is My DI Conductivity Light Lying? Part Two
water brings us back to insanely long recovery times.
In our Winter 2024 issue Jeff complained, “I’m having
an issue with my Wire EDM dielectric system that is
operator or maintenance department advance warning
baffl ing me. The red/green conductivity light affi xed
of the DI bottle’s imminent demise, and, therefore, we
to my resin bottle is showing green, yet my machine
have to look at the situation in a different light. (No pun
display indicates that, no matter how long I wait, my The whole point of a conductivity light is to give the
intended.)
conductivity is higher than it should be.”
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My recommendation is that Wire EDM’ers should be
In my reply, I suggested that the resistivity ohm
using a 10 µS (or 100 KOhms) light in order to get
rating of the conductivity light relative to the machine
some advance warning of the need to change the DI
conductivity set point might be the issue, but deferred
bottle, before recovery times substantially increase.
that discussion to this Spring issue.
But, readers may rightly ask, “Am I giving up a substan-
Roger Kern’s reply:
tial amount of DI bottle life by changing at a 10 µS DI
bottle output level?”
Conductivity/Resistivity
I had the same question, so I reached out to Larry
Depending upon the preference of the machine tool
manufacturer, the electricity conducting properties of
foremost ion exchange companies for his take on this.
deionized water are measured and reported by two
parameters: Gottlieb, President of ResinTech, one of the world’s
He responded by sharing the graph shown in Figure 5
and an explanation:
• Conductivity: µS/cm
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According to Larry, as is readily apparent from the
• Resistivity: Ω
graph, the plot of resistivity vs. capacity is a non-linear
function. There-
The resulting measurements of these parameters
fore, changing out
for the same water sample are the reciprocal of each
the DI bottle at 10
other as shown in the following examples:
µS vs 20 µS will
make very little
• 1 µS/cm = 1,000,000 Ω
difference in resin
• 20 µS/cm = 50,000 Ω
operating capaci-
Conductivity Control Realities
In my opinion,
A freshly regenerated resin tank will produce water
that’s a small
with a conductivity of less than 1 µS (or resistivity ty, less than 3.5%.
price to pay for
greater than one megohm). Since most machines
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both the sub-
operate in the range of 15 µS, the 1 µS water from
stantial reduc-
the resin bottle will quickly restore the machine to its
tion in recovery
conductivity set point when make-up water is added.
Figure 5
time near the DI
(Depending on the source, the conductivity of the
bottle’s end of life and the security of changing out DI
make-up water can range from 40 µS for municipal
bottles in a timely fashion before production is inter-
water from a surface reservoir to as much as 200 µS
rupted as some machines will actually shut down if
for either domestic or municipal well water.) Howev-
conductivity levels exceed a set limit.
er, as the resin bottle is used, the conductivity of its
So, to me, changing over to a 100 KOhms light is a
output water will steadily decline. Thus, the recovery
times of the DI system will gradually increase due to
offer conductivity lights with that specifi cation as a
the increasing conductivity of the bottle output water.
standard item.
The most common conductivity lights have a switch
point of 20 µS (or 50 KOhms). Therefore, it is entirely no brainer. However, there are very few suppliers that
If you contact me at my website listed below, I’d be
possible that you could observe long recovery times,
pleased to share the source from which I purchased
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and eventually no recovery at all, to the 15 µS control
mine.
point, with a green light as the conductivity of the
As the renowned (and now deceased) radio commen-
bottle output water approaches the set point
tator Paul Harvey used to say at the end of his broad-
conductivity of your machine.
casts, “Now you know the rest of the story.”
So here’s the rub. Almost all conductivity lights being
promoted, sold and used in our industry are 50 KOhms! Roger Kern | Principal Consultant
Yet in reality, they’re useless at predicting the time to
International Technology Professionals
exchange the DI bottle.
www.itp4edm.com
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EDM Today, Spring 2024 Issue