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Kamal:<br>
<br>
Let me start by saying that I've never actually tried to calibrate
meters of this type. Why? Because it is not easy. Here is a good
reference article from over 15 years ago that describes the process
in great detail including the fact that the standard conductivity
solutions only go down to about 5 uS/cm ... which isn't very close
to the 0.06 uS/cm you are hoping to measure. They also talk about
separating the whole calibration process into the steps of
calibrating the meter itself (easy), the temperature probe
(reasonably easy), and the "cell constant" of the probe itself
(hard). A number of you will recognize that the author of this
paper works for a company that makes and sells resistivity probes
and monitors. This is not intended to be an endorsement of that, or
any other, company ... but, I think, indicates that detailed
calibration of DI resistivity monitoring systems is typically found
primarily in the companies that make and sell such instrumentation
rather than by the folks that use such instrumentation.<br>
<br>
Their solution for high-precision calibration was to measure UHP
water over a range of temperatures as a means of determining and/or
calibrating the cell constant. If you read this article, however,
you will conclude that this is not a procedure for the faint of
heart.<br>
<br>
In recirculating DI systems, I believe that it is more common to
have continuous resistivity monitoring on both the supply side and
return side of the system. In our case, we typically see supply and
return resistivity readings about 17.7 MOhm-cm or higher ... but
that rarely, if ever, read the theoretically expected 18.2 MOhm-cm.
In fact, it is not uncommon to see a return resistivity that is
slightly higher than the supply-side resistivity ... which would
seem unlikely.<br>
<br>
Then, on an occasional basis ... probably not as frequently as we
should ... we (well, a third-party analytical laboratory) collect
samples and have them measured for particle content, bacteria grown,
total oxidizable carbon, dissolved silica, and a 30-element
mass-spec analysis for metal levels in the ppt range that is
commonly used for DI systems. In short, there are lots of things
that CAN be wrong with DI water that are not seen by even an
accurate resistivity measurement. In other words, as long as our
resistivity readings are on the order of 17.5 MOhm-cm or above on
both supply and return lines, I, for one, don't worry about the
resistivity aspects of our water. In fact, earlier this week, I was
comparing these DI analytical test results with another frequent
contributor to this forum from the Bay Area institution with the
longest history as a university laboratory in this field.<br>
<br>
Finally, when you say "periodic monitoring" do you mean that you
have a probe in a continuously recirculating loop and you want to
look at the resistivity of that periodically ... or that you
occasionally collect a sample of water and are trying to measure
it's resistivity? If it is the latter, that can be tricky: when
exposed to air, DI water absorbs CO2 which forms carbonic acid that
can cause your resistivity numbers to degrade.<br>
<br>
My guess is some of the folks that run newer labs than ours will
have more details about the way that they monitor the DI water in
these newer operations.<br>
<br>
Let me know if you have any additional questions.<br>
<br>
John<br>
<br>
<div class="moz-cite-prefix">On 1/30/2015 2:01 AM, Kamal Yadav
wrote:<br>
</div>
<blockquote
cite="mid:CAOfoCyiJ3FdsX3LDbhbkqD0A_ADdsfu234FRDPoCdiJrvqZzew@mail.gmail.com"
type="cite">
<div dir="ltr">Dear All,
<div><br>
</div>
<div>What is the best way to calibrate conductivity meters for
DI water resistivity periodic monitoring.</div>
<div><br>
</div>
<div>Standard known conductivity solutions are available but
which one is good and stable for this range of measurement.
[18 MOhm-cm or ~ 0.06 uS/cm]</div>
<div><br>
</div>
<div>Thanks a lot!<br clear="all">
<div><br>
</div>
-- <br>
<div class="gmail_signature">
<div dir="ltr">
<div>Thanks,<br>
</div>
<div>Kamal Yadav</div>
<div>Sr. Process Technologist<br>
</div>
<div>IITBNF, EE Department, Annexe,</div>
<div>IIT Bombay, Powai</div>
<div>Mumbai 400076</div>
<div>Internal: 4435</div>
<div>Cell: 7506144798</div>
<div>Email: <a moz-do-not-send="true"
href="mailto:kamal.yadav@gmail.com" target="_blank">kamal.yadav@gmail.com</a>,
<a moz-do-not-send="true"
href="mailto:kamalyadav@ee.iitb.ac.in" target="_blank">kamalyadav@ee.iitb.ac.in</a></div>
</div>
</div>
</div>
</div>
<br>
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