[labnetwork] Conductivity Standard Solution

[John Shott]

Kamal:

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.

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.

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.

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.

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.

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.

Let me know if you have any additional questions.

John

On 1/30/2015 2:01 AM, Kamal Yadav wrote:
> Dear All,
>
> What is the best way to calibrate conductivity meters for DI water 
> resistivity periodic monitoring.
>
> 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]
>
> Thanks a lot!
>
> -- 
> Thanks,
> Kamal Yadav
> Sr. Process Technologist
> IITBNF, EE Department, Annexe,
> IIT Bombay, Powai
> Mumbai 400076
> Internal: 4435
> Cell: 7506144798
> Email: kamal.yadav at gmail.com <mailto:kamal.yadav at gmail.com>, 
> kamalyadav at ee.iitb.ac.in <mailto:kamalyadav at ee.iitb.ac.in>
>
>
> _______________________________________________
> labnetwork mailing list
> labnetwork at mtl.mit.edu
> https://www-mtl.mit.edu/mailman/listinfo.cgi/labnetwork

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