[labnetwork] Conductivity Standard Solution
John Shott
shott at stanford.edu
Mon Feb 2 12:59:50 EST 2015
Kamal:
OK, I understand that you do not have a circulating system. I suspect
that this is a "once through" system where water goes through a certain
number of polishing beds, UV, and final filters before heading directly
to your DI taps. I've run systems of that type.
I personally don't think that calibration of your resistivity monitors
is the thing that you should worry about. Even if your conductivity
measures 0.04 uS/cm ... which, yes, is technically impossible because at
room temperature water can be no less than 0.055 uS/cm ... that is
indicative of high-resistivity water. If the conductivity increased to
0.08 uS/cm, however, then you know that something has changed.
I would suggest that you try to get the appropriate adapter in your
supply line so that you can install a permanent mounting point for your
resistivity monitor. In that way, you can look at your conductivity any
time that you are flowing water. You can also answer the question in a
once through system: how long after I begin flowing water, does the
conductivity fall to the point that I am getting good resistivity
water. Most importantly, if you normally see a conductivity of 0.04
uS/cm, and it increases to 0.08 uS/cm, for example, you know that
something has changed. That might be indicative, for example, that it
is time to change your resin beds.
While absolute calibration is nice, with all due respect to my friends
at NIST, not all instruments need to be fully calibrated to provide
useful diagnostic information. I believe that this may be one of those
cases where being able to detect a change in conductivity may be more
important than the absolute accuracy of the conductivity measurement.
Good luck,
John
On 2/2/2015 4:22 AM, Kamal Yadav wrote:
> Dear John, Noah, and Miller,
>
> Thank you for your response. I have gone through the attachment sent
> by John and Noah, which is identical and discusses about the topic in
> detail. I did not know this one is going to be difficult as well...
>
> We do not have a circulating DI water loop. We have a conductivity
> meter using which we try to periodically check the DI water
> resistivity in flowing condition in a beaker waiting for sufficient
> time. I was happy as it was showing 0.04 uS/cm :) as that would be
> around 25 Mohm-cm much more than 18.2 Mohm, until a faculty pointed
> out that it is theoretically impossible at that temperature and is
> evident from the attachments you sent.
>
> We have another resistivity meter in Solar Cell Center, that one is
> new and gives good reading something on which we can believe, so far.
> Calibrating that meter after sometime need to be identified as
> informed by the supplier [Merc Millipore]. They do not know as of now!!
>
> *Two Options I can see: [John, Noah, Miller, please comment].*
>
> 1. These days 1 uS/cm standard is also available at NIST. But still
> far from ~ 0.055. The attachment claims ASTM standard even at 100
> uS/cm can be used to calibrate UPW range resistivity meter? I am
> little confused whether I can use the standard, and whether it will be
> stable with its conductivity. We are ok as long as it shows more than
> 16 or 17 MOhm,..but certainly not 25 :). The good meter from Solar
> cell lab shows around 15 Mohm-cm so we know it may be accurate. Since
> it is flowing DI water in a beaker it would be little far from 18.2
> compare to John's 17.5, I suppose!
>
> 2. Send out the meter to an outside agency, will work if agency is in
> India, otherwise I need to see how much it cost, which may be equal to
> buy a new one.
>
>
> Thanks,
> Kamal.
>
>
>
>
>
>
>
>
>
>
>
> On Sat, Jan 31, 2015 at 12:03 AM, Noah Clay <nclay at seas.upenn.edu
> <mailto:nclay at seas.upenn.edu>> wrote:
>
> Kamal,
>
> Personally, I would send out your meter(s) for calibration by an
> expert. That said, here’s a reference from a company in the
> Boston area (google search: "calibrate ultrapure water
> conductivity meter”)
>
> http://www.snowpure.com/docs/thornton-upw-resistivity-measurement.pdf
>
> Apparently, one can purchase standards from NIST for this (as
> stated in the above link), but I’m not sure if they have a
> standard in your range.
>
> Here’s another link from the same search/query:
>
> http://www.thermoscientific.com/content/dam/tfs/ATG/EPD/EPD%20Documents/Application%20&%20Technical%20Notes/Water%20Analysis%20Instruments%20and%20Supplies/Lab%20Electrodes%20and%20Sensors/Ion%20Selective%20Electrodes/AN-PUREWATER-E%20RevA-HIGHRES.pdf
>
> Best of luck,
> Noah Clay
>
> /Director, Quattrone Nanofabrication Facility/
> /School of Engineering & Applied Sciences/
> /University of Pennsylvania/
> /nano.upenn.edu <http://nano.upenn.edu>/
> /
> /
>
>> On Jan 30, 2015, at 11:12 AM, John Shott <shott at stanford.edu
>> <mailto:shott at stanford.edu>> wrote:
>>
>> 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 <mailto:labnetwork at mtl.mit.edu>
>>> https://www-mtl.mit.edu/mailman/listinfo.cgi/labnetwork
>>
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>
>
>
> --
> 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>
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