|Ask the Experts|
January 29, 2021 - Updated
June 5, 2012 - Originally Posted
Deionized Water Sample Testing
We have an in-line sensor connected to a resistivity meter at the exit pipe of our deionized water system. The resistivity readings of the deionized water are above 15 megohms. The meter is calibrated per the manufacturer recommendations and the sensor has been verified with acceptable results.
We have taken samples of the deionized water to an outside lab for testing. The resistivity results of the samples tested by the outside lab have been approx. 1 megohm.
Is there any explanation for this big discrepancy? Can the deionized water samples change by exposure to the atmosphere? How should we transport samples of deionized water to lab for testing?
|Expert Panel Responses|
That is exactly what is happening. As soon as the DI water comes out of the system and is exposed to the atmosphere, it dramatically loses resistivity as it quickly becomes ionized again.
It is also true that it will lose resistivity the moment that it comes in contact with any container due to contaminants inherent in the bottle. We have a DI water system that we use for making 18.2mOhm deionized water for use with our ionic chromatography testing laboratory where we test the cleanliness of printed circuit board assemblies.
We have a resistivity/conductivity meter and it reports the exact same thing.As soon as the water (which is measured by the instrument at 18.2mOhms) leaves the system, it very quickly drops down to about 1mOhm so what you're seeing is typical.
Sales & Marketing Manager
Technical Manufacturing Corp.
Deionized water is a very "hungry" acid. However, it's hunger is very easily satisfied.DI water fresh from a DI bed will easily be >15 meg Ohm resistivity. Once exposed to the air, DI water will quickly absorb (eat up) CO2 from the air satisfying its acidity and reducing its resistivity very quick.
This makes getting an "outside lab" report somewhat difficult, unless you can find a way to draw,transport and test the sample without exposing it to air.
Senior Project Engineer
Electronic Controls Design Inc
The De-ionized water will absorb Carbon Dioxide in the air and this will deteriorate the megohm readings of the material. Since the DI water is so clean it will attack anything to change chemically. You need to be able to transport the water in a specialized container to make sure it is sealed from the environment.
For more information visit: http://www.tmasc.com/di%20water%20specs.htm
I would recommend doing a second test with a different piece of test equipment directly at the site to verify the goodness of the DI system. One of the recommendations from many of the sites is to maintain the correct DI readings it is recommended that the DI water is continuously processed through the DI columns.
So to take it to another location it will deteriorate over time which is probably why you are experiencing the change in DI values.
Vice President, Technical Director
Absorption of ionic species such as CO2 from the atmosphere as well as very small amounts of contaminants from containers can dramatically lower the resistivity of deionized water. If your meter is calibrated, rely on it. The delay in sample processing will introduce very large discrepancies that make lab testing less than useful.
What lab testing can tell you is whether there are non-ionic dissolved solids, such as organic compounds, in the water. Normally, this is not an issue, but if you suspect that there might be contaminants present, testing may be worthwhile.
As to the question about DI Water Sampling: It is almost impossible to take an ultra-pure water sample to the lab and compare that to the reading from the closed system in the facility. It takes so little contamination -- from either in the method of taking the sample, or in the sample container, or in the exposure to the atmosphere, or in the analysis method in the lab -- to change the resistivity from 15 MOhms to 1MOhm.
If you think about how much dissolved solids are in ultra-pure water:
So to drop from 15 MOhm water to 1 MOhm water, we only need(0.500 - 0.034 = 0.466 mg/L). If the sample was a typical 500 mL bottle,then we only needed 0.233 mg of TDS either in the bottle, or added to the sample during the transfer from the in-plant system to the analytical instrument in the lab. That's not very much contamination!
DI water will change within minutes of exposure to the atmosphere. Ultra-pure water is one of the most aggressive solvents and will scavenge anywhere for ions. It will also absorb CO2 from the atmosphere which will drop the resistivity. There is almost no way of taking a clean sample to an outside lab for analysis.
I was told a few years ago that a properly regenerated set of ion-exchange columns should give you between 16 and 18 MOhms, but there was no way to use a sample to calibrate the resistivity meters. I don't know if that information still holds true.
There was a related (although not exactly on this question) previous discussion on Circuitnet in 2009 on this topic: http://www.circuitnet.com/experts/59891.shtml
Corporate Product Compliance Manager
TriQuint Semiconductor, Inc.
You indicate that you are getting a reading of 15 mOhms. Is that at the discharge from the point where the water is returning to the recirc system or at the entry into the machine from the DI system?
15 mOhms is very good quality DI water.Di water exposed to the atmosphere will degrade as it absorbs ions from the surrounding air.Please clarify where you are taking/getting your readings from.
Exposure to atmosphere will definitely impact the resistance of the water as will the container you are using -- but this is a major reduction in resistivity.
Have you asked to see the measurement uncertainty calculations for the equipment that the outside lab is using?
S T and S Testing and Analysis
About the disparity of the test results - Remember the meter is sampling from a closed system. You're securing a second sample and "breaking" the system, so it's important to compare apples to apples as closely as you can. Make sure you're sampling from the same area as the monitor. Any significant length of plumbing could affect the results.
If there are any metal pipes or unions the water is traveling through prior to the sample point or if the sample valve is metal, the water will a) be acidic due to metal ion pickup and lack of buffering and b) will show a decrease in resistivity. PVC or polypropylene are common materials for DI water plumbing. Eliminate any sources of metal in the system.
Contaminants from the transport container can certainly cause reading differences. In other words, don't send the sample to the lab in a Coke bottle (yes, we've seen this!). A good reference for water sampling techniques can be found in Standard Methods for The Examination of Water and Wastewater. Sterile, sealed water sampling containers are available through most lab supply companies.
Ask your lab about the test method, test equipment, and range limits. Differences in methodology and test equipment can result indiscrepancies, although a 14 meg swing is huge. Contamination can also occur at the lab, so check on their protocol. Also question your water supplier and get advice from them. You could ask a competing supplier to test the water from the same point you do. This can usually be done at no charge.
As far as being exposed to the open atmosphere - yes, it can absorb contaminants from the air and through improper handling (ie sulfur compounds, ammoniated compounds, salts from your hands etc) that will affect the outcome. Any ionic species that can be picked up by the water will cause large swings in the resistivity due tolack of (initial) buffering capacity of the water.
I don't have direct DI sampling experience, but do know that water samples for just pH measurement can change over time from absorption of air constituents like CO2. That is why EPA requires analysis of samples for pH within 15 minutes of sample collection.
Also, the type of sample container used is crucial to obtaining accurate lab results. I don't know what type of sample container you used for your outside lab testing, but I highly recommend that you use glass, and not plastic. It is known that certain constituents in plastic containers, like the plasticizer DEHP. DI water is very "hungry" and will dissolve anything it contacts if it can.
Finally, re: sample containers, you need to ensure that there are nopreservatives in the clean (unused) sample container. Depending on the analysis desired, some labs will supply sample bottles with a preservative in the container. Also, you should check to see what type of sample bottle lid material is used. I recommend one of Teflon since it is inert. Hope these tips help.
Keep in mind that deionized water, cleaned to high levels, is a very hungry solvent and will pull in ionic material from wherever it can.
The most common source is carbon dioxide from the air. The degradation from 15 megohm-cm to 1 megohm-cm is most likely from this. You might also have a container that contributed low levels of ionics.
Principal Materials and Process Engineer
P.C. the DI water will always read highest at the source. DI water is only DI water in its purest form at the source.
Once transported through pipe, sprayed in air, heated or even exposed to various metals the quality level of where it started to where it is drops rapidly. Deionized water is exactly that - water that has essentially been stripped of all of its ions.
Water likes to be balanced in its natural state, however, and this means that it adds ions to itself to achieve that goal. Therefore, DI water grabs ions from everything it touches that can be dissolved or absorbed. It is about a close as you can get to a Universal Solvent.
Honestly I see you spending money for tests that can be easily done in house with a simple meter. I have seen some customers put a tap off the feed line just before it enters the machine where they can grab a sample and test it with a resistivity meter. Now with that said;
IPC-AC-62A,Chapt. 10.6, "Good quality deionized water is the preferred rinse medium . . . Water in the 1 to 5 megohm-cm region will be satisfactory for most operations. The temperature of the rinse water should be as high as possible, but comparative with parts and process."
For practical purposes, the maximum level of deionization of water results in resistivity of approximately 18.2 megohm-cm. There is are latively small difference in parts per million (TDS) between 1 megohm and 18megohm water. There is an exponential relationship that has a significant costimpact as well. Operating a system at 18 megohm (or in your case 15) will be substantially more costly than operating one at 1 megohm, yet the benefits in cleanliness will be minimal. Typically, DI water above 1 megohm is a sufficient supply for the final rinse of a cleaner.
In summary, the resistivity of the feed water into final rinse stage of the washer is fine at the 1 to 2 megOhm range measured at the source.Or a TDS (total Dissolved Solids) of 0.4 mg/l as recommended by IPC standards.Once it reaches the actual rinse tank however this number is way lower.(typically 500 to 1000 k-Ohm will suffice)
If you would like to discuss this further do not hesitate to contact me.
Technical Expert Sales Support
It is technically possible for water to absorb CO2 from the atmosphere which will form acid, that in turn will decrease the resistance of the water, this process is however relatively slow and requires exposure to air.
A more likely explanation is the ionic cleanliness of the container used to hold and transport the samples. Water will leach ionic material from many/most materials given intimate contact and time.
Senior Applications Chemist
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