Letter 24005

DI Water Resistivity is varying

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I work in a small laboratory. Our DI water supply is kept in our lab in a 5 gallon container. Our initial requirement was to maintain at a minimum resistivity of 50,000 ohm-cm. We are now instructed to maintain at a minimum resistivity of 500,000 ohm-cm. Is there something I can add to our 5 gallon water supply or something I can connect to this water supply to increase the resistivity?

Gustavo M
sheet metal manufacturing and finishing - Duncan, Oklahoma

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You mentioned that your water is in a 5-gallon container. I am guessing that this is bottled water that you are buying at the store. Is that right? If so, it is likely purified by reverse osmosis. Water of this type will be insufficient for meeting your new resistivity requirement, so you will need to come up with something new. If you are using less than a few gallons per day, the choices that you have are not very wallet friendly. There are various water purification devices that can be purchased from a scientific supply catalog. They need to provide Type II water (that's double the resistivity that you must have). This will cost you between $2,000 and $3500 dollars. Your other alternative is to contact a water purification company in your local phonebook. They would set up a rental agreement for a small, under-the-sink, ion exchange canister that can be tied directly to your water faucet. They will swap it out once a month for a new one. These canisters will deliver reliable Type II water. Expect to pay $100 per month for this service. If that is still above the lab budget, maybe you can work out a deal to purchase some Type I or II purified water from a larger lab nearby. If so, you will need to take frequent trips because, as you probably have realized, purified water does not remain pure when it is setting around.

Jon Barrows
aerospace jobshop
Tulsa, Oklahoma

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The easiest way for you to maintain the resistivity at greater than 500,000 Ohm-cm is to purchase a disposable cartridge of mixed-bed ion exchange resin and circulate the water through it. The cartridges are available from lab supply equipment companies.

Lyle Kirman
water treatment systems - Cleveland, Ohio

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I believe anything that you add, will decrease the resistivity, not increase it. It doesn't take long for DI water to lose its quality, unless its kept very well sealed. If I were you, I would pump it through a unibed just prior to using it, then you'll likely have 16-18 megohm water at your disposal, as you need it.

Marc Green
anodizer - Boise, Idaho

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Jon - The budget for the lab is very, very, low - this is an internal lab for a manufacturing company. Thank you for the input though.

Lyle - the disposable cartridges - I have asked around our Lab Suppliers around here - none of them knew anything about this - could you please recommend someone that I may go to?

Marc - As noted above, my budget for this lab is very low. I would not be able to afford your idea. Thank you for the input though.

I apologize, I failed to explain about my 5 gallon container. The DI water supply is not plumbed into a faucet. The reason I have it in a 5 gallon container is that we have water treatment area for our finishing line. We use DI water for our tanks, and so I just pull a 5 gallon sample every week for my testing.

Gustavo M
- Duncan, Oklahoma

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I suspected that you had a budget problem. I believe that the disposable cartridges that Mr Kirman mentioned are designed to operate as part of a purification system. If you are going to meet your new requirement, there is not going to be any way around spending some money. I guess the cheapest thing you could possibly do is to construct your own mixed bed column and run your water through it by gravity. This would only be practical if you only need to use about a gallon or so per day. If you are going to do this, you will need to spend some time getting educated on ion exchange resins theory and use. Resins are not cheap, but if you learn how to take care of them and regenerate them properly, they should last you a long time.

Good Luck.

Jon Barrows
aerospace jobshop
Tulsa, Oklahoma

December 27, 2008

First, find out exactly why the new requirement has been set!

It is not uncommon for people to specify much higher standards than are actually required.

The rise of a full order of magnitude is suspicious in that regard.

Without knowing what your lab is actually using the water for, it's not possible for us to even guess if it is correct.

Track down the person who set the requirement and talk with them--no matter what you do to reach the requirement, it will cost the company either in direct costs, labor or continuous maintenance costs.

Track it to the source, and find out the actual need. If it is a real need, then you will have to find a source of money to implement it, and that means your budget must be increased both for initial and continuous maintenance.

Point out that it is not 10x more expensive to drop resistivity that low, but more like 1000x. Right now your cost is ~%5 a week with just the cost of the jug up front.

Since you have been using DI from your fab line, and it is likely that your lab work is related to that line, there is a chance that the fab line should also have a higher standard, in that case, the investment will be substantially more expensive (although your cost per gallon will drop.)

Charles M. Barnard
- Menomonie, Wisconsin