Letter 24005

DI Water Resistivity altering [Oklahoma] 

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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 Martinez
Sheet Metal Manufacturing and finishing - Duncan, Oklahoma, USA

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 cannister that can be tied directly to your water faucet. They will swap it out once a month for a new one. These cannisters 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

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

I believe anything that you add, will decrease the reisitivity, 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 youll likey have 16-18 mgohm water at your disposal, as you need it.

Marc Green anodizer - Boise, ID, USA

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 Martinez
- Duncan, OK, USA

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

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