Re: [at-l] Keeping filters from freezing

[Tim Hewitt <thewitt@fairchildsemi.com>]

OIdMaster@aol.com wrote:
> 
> In a message dated 1/8/99 7:28:36 PM Eastern Standard Time,
> thewitt@fairchildsemi.com writes:
> 
> << At those temps iodine is totally ineffective, not less effective. How about
> 4 hours of contact time at 34 degrees! >>
> Pardon my ignorance, but iodine doesn't work at cold air temps? Does it fail
> to be effective on very cold water? ( For instance the icy brooks of the 100
> mile in May? ) Is very cold stream water 'healthier' than the warmer waters of
> summer?
> Please excuse so many questions, all at one time.
> ~OM

Cold water requires dramatically longer contact times for iodine to kill the
nasties. 

The chart I have in front of me, put out sometime in the distant past by the US
Military for a 2% iodine solution, says:

Clear water:

  Water              Contact
  Temperature (f)    Time (minutes)

  69-65               10 (min contact any temp)
  64-60               20
  59-55               40
  54-50               60
  49-45               90
  44-40              120
  39-35              180
  34-30              240

I like this part the best:

   "For cloudy or suspect water sources, double all doses and contact 
    times. Note that iodine alone is not capable of making all water 
    sources potable. Severe lower G.I. conditions may result from 
    drinking contaminated water. If you suspect contact with a contaminated
    water source, seek medical attention at your earliest opportinuty."

A G.I. with G.I. conditions... hark - a sense of humor in the military?  Nah.
Double the contact times for cloudy or suspect water? Does that really mean 8
hours of contact time for cloudy water at near freezing? Wow!

I don't know how a 2% solution of whatever iodine the military issued in the
past (no idea the date of this chart) compares to modern methods of iodine
delivery, but the longer exposure times for colder or dirtier water must still apply.

As for cold water being safer to drink, I don't know that there is any truth to
that. Certainly giardia cysts can survive in freezing conditions. I'm sure other
nasties can as well.

Spring runoff water will help to make your water fall into the "cloudy" category
above, requiring more iodine than clear water.

-Paddler
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