Graham
The basic unit of conductance is the siemens, which is 1/(resistance in ohms).
Therefore conductivity is the siemens-centimetre which is 1/(resistivity in
ohms-cm). The microsiemens-cm is therefore 1/(resistivity in megohms-cm),
usually related to the international standard of 20°C or 293,15K. Unfortunately
the TC of water is not linear, being approx -3%/°C at 0,02 uS-cm, dropping to
-2.3%/°C at 10 uS-cm.
You will note I use a hyphen and not a multiplier/divisor before the cm. This is
often incorrectly misconstrued. The definition of resistivity or conductivity is
the resistance or conductance across opposite faces of a cube whose edges are 1
cm long. As there are twelve such edges, there is obviously no mathematical
relationship between the resistance or conductance and the dimension of any one
edge, and hardly more if you use all 12 edges. The hyphen then signifies this
non-mathematical relationship. This would have been clearer if someone, at the
start of the century when these units were first expressed, had used a subscript
e.g. ohm(subscript cm) to indicate that it was related to a cube of a cm edge.
This is probably a pedantic statement but, there, I'm a pedant! If you wish to
see the wisdom of this pedantry, think of a 1 cm edge cube with a resistance of,
say, 1 ohm. Then translate this into resistivity in ohm-inch, which was hitherto
much used.
Hope this helps
Brian
Collins, Graham wrote:
> Brian
> Forgive my ignorance, what is the unit for conductivity? "uS-cm" - the cm
> part I get, what is uS? I'm used to resistivity, haven't used a
> conductivity number much...
>
> Thanks for you help!
>
> - Graham Collins
>
> -----Original Message-----
> From: Brian Ellis [mailto:[log in to unmask]]
> Sent: Tuesday, December 14, 1999 1:34 PM
> To: TechNet E-Mail Forum.; Collins, Graham
> Subject: Re: [TN] water, water everywhere and not a drop to drink...
>
> Graham
>
> There is distilled water and distilled water. It depends on the installation
> used. Even
> triple-distilled in quartz water will not have as low a conductivity as a
> good DI
> water, but may have lower impurities of other natures, especially
> particulate matter.
> However, single distilled water from an iron still would be definitely
> suspect.
>
> Unless it has been distilled more than once or in a reflux still, then a
> number of
> impurities can be carried over. A steam bubble bursting at the surface will
> throw up
> microdroplets of contaminated water into the steam. A single distillation of
> sea water,
> for example, with 35 000 ppm dissolved solids will produce a water of about
> 1 000 ppm
> in a non-reflux still. Multistage solar flash distillation of sea water
> always is done
> over three stages to produce potable water (c. 500 ppm NaCl max, typically
> 100 uS-cm).
> True RO + DI water is the most economic reasonably pure water.
>
> If you buy distilled water, get a written certification of a conductivity of
> less than
> 1 uS-cm at the time of packing (it may rise to about 5 if not consumed
> rapidly).
>
> Brian
>
> Collins, Graham wrote:
>
> > Good day Technetters!
> >
> > A water / chemistry question for the TechNet today. Is distilled water an
> > acceptable substitute for DI water? My non-chemistry mind thinks so but
> I'd
> > like a more qualified opinion.
> >
> > Why? We are looking to up our cleaning capacity, and while the DI set-up
> we
> > have would keep up with the running requirements of a bigger cleaner, it
> > would have a hard time filling the tank for start-up. So I can either buy
> a
> > bigger DI set-up, or buy DI water as required. Our local filtering
> supplier
> > claims distilled will do, but I question his expertise. Is he correct?
> >
> > As always - thanks, and have a Cool Yule!
> >
> > Graham Collins
> > Process Engineer,
> > Litton Systems Canada, Atlantic Facility
> > (902) 873-2000 ext 6215
> >
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> --
> Brian Ellis
> Protonique SA
> PO Box 78
> CH-1032 Romanel-sur-Lausanne, Switzerland
> Voice: +41 21-648 23 34 Fax: +41 21-648 24 11
> E-mail: [log in to unmask]
> URL: Technical and consultancy divisions:
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> Web services division:
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--
Brian Ellis
Protonique SA
PO Box 78
CH-1032 Romanel-sur-Lausanne, Switzerland
Voice: +41 21-648 23 34 Fax: +41 21-648 24 11
E-mail: [log in to unmask]
URL: Technical and consultancy divisions:
http://www.protonique.com
Web services division:
http://www.protonique.com/webserv
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