Brian,
Thanks for clearing up the "-cm" part of resistivity. I have always
misconstrued it as a unit of distance between probes, in which case it
should be "/cm". It certainly explains why I could never see a change
in resistance when changing the distance between a multimeter's probes.
(The lesson I have learned today is that I must use the correct
equipment to get a correct measurement!!)
>-----Original Message-----
>From: Brian Ellis [SMTP:[log in to unmask]]
>Sent: Tuesday, December 14, 1999 1:06 PM
>To: [log in to unmask]
>Subject: Re: [TN] water, water everywhere and not a drop to drink...
>
>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:
>> http://www.protonique.com
>> Web services division:
>> http://www.protonique.com/webserv
>
>
>
>--
>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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>If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
>847-509-9700 ext.5365
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