TECHNET Archives

February 2004

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 27 Feb 2004 05:43:38 -0800
Content-Type:
text/plain
Parts/Attachments:
text/plain (199 lines)
        Brian:
        Good article, I appreciate your clarification and you are very
right. We all tend to oversimplify and in the process cut off parts of
descriptions and communicate in a chopped up language to  express ourselves
faster and to the point. The DI probes are one centimeter apart.
        Ramon

> -----Original Message-----
> From: Brian Ellis [SMTP:[log in to unmask]]
> Sent: Friday, February 27, 2004 3:59 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] Conductivity and Resistivity Correlation
> 
> OK, Steve, here goes.
> 
> As there appears to be confusion here between resistance and resistivity
> and between conductance and conductivity, let me put this matter
> straight, to start with. Let's start with resistance. You have all seen,
> say, a 10 kilohms resistor, as you mount such beasts on your PCBs every
> day. This has a resistance of, of course, 10 kilohms (±n%) and this
> means that if you pass a current of 1 mA through it, you will have a
> potential difference of 10 V across it (or vice versa). The conductance
> of the same device is 0.1 millisiemens (a siemens is the reciprocal of
> the ohm). Note that the mho is a totally obsolete unit equivalent to a
> siemens, but has not been considered as acceptable for many decades and
> is not even mentioned in ISO standards on units. It was coined in the
> late 19th century. It is as logical a name as using snemeis for
> resistance :-)
> 
> Historically, resistivity was used to describe a material property
> before conductivity, although the latter makes a lot more sense because
> it is a linear function. Conductivity can be defined as the ability of a
> material to conduct electricity and is the conductance of a cube of unit
> length of each edge of the material across opposite faces. The
> resistivity is therefore the resistance of a cube of unit length of each
> edge across opposite faces. Notice "of unit length of each edge". This
> is important. It means that the resistivity and the conductivity depend
> on the size of the cube. This has led to confusion in the way they are
> expressed, because the mathematical relationship between the electrical
> and the linear values is not evident. The resistivity of a non-cubic
> block of a material may be expressed as (R.l)/(w.h), where the length,
> width and height are obviously all in the same units and the
> conductivity is (G.w.h)/l . So if we put a mathematical relationship
> into the unit, such as ohm/cm, does the cm refer to the l, the w or the
> h? I've seen metric resistivity expressed as ohm.cm, ohm/cm, ohm.cm3,
> ohm/cm3 and ohm-cm and conductivity in all these variants as well, such
> as S/cm etc. With the possible exception of the ohm-cm and S-cm, they
> are all wrong or, at the least, misleading. Then, of course, I've also
> seen resistivity on ohm.inch etc. and this is different from ohm.cm. As
> I write this, I've got in front of me an electrical engineer's handbook
> from 1927 where there is a resistivity table of elemental metals where
> the column is headed: "Specific Resistance in legal Microhms at 0°C per
> c.m. cube". On the following page, there is one of alloys, where it is:
> "Specific resistance in legal microhms", with no dimensional mention. A
> few pages later, there is a table on insulating materials, where it is:
> "Megohms per Square Inch-Mil". However, in the text, it states: "The
> electrical resistances of porcelain and glass are given as 200 x 10^10
> megohms (porcelain) and 20,000 x 10^10 megohms (glass) per cm. cube.".
> Obviously, there was as much confusion then as there is now :-). N.B.
> all the quotations are textual re punctuation and case. Note that the
> terms specific resistance and specific conductance have been dropped by
> ISO because "specific" is now permitted only related to mass.
> 
> My physics text book I had as a 1st/2nd/3rd year undergrad has a
> resistivity table and it gets out of the problem by giving just the> 
> figures, although they are in ohm-cm at 18°C. However, my physics book
> as 4th/5th year undergrad also has a table headed "Resistivity in ohm-cm
> x 10^-6" with a separate column for temperature, as they vary from 0°C
> to 100°C, via 18°C and 20°C. These date from the 1940s.
> 
> My Handbook of Chemistry and Physics, 65th Edition (1984-1985), gives:
> "Resistivity microhms-centimetres 20°C" for metals, "Microhm-Cm" for
> elements but simply "µ-ohm" for alkali metals. However, for ionic
> conductivites in solutions, it uses "10^-4 m2 S mol^-1"!!!
> 
> ****IMHO****, the correct way of expressing resistivity and conductivity
> would be ohm<subscript cm> or siemens<subscript cm>, although I would
> accept ohm-cm and siemens-cm, as used in my 4/5y physics book and the
> "Handbook", because the - does not signify a mathematical relationship,
> as , or / would do.
> 
> Now, the original question is about water purity. Water has an enormous
> and non-linear temperature coefficient of conductivity, so temperature
> does play an important role. At usual conductivity values, the TC is
> about -2%/°C but rises to about -3%/°C at high purities (e.g., <0.1
> µS-cm). This TC itself is non-linear, as well. In the USA, the reference
> temperature is usually 25°C, but 20°C in the rest of the world.
> Commercial conductivity/resistivity meters for tap water are often not
> temperature compensated but the better ones are, usually at a fixed
> value of -2%/°C, although some expensive lab meters do allow you to set
> the TC and the ref temp.
> 
> Very obviously, temperature can therefore be a big source of errors
> depending on the quality of the instrument.
> 
> Sorry for the rant, but having manufactured ultrasensitive compensated
> conductivity meters for the Contaminometer in a former life, I do have a
> little experience in the subject.
> 
> Brian
> 
> 
> Steve Gregory wrote:
> > Hi All!
> >
> > Is there any chart, formula, or conversion that I can use to correlate a
> > conductivity reading to a resistivity reading?
> >
> > The reason I'm asking is that I have a new batch cleaner that has been
up and
> > running the last three weeks or so. One of the neat things with the
cleaner
> > is that during the rinse cycle, it monitors the rinse water resistivity
to a
> > setting that you program in the machine. If it reaches that
pre-programmed
> > resistivity setpoint before completing the number of rinse cycles that
you program,
> > it will terminate the rinse cycles and go into the dry cycle. If it
never
> > reaches the resistivity setpoint within the number of rinse cycles that
you've
> > programmed, it will display "FAILED" on the touch screen. You can then
run
> > another cycle, or start trying to figure out why the boards didn't come
clean.
> >
> > Well, everything has been going fine with the cleaner up until
yesterday.
> > Every batch of boards I ran failed the resistivity setting I had in the
machine.
> >
> > The manual recommends a setting between 350-750 kohms, I've had my
setting at
> > 550 kohms from the beginning, and everything has been fine. I've had to
lower
> > the setting to 400 kohms so the batches would pass. It's still within
the
> > 350-750 kohm range, but I'm curious why all of a sudden things won't
pass at 550
> > kohm like it's been doing the few weeks.
> >
> > So I called tech support for the cleaner, and they feel that my input DI
> > water may be going bad. I have a US Filters reverse osmosis filtering
system that
> > dumps that water into Ion exchange columns and that water goes directly
to my
> > cleaner. The DI system displays the conductivity of the output water in
µS
> > (micro-siemins). I want to correlate that to resistivity to see if my DI
water is
> > truly going bad.
> >
> > Thanks!
> >
> > -Steve Gregory-
> >
> > ---------------------------------------------------
> > Technet Mail List provided as a service by IPC using LISTSERV 1.8e> 
> > To unsubscribe, send a message to [log in to unmask] with following text
in
> > the BODY (NOT the subject field): SIGNOFF Technet
> > To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
> > To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
> > Search the archives of previous posts at:
http://listserv.ipc.org/archives
> > Please visit IPC web site
http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information,
or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
> > -----------------------------------------------------
> >
> >
> 
> ---------------------------------------------------
> Technet Mail List provided as a service by IPC using LISTSERV 1.8e
> To unsubscribe, send a message to [log in to unmask] with following text in
> the BODY (NOT the subject field): SIGNOFF Technet
> To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
> To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
> Search the archives of previous posts at: http://listserv.ipc.org/archives
> Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
for additional information, or contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
> -----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------

ATOM RSS1 RSS2