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-
>
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