Sorry, I forgot one important practical point: are the electrodes clean? If you have an accumulation of crud, lime, flux residues etc. on them, you will get apparently lower conductivites/higher resistivities. Brian Brian Ellis wrote: > 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 -----------------------------------------------------