View Full Version : Conductivity
May 3rd, 2002, 01:15 PM
This is stretching the definition of "PLC-Related" a bit, but since I'm connecting it to a PLC, I'll give it a shot :D Perps somebody here has dealt with one of these devices before.
Does anybody know how a fluid conductivity meter works? I have the basics, submerge a magnetic probe, read current induced in pickup coil, yadda yadda, but what can the thing actually detect ? Only ferrous materials? Soap? Oil in suspension? Any contaminant at all?
May 3rd, 2002, 03:10 PM
I am not sure what you are wanting to know. A conductance meter doesnt detect elements, it just measures the ability to conduct. You dont need ferrous materials to conduct but at some point would make connection easier. Water (pure H2O) is not a good conductor but add salt and you have a good conductor. Some of the old guys may remember saltwater tank DC drive systems. Basically water with minerals or mixed chemicals or combinations thereof can become conductive. I worked on a system where they waterproofed cloth (used for surgical gowns) and we had to monitor conductance.
May 3rd, 2002, 04:33 PM
Yep, Ron's got it basically right. The key word is IONS
To conduct electricity, a single electron doesn't move from one end of the wire to the other (so far as anyone knows - no one's been able to paint one blue to watch it to verify this), but goes to the nearest metal atom, knocks an electron loose (from the higher orbitals), and takes its place. The new electron (travelling "downhill" based on electrical potential) knocks the next one loose, etc.
Water molecules hold onto electrons pretty well, which is why it's a lousy conductor. It grabs electrons so well, in fact, that it can pull apart some molecules where the atoms aren't so good at holding onto them. This is called "dissolving". Thus, instead of having neutrally charged sodium and chloride atoms, or even whole molecules floating around, you get Na<sup>+</sup> and Cl<sup>-</sup>.
Since the CL has an extra electon, it makes it a great conduit for electron flow (=conductivity). The more chloride ions, the better the conductivity.
It doesn't have to NaCl salt - any impurity in the water is going to get pulled apart some - even oils (only less so) or disolved oxygen. This is what makes conductivity meters such a pain to deal with - what they measure is very non-specific. If the water is agitated, or the "hardness" goes up, you'll get a change in conductivity that is not a function of whatever you are trying to measure.
May 3rd, 2002, 07:26 PM
All above statements are exact but...
There is a little more to it. When you have a known process, say you mix some stuff and end up with a magic potion... This potion could have some conductance characteristics which will vary in function of something you know! Hey this is great. Just add a cheap meter whit close to no maintenance and you monitor your process.
How else could you monitor on-line a mixture of water-yeast and sugar... and follow the chemical transformation of this product. Can't go cheaper than this.