question about water/citric acid conductivity:

At my company we have two sputtering machines that deposit metals or insulators or just about anything on a substrate in a good vacuum using either RF or DC technology. The machines are water cooled and we use distilled water in the chiller loop.

We found out citric acid powder added to the water at about 10% by weight ratio and running for several hours would take out the rust powder and so forth.

It certainly did, great job in fact, polyflow tubing that was brown with rust deposits got cleaned to brand new looking and so forth.

But when I measure the resistance of the mixture with a DVM (Fluke 77) I found it near impossible to measure correctly because the metal parts inside did some kind of galvanic action that made for about 1/3 volt which upsets the DVM resistance reading.

Anyone here know how to measure the resistance of water with citric or other acids in it? I made up a 10% solution of citric acid in a small container, 1 liter and tried to measure the resistance of the water and it started out at some figure like 10 Kohms but then the reading started going up and then to my surprise, going back down after a few minutes, getting to under 3000 ohms and falling but putting the DVM in millivolt mode showed I was reading about 80 millivolts which will also upset the resistance measurement. The DVM sends out a small voltage which it puts across a resistance and then reads the resultant current and converts that to a resistance reading so any voltage generated by the resistor to be measured upsets the actual resistance reading.

So how can I be generating a millivolt signal when I am using standard voltage probes, which have the exact same metal in both positive and negative leads? I thought you had to have dis-similar metals to generate a galvanic voltage.

And how do you measure the resistance of a water/acid solution accurately?

It is a big deal because the cooling water touches the target to cool it which can get very hot and any conductivity in the cooling water sucks off some of the energy used to make the sputtering plasma so the higher the cooling water resistance, the more energy is used for the actual sputtering process.

These links are probably useless for you, but they were kind of interesting:

http://www.coleparmer.ca/TechLibraryArticle/79
http://www.snowpure.com/docs/thornton-presentation-conductivity-measurement.pdf
http://www.mbhes.com/conductivity_measurement.htm

Originally posted by Soothfast
These links are probably useless for you, but they were kind of interesting:

http://www.coleparmer.ca/TechLibraryArticle/79
http://www.snowpure.com/docs/thornton-presentation-conductivity-measurement.pdf
http://www.mbhes.com/conductivity_measurement.htm

Actually, they were very interesting. I did not know of the inductive method which seems much better than simple 2 electrode methods since that latter can also register galvanic action which upsets a simple DVM resistance measurement.

I think I figured a way to overcome the galvanic action though, generate a reverse voltage with a variable reference voltage in series with the DVM readings.

For instance, I measured a voltage of several hundred millivolts in our machine where it was just cooling water flowing in the system, no plasma activated or such. So I figure if I introduce the opposite voltage which I can do with a small battery and a variable resistor, I should be able to cancel out that galvanic voltage. I measured or tried to measure the current generated by that galvanic voltage and it was below my Fluke DVM's range so a small battery should be able to cancel out that voltage and what is left should be the base resistance of the cooling water.

I'll let you know if that works.

Originally posted by sonhouse
Actually, they were very interesting. I did not know of the inductive method which seems much better than simple 2 electrode methods since that latter can also register galvanic action which upsets a simple DVM resistance measurement.

I think I figured a way to overcome the galvanic action though, generate a reverse voltage with a variable reference vol ...[text shortened]... s left should be the base resistance of the cooling water.

I'll let you know if that works.

Come on just tell us. You are fiddling with a bagdad battery arent you?

Originally posted by joe beyser
Come on just tell us. You are fiddling with a bagdad battery arent you?

They are cheaper there🙂 Much better than those old fashioned Li-ion ones we pass off as batteries here🙂

Originally posted by sonhouse
They are cheaper there🙂 Much better than those old fashioned Li-ion ones we pass off as batteries here🙂

Heck yea. When the cordless drill stops, just empty out the old juice and squirt in some new lemon juice into the reservoir built into the handle. Viola!!

Originally posted by joe beyser
Heck yea. When the cordless drill stops, just empty out the old juice and squirt in some new lemon juice into the reservoir built into the handle. Viola!!

You may be on to something here! Look at this link, very tiny fuel cell made of some kind of glass, invented by Yale engineers:

http://scitechdaily.com/yale-engineers-develop-micro-fuel-cells-made-of-bulk-metallic-glasses/

Originally posted by sonhouse
You may be on to something here! Look at this link, very tiny fuel cell made of some kind of glass, invented by Yale engineers:

http://scitechdaily.com/yale-engineers-develop-micro-fuel-cells-made-of-bulk-metallic-glasses/

Nice!!! I want to look into this further.

Originally posted by joe beyser
Nice!!! I want to look into this further.

BTW, I tested the PH of the citric acid solution we were using to clean the chiller system and did as close to 10% as I could, by volume, 100 ml of citric acid powder to 1 liter of water and the result was a PH of 1.71 (We have in our shop a couple of calibrated PH meters with various PH level standard fluids so we can see how well the PH meter is responding) The cal fluid at Ph 7.00 read 7.00 on the meter so I could see it was pretty well calibrated. 1.71 is pretty acidic all right. Using a cheapo DVM which is surprisingly accurate on the resistance scale, just sticking the probes in that solution gave a resistance reading of about 300 ohms. Then switching to dc volts, it read about 18 millivolts, the galvanic action of the two probes in response to the fluid and that will upset the actual resistance reading. So my next move when I get time away from the other projects going on at the same time, is to make a simple voltage reference for cancelling out that voltage, 18 mv in that liquid but ten times higher in the actual machine.

Originally posted by sonhouse
BTW, I tested the PH of the citric acid solution we were using to clean the chiller system and did as close to 10% as I could, by volume, 100 ml of citric acid powder to 1 liter of water and the result was a PH of 1.71 (We have in our shop a couple of calibrated PH meters with various PH level standard fluids so we can see how well the PH meter is respondin ...[text shortened]... or cancelling out that voltage, 18 mv in that liquid but ten times higher in the actual machine.

If you find the exact specs of the meter you should be able to calculate the difference the 18 mv would make. Add or subtract that value and viola!!! In testing resistance the meter still works like a volt meter only the meter is supplying the potential and makes a votage divider ckt between meter and ckt you are testing. How many ohms of resistance per millivolt does your meter show?

Originally posted by sonhouse
BTW, I tested the PH of the citric acid solution we were using to clean the chiller system and did as close to 10% as I could, by volume, 100 ml of citric acid powder to 1 liter of water and the result was a PH of 1.71 (We have in our shop a couple of calibrated PH meters with various PH level standard fluids so we can see how well the PH meter is respondin ...[text shortened]... or cancelling out that voltage, 18 mv in that liquid but ten times higher in the actual machine.

A person can experimentally find out the value by measuring the value of any ol resistor and apply 18mv across the resistor. This will tell you how much difference to expect.

Originally posted by sonhouse
BTW, I tested the PH of the citric acid solution we were using to clean the chiller system and did as close to 10% as I could, by volume, 100 ml of citric acid powder to 1 liter of water and the result was a PH of 1.71 (We have in our shop a couple of calibrated PH meters with various PH level standard fluids so we can see how well the PH meter is respondin ...[text shortened]... or cancelling out that voltage, 18 mv in that liquid but ten times higher in the actual machine.

How about using non metallic electrodes, or gold plated electrodes?

Originally posted by joe beyser
How about using non metallic electrodes, or gold plated electrodes?

If my voltage cancellation idea works it won't matter what electrodes are used. News at 11.

Originally posted by sonhouse
If my voltage cancellation idea works it won't matter what electrodes are used. News at 11.

Ok it is way past 11 and the news didn't come on.

Originally posted by joe beyser
Ok it is way past 11 and the news didn't come on.

I have several engineering projects going on at once, right now at the end stage of a large ultrasonic cleaner drain project, flip a switch and a valve opens and a pump comes on, I got that to work but there seems to be a leak in one of the valves or something. I designed it and built it, so I get to make it work🙂 funny how that works out!

Another project I just finished was getting a TC gauge to read the inside of an RF magnetron target on one of our sputtering machines, the PHD running the development project messed up the TC gauge cable, not knowing how I installed it so I have to do some more mods on the target to get the cable out. That is part of the same problem we had with the voltage being developed by the target, the TC gauge had to be the insulating type, it contacts part of the target that is biased at around 400 volts which would definitely upset a thermocouple.

And also learning other parts of the development program, running substrates on the MAT-VAC sputtering tool which I have not done as of yet. Been busy on the other sputtering machine here, worked on that one continuously for a year getting it in shape to do industrial sputtering. That was a job, I can tell you!