Question regarding water resistance changing with voltage

Thread Starter

effectivedragon

Joined Nov 28, 2022
3
Hi, recently I have been trying to find out regarding resistance of water at high voltages. I may seem oddly obsessed with this but it is forming a big part of a science project I am doing revolving around electrical safety with water and wet things, using detailed resistance values to back things up.





I have studied these videos a fair bit and calculated resistance values at different voltages as per the video with AC, monitoring the resistance change. Here is what I found;

25V = 170000 ohms
49.8V = 171724 ohms
100V = 166666 ohms
150V = 166666 ohms
200V = 166666 ohms
225V = 164306 ohms
250V = 163265 ohms
287V = 163068 ohms

So for this I have come to the conclusion that whilst it doesn't quite follow ohms law the resistance does not decrease by much at all and stays in a fairly small range (in the end only by like 8,000 ohms.) My question is is this what would normally happen that it only strays from ohms law by a little bit and what is the reason it strays from it but not by much? Also why does resistance remain at the same value for a bit? And would increased salts or any other substance in the water affect how much it deviates from ohms law?

Hope I have made the question clear and understandable :D
 

crutschow

Joined Mar 14, 2008
31,151
Also why does resistance remain at the same value for a bit?
So it it basically following Ohm's law.
Why would you expect it not to?
And would increased salts or any other substance in the water affect how much it deviates from ohms law?
Distilled water does not conduct electricity.
It's the dissolved salts and minerals that cause it to conduct, so the more salts you add the lower the resistance.
For example, sea water, has a very low resistance due to all the dissolved salt.

Since there is such a variation of water resistance depending upon the amount of dissolved salt, from a safety point-of-view, all water should be considered dangerously conductive, as essentially being a short-circuit.
 

MrChips

Joined Oct 2, 2009
27,727
We cannot see your experimental setup.
What is your measure of uncertainty?
Your results show about ±3% uncertainty.
 

Wendy

Joined Mar 24, 2008
23,082
Ultra pure water (AKA deionized water) is not conductive. However, all water has something dissolved in it which will make it conductive. So in real life it is best to treat water as a conductor. This also includes things like downed power lines in a puddle.
 

WBahn

Joined Mar 31, 2012
27,942
Ultra pure water (AKA deionized water) is not conductive. However, all water has something dissolved in it which will make it conductive. So in real life it is best to treat water as a conductor. This also includes things like downed power lines in a puddle.
I'm more than willing to give downed power lines a wide berth no matter what they are resting on. ;)
 

nsaspook

Joined Aug 27, 2009
10,715
Because water isnt a stable compound and seen a few say that as voltage increases the chemicals change which affects conductivity...
True that. I deal with Ultra pure DI water cooling systems that handle 100's of thousands of volts. We constantly filter and deionize (with Ion exchange resins) the wafer with special filters to get better than 10M/cm resistance. Stopping the filter pumps for a few minutes causes a easy to see water quality change on the monitoring systems.

https://en.wikipedia.org/wiki/Ultrapure_water#Applications_in_semiconductor_industry
https://www.yokogawa.com/us/library...ter-and-the-difficulties-with-ph-measurement/
 

Wendy

Joined Mar 24, 2008
23,082
Where I saw it used was cooling for a Xenon tube that was continuously fired to pump a Yag rod That was used in a laser to trim thick film resistors on a ceramic substrate.
 

DickCappels

Joined Aug 21, 2008
9,524
I once worked for a small company that made power supplies, deflection circuit, etc. for the semiconductor industry. One day we were testing a several kW power supply, using a plastic pipe with tap water running through it. We managed to finish the test but because of boiling the the water's effective resistance was all over the place. Not a good dummy load, but the price was right.
 

Thread Starter

effectivedragon

Joined Nov 28, 2022
3
I once worked for a small company that made power supplies, deflection circuit, etc. for the semiconductor industry. One day we were testing a several kW power supply, using a plastic pipe with tap water running through it. We managed to finish the test but because of boiling the the water's effective resistance was all over the place. Not a good dummy load, but the price was right.
How much did the resistance change by and what voltage was this?
 

Danko

Joined Nov 22, 2017
1,493
Electrodes and water are affected by electrolysis on AC current.
It is why you see deviation of resistance.
ADDED:
You need also to stabilize temperature of water.
Increasing temperature on 1 °C (for example because of current),
decreases resistance of water on 2-2.5 %.
 

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