Urgent help with resistance of water

Have you seen these...

https://youtu.be/MF9DCNkaE8I

https://youtu.be/3-3M3r69pOQ

Think so but the electrodes are fairly far from each other here would it be similar resistance difference if they were closer

^. Water itself, free of any/all possible ions and contaminants, is usually regarded as a good insulator (though that is a relative term.) Even the slightest concentration of ions, though, and your water will rapidly become a very good conductor.

The amount of current that can flow will be somewhat proportional (it's been a damned long while for me since I've done much physics) to the number density of charge carriers, and the amount of charge they each carry- and these will be the ions in the water. They're going to be responsible for most of the conductivity available in water at relatively low voltages. Quickly googling-

"The electrical resistivity of water (0.2 Ω·m sea water, 2 to 200 Ω·m drinking water, 180000 Ω·m deionized water at 20°C)" first result.

Further googling gives rough guesses of-

"between 33-37 grams per liter"
"less than 600 mg/L is regarded as good quality drinking water."

for sea/tap water, with tap water thus being roughly 60 times less salty- 60*0.2 ~ 12, which already gets us into the above spec for drinking water (which would likely have much less of other forms of ion, and less variety I would expect... anyway, just to demonstrate that the proportionality does somewhat follow from common sense.) Good quality distilled water can reach megaohm-meters in resistivity, mind you- those are ballparks.

The 240V we get at mains is AC, too- not DC. That could impact the dangers of being shocked- though to my understanding, getting whipped by AC is more dangerous, it could have different effects in terms of current flow in the water (fluctuating current directionality/magnitude.) If you wanted to do it as an experiment, you could use a hall effect current sensor around a small channel of water and measure current w.r.t voltage. You could vary salinity/etc for distilled water to measure effect of solute concentration on resistivity, too. Note that the effects are generally temperature dependent.

I should also add that it'd be best to be cautious if designing an experiment to work with mains. If you get your own rectifier setup for DC, make sure that it's rated for whatever current you aim to use (you can use regulators that self-limit for safety, for example. I blew a rectifier trying to power a fan array before I took the effort of slapping a decent regulator on it.) You can also use resistors in series + use a voltage divider to measure resistivity more safely. If you gun straight for the AC instead of DC-rectification prior... well, I've no experience in doing that so can't offer advice.

I want to avoid doing anything with 240V... Doesn't the electrode inject ions? I guess not enough to cause a massive difference...

With lots of consideration I have two ways to proceed and queries on both. First stage is to use fluid dynamics or other simulation software that can give me an accurate result on this. However OpenFOAM is extremely complicated to use and there is no way I can learn it in time to get a valid answer. My question is is there any CFD software or virtual labs that is accurate where I can do this experiment virtually (also that has a free trial)? I’ve investigated Labster, is this an accurate piece of software?

Secondly I am coming to the conclusion that I am probably going to have to do this experiment for real. But this fills me with a lot of anxiety, as you’d probably expect handling 240V would do. Ideally I would like to do it in an environment for this sort of thing but the lab at my uni is shut for the summer (spent too long trying to avoid doing this experiment for real) so that takes that option away. What’s the best environment to do this in, is there anywhere that is designed for this sort of thing, should I do it with an experienced electrician (I don’t know many…)

Or would it be better to do neither and just assume the resistance of water at 240V is the same at 9V? In past posts I have done on various forums, I have been told I should do some sort of experiment to prove that because of factors like electrochemistry changing...

For your first paragraph- simulating it would inevitably be a pain. That being said, I expect some kindly people have posted pre-made models already- that's usually the case as with all things- so it wouldn't be too bad to do. I've used some simulation softwares before (Enzo + ANSYS) and it's quite fast to pick them up when you know the basics. The issue of this is that it isn't just CFD, you have to include the electrical aspect- I've never done that personally.

If you want to do this practically, here's some advice: do it all remotely. Your shutoff switch should be remote, you should be behind some glass when you do it, you should have a remotely-activated mechanism to douse water as required, etc etc. As long as you get a good safety plan in mind and run it past someone with electrical knowhow, mains is nothing to worry about- I'd be more worried about triggering the fuse board in your property than the actual experiment itself (that'll hurt if you're not on modern fuses and instead use wire fuses, as my old flat did. Fire risk and expensive.) Do your splash test, record your measures remotely with a data logger, and record it all on camera (and since this is "real" you will need to repeat it for errors/etc quite a bit.) I'm still unsure on what exactly it is you're trying to do, but if you want to measure the current across a splash at some distance/etc, then I would think it fairly straightforward to "remote-ize" the experiment. If you don't do it all remotely, and instead opt to do the splash yourself- well, that's a bit of a safety risk.

If you want to make an assumption, you have to justify it. Unless other people have made that assumption before and it has been accepted and you can safely reference it, it needs to be justified through some way.