Pool water 'power supply' strange behaviour

Thread Starter


Joined Sep 29, 2018
Could someone help me explain what's happening below in 1b & 2b - my existing knowledge can't articulate this.

1) I put a +5.1V lead from my power supply into a small sample of pool water in a cup.
1b) My multimeter reads +5.6V when submerged in the sample but +5.1V if the MM & supply leads make contact.
2) I place the anode of an LED in the same sample. The LED lights up.
2b) MM submerged in the sample now reads +4.9V .

Shared ground for power supply/MM/LED. Specifically, why wouldn't the MM show 5.1V in 1b & the same value in 2b?

Thanks in advance!


Joined Jan 8, 2017
You need to be a bit more pricise in your wording. I di not think that you realy submerge your multimeter in the sample. I think you probably mean that the positive probe is submerged in the sample. I think the 5.6 volt reading is due to electrolytic effects caused by the wires being made of different metals (Or plated with different metals.) This is creating a very simple battery which adds 0.5 volts to the supply. Note your meter will take almost no current as it probably has 10 Meg ohm input resistance. (So for 5 volt input the current would be 0.5 uA.) The voltage drops down to 4.9 volts due to the current drawn by the LED causing a voltage drop across the resistance of the sample.



Joined Sep 9, 2010
2) I place the anode of an LED in the same sample. The LED lights up.
It's a little surprising to me that even pool water will conduct enough current to light the LED. I suspect it's only 1 mA or so and dimly lit? You didn't say how the lead from the power supply is terminated. Stranded copper could provide quite a bit of surface area for current but the current still has to enter the lead of the LED. Does your pool water utilize salt? That changes everything.


Joined Apr 21, 2014
I can see the effect Les is describing if it is either types of pools. The salt water pool would have intrinsic free ions (Na+ and Cl-) but the chlorinated pool would also have excess Cl- which would be conductive.


Joined Feb 8, 2018
Given that a LED could be lit through the electrolyte, I suspect we are talking about "salt water pool" water. Pool chlorination systems that generate sodium hypochlorite in-situ by electrolyzing sodium chloride (actually replenished with hydrochloric acid as hydrogen is lost in electrolysis and chlorine slowly lost to the atmosphere and by reacting with assorted goop taken out by filtering, leaving an excess of hydroxide ion) have become very common, both for private and public pools. The salt concentration is typically in the range of 0.4 to 0.5% which makes a fairly decent electrolyte - not surprising, give the original intent.