On Fri, 30 Nov 2018 01:22:13 -0000, Rod Speed wrote:



"Bruce Farquhar" wrote in message
news
On Thu, 29 Nov 2018 21:10:43 -0000, Rod Speed
wrote:

Bruce Farquhar wrote

Those cheap Chinese water heaters which are simply two plates
of metal, one on live and one on neutral, how can they work?

The current flows thru the water and that heats it.

Because the resistivity of tap water apparently
varies from 2 to 200 ohm metres. That's a
range of 100 fold in possible power output.

That only affect how long it takes to heat the water.

I assume the resistivity changes due to
impurities like lime in hard water areas?

Yep.

My point is, a 2.5kW heater could end up 25W and be totally useless.

Sure, but that's the risk you take with that approach but it has other
advantages, most obviously the price and lack of maintenance.

I like bigclive's warning that it's undesirable to use it in the baby's bath while the baby is in it.

A factor of 100 is a LOT.

But isnt actually anything like that great in real life with tap water.

I wonder what real life values are? The 2 to 200 ohm metres was from a google search and it related to "drinking water", without any further specifications.