Author Topic: Water Resistance: Tap Water vs. Salt Water  (Read 6317 times)

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Offline SoerenTopic starter

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Water Resistance: Tap Water vs. Salt Water
« on: February 07, 2011, 07:15:26 PM »
Hi,

Water resistance has been discussed from time to time and I thought it was about time getting some numbers on the subject.
So here's a couple of unscientific yet controlled measurements on tap water as well as "sea" water 35g/l (made from tap water and table salt).

The probes used:


Tilted to better show the setup


All measurements was made with the probes vertical.

As can be seen, the probes are held apart with a piece of plastic cut from packaging material, folded and punched with 2 holes each going through two layers (to get the probes parallel)
Distance between holes/probes was made 10mm ("air" between the tubes).

To sum up...
Material: Brass tube
Dimensions...
  Diameter: 5mm outside, 4mm inside (to fit banana plugs)
  Length: 102mm
  Spacing: 10mm

For the measurements, the probe pair was submerged 61mm, and thus had a submerged area of 958.2mm^2 (9.582 square centimeters or roughly 1.5 square inches) each.

Water resistance measured
Tap water (rather hard water): 18k5 after about 3..5 minutes.

Salt water (35g/l of same hard tap water): 3k0 after about 3..5 minutes.
Shortly when inserted 700..800 Ohm, but in much less than a second, so may be the cheap DMM playing games. quickly rising and in less than a minute it was around 1k3.

In tap water it started lower as well and my first thought was that the brass probes was forming a layer of oxide, but lifting the probes out of the water and submerging them again yielded the same results repeatedly - I have no immediate explanation and don't wanna shoot off a wild guess.


An important fact to remember is, that the the area of immersed material as well as the distance between the areas of different polarity will be different in most cases, so this is merely to have a ball park figure of the difference.

The probes I used have a relatively large area compared to most of the stuff discussed and less area means higher resistance. The distance is important too - less distance means lower resistance.

Hard tap water means a lower resistance than softer water, but short of being able to measure the actual lime content, there's no way to be sure of the magnitude, so just expect somewhat higher readings if you have softer water than me.


Distilled/de-mineralized water would ideally be an insulator, but dip a finger, a tea spoon or a probe in it, or just subject it to ambient air for a short while and you have added ions, so never think of this as an isolator, as it's not and while it start out high ohmic, resistance will lessen as ions are added.
« Last Edit: February 07, 2011, 08:38:49 PM by Soeren »
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

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Re: Water Resistance: Tap Water vs. Salt Water
« Reply #1 on: February 10, 2011, 08:07:27 PM »
Thanks Soeren. This would have been a huge saver for me about ~3 years ago ::) :P

That said, tap water varies quite a lot with every city/country. Politicians all have their own ideas on purity and what additives are best to strengthen my teeth . . .

Quote
In tap water it started lower as well and my first thought was that the brass probes was forming a layer of oxide, but lifting the probes out of the water and submerging them again yielded the same results repeatedly - I have no immediate explanation and don't wanna shoot off a wild guess.
I haven't measured salt water, but with de-ionized water I get a fairly stable value on my expensive multimeter. It was fairly sensitive to probe separation distance, but it looks like with your setup the probes are fixed pretty good . . .

When a multi-meter makes measurements, it'll send out small amounts of power into your circuit. This is why voltage will change when you measure it across an unpowered capacitor . . . maybe somehow something in the water was holding a charge? (just a random uneducated guess)

 


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