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'[EE]Resistivity of water.'
2011\09\08@053550 by cdb

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I've been looking at the specifications for a domestic water heater and in the sales bumph they specify the minimum water resistivity required (2K795 x inches or 1K1 x cm's). I've never seen such a specification before for this kind of water heater nor the unit resistivity per area/length.

I can find no documents on the interrent apart from those referring to measurement devices for analytical purposes.

Colin
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cdb,   8/09/2011
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spam_OUTcolinTakeThisOuTspambtech-online.co.uk

2011\09\08@055556 by Brendan Gillatt

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On 8 September 2011 10:36, cdb <.....colinKILLspamspam@spam@btech-online.co.uk> wrote:
>  I've been looking at the specifications for a domestic water heater and in
> the sales bumph they specify the minimum water resistivity required (2K795
> x inches or 1K1 x cm's). I've never seen such a specification before for
> this kind of water heater nor the unit resistivity per area/length.

I'm assuming this is to ensure shorts don't occur across the heating
coil. Most manufacturers will probably just assume this is safe.

> I can find no documents on the interrent apart from those referring to
> measurement devices for analytical purposes.

Electrical resistivity (or conductivity^-1) varies as a function of
salt concentration and temperature--two things that are not absolute
in domestic water supplies. NB: I refer to "salt" here as any ionic
salt, not just table salt (NaCl).

I found some information at
http://www.fao.org/docrep/T0667E/t0667e05.htm concerning the
conductivity of water at various concentrations of NaCl at 25 deg C.

Regards,
Brendan

2011\09\08@065117 by Spehro Pefhany

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At 05:36 AM 9/8/2011, you wrote:
>  I've been looking at the specifications for a domestic water heater and in
>the sales bumph they specify the minimum water resistivity required (2K795
>x inches or 1K1 x cm's). I've never seen such a specification before for
>this kind of water heater nor the unit resistivity per area/length.
>
>I can find no documents on the interrent apart from those referring to
>measurement devices for analytical purposes.

IME, water conductivity (the reciprocal of resistivity) is often used as a
measure of water purity (like TDS). The units are siemens/unit length (for
example uS/cm = 1/Mohm-cm).

Resistivity is rho, where R = rho * L/A in the measuring cell.
So rho = R * A/L, and you can see the units are ohms per unit length.

Impure water can corrode the system and will cause scaling to quickly build
up on the heaters, leading to poor heat transfer. They spec it so they can
void your warranty if your water quality sucks and kills their product
prematurely.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2011\09\08@121242 by Yigit Turgut

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On Thu, Sep 8, 2011 at 12:55 PM, Brendan Gillatt
<.....brendanKILLspamspam.....brendangillatt.co.uk> wrote:
> On 8 September 2011 10:36, cdb <EraseMEcolinspam_OUTspamTakeThisOuTbtech-online.co.uk> wrote:
>>  I've been looking at the specifications for a domestic water heater and in
>> the sales bumph they specify the minimum water resistivity required (2K795
>> x inches or 1K1 x cm's). I've never seen such a specification before for
>> this kind of water heater nor the unit resistivity per area/length.
>
> I'm assuming this is to ensure shorts don't occur across the heating
> coil. Most manufacturers will probably just assume this is safe.
>
>> I can find no documents on the interrent apart from those referring to
>> measurement devices for analytical purposes.
>
> Electrical resistivity (or conductivity^-1) varies as a function of
> salt concentration and temperature--two things that are not absolute
> in domestic water supplies. NB: I refer to "salt" here as any ionic
> salt, not just table salt (NaCl).

It depends on the water quality as whole. Domestic water contains
dozens of materials like ; arsenic, mercury, copper, lead and aluminum
to name just a few interesting ones.Anything solvable in H2O will
result in a boiling point elevation.

2011\09\09@123752 by mark springer

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"At 25°C the accepted values with their uncertainties for
conductivity, ê and its reciprocal, resistivity, ñ, are:
ê = 0.05501 ± 0.0001 ìS/cm at 25.00 °C
ñ = 18.18 ± 0.03 MÙ·cm at 25.00 °C"

Standardized Result:  18.18 MÙ·cm
unit resistivity per area/length.??
http://hypertextbook.com/facts/2006/SamTetruashvili.shtml




On 9/8/11, cdb <colinspamspam_OUTbtech-online.co.uk> wrote:
{Quote hidden}

>

2011\09\09@124122 by mark springer

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unit resistivity per area/length

resistance has to be ? expressed over a Volume. can you have
dimensionless resistance that is not infinite? I do not see it

On 9/9/11, mark springer <KILLspammbspringer133KILLspamspamgmail.com> wrote:
{Quote hidden}

>> -

2011\09\09@133314 by Isaac Marino Bavaresco

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In semiconductor wafers it is used the "resistance of a square", that
is: if you have a square piece of semiconductor with two electrodes in
two opposite sides, the measured resistance is a constant, irrespective
of the measure of the side of the square.

Bigger squares have more power dissipation capability.

To make resistors with different values just make the areas rectangular.

To me it seems that for water something similar could be used, such as
"resistance of a cube", using a cubic recipient with two opposite faces
being metallic electrodes.


Isaac


Em 9/9/2011 13:41, mark springer escreveu:
{Quote hidden}

>>> --

2011\09\09@192004 by Adam Field

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2011/9/9 mark springer <EraseMEmbspringer133spamgmail.com>:
> unit resistivity per area/length
>
> resistance has to be ? expressed over a Volume. can you have
> dimensionless resistance that is not infinite? I do not see it

Resistivity of metal films used for capacitor construction is measured
in ohms per square, and it is dimensionless.

See here for a good explanation:
http://www.esdjournal.com/techpapr/ohmmtr/ohm.htm

I don't know how that relates to water though, which would have to be
ohms per cube

2011\09\09@195450 by peter green

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>  I don't know how that relates to water though, which would have to be
> ohms per cube.
>   No the unit for resistivity of solids is ohm-meters.

http://en.wikipedia.org/wiki/Resistivit

2011\09\10@111844 by Ing. Marcelo Fornaso

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Salty water resistivity is no-ohmic.

It is related to ions movements, and so you'll get different values when measured with CC or CA, and also depending on the sensing current frequency and amplitude, among others (temperature, salt content, type of ions present, and a long etc.)

You can do a simple experiment. Fill a glass with tap water and put your tester probes into it. Read the "resistance". You will see it is changing. Then shake the water a little and see what happens. Then remove the tips from the water, wait some seconds and put them back in contact with water. 
You will see ever changing resistance values. That is due to salt ions traveling from one tip to the other (your "electrodes"). If you wait long enough you can see litle bubles starting to accumulate in one or both of your tester tips: You are producing water electrolysis...

To have a kinda usable resistance value use alternating current at a frequency of some kilocycles.

Hope it helps.

Marcelo Fornaso


________________________________
From: Adam Field <RemoveMEadamEraseMEspamEraseMEbadtech.org>
To: Microcontroller discussion list - Public. <RemoveMEpiclistspam_OUTspamKILLspammit.edu>
Sent: Friday, September 9, 2011 8:20 PM
Subject: Re: [EE]Resistivity of water.

2011/9/9 mark springer <RemoveMEmbspringer133TakeThisOuTspamspamgmail.com>:
> unit resistivity per area/length
>
> resistance has to be ? expressed over a Volume. can you have
> dimensionless resistance that is not infinite? I do not see it

Resistivity of metal films used for capacitor construction is measured
in ohms per square, and it is dimensionless.

See here for a good explanation:
http://www.esdjournal.com/techpapr/ohmmtr/ohm.htm

I don't know how that relates to water though, which would have to be
ohms per cube

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