Simple salt-water detector, go/no-go type discrimination or

[Bill Sloman]

On Saturday, August 10, 2019 at 11:29:19 PM UTC+10, John Larkin wrote:

On Fri, 9 Aug 2019 20:35:52 -0700 (PDT), "John Miles, KE5FX"
jmiles@gmail.com> wrote:

On Friday, August 9, 2019 at 6:22:52 PM UTC-7, Bill Sloman wrote:
You drive one toroid, and monitor the current out of the other toroid. In air - if you've got the windings non-progressive - there nothing coming from the second toroid.


Why non-progressive? Ordinary toroids should behave that way too, no?

With a progressive winding, you have one turn in the plane of the toroid, and it couples the other other progressively wound toroid.

The two toroids interact even without the conductive liquid.

I was thinking that conductive water would present a resistive shunt
load to a single toroid. The circuit could be simple.

You weren't thinking very hard. Two toroids is the classic solution to this particular problem, and it still works with very low conductivity fluids.

Exciting an inductor is never all that simple, even at the best of times.

Separating the current flowing in the resistive shunt from the current that might be induced in the core might not be all that simple either.

--
Bill Sloman, Sydney

 

[Clifford Heath]

On 10/8/19 8:29 pm, tabbypurr@gmail.com wrote:

On Saturday, 10 August 2019 09:07:17 UTC+1, N_Cook wrote:
On 09/08/2019 18:09, George Herold wrote:
On Friday, August 9, 2019 at 11:08:02 AM UTC-4, N_Cook wrote:

Any ideas. It will not be permanently immersed in salt-water, just the
odd sampling. Ancient unmaintained sewer system has breaches to tidal
sea-water at high tides from cracked and dislodged pipe-runs. Need a way
to monitor , firstly that it is partly at least seawater and then some
idea of concentration , to gauge how the situation worsens. At the
moment just the noise of the 1m fall at a manhole from the high-level
local system to the low-level district sewer , coincident with
high-tides, indicates it is mainly sea-water at thoe times. And
photographically to gauge the flow. Just needs a probe of some sort to
pass thru an air-balancing vent in a manhole, perhaps 1 inch x 4inches
and 6 to 10 feet below the surface.

--
Monthly public talks on science topics, Hampshire , England
http://diverse.4mg.com/scicaf.htm

Simplest might be some stainless steel probes and AC conductivity..
at ~1kHz or something. Maybe a SS tube with SS rod down the center,
or some other way to fix the geometry.

George h.


Thanks all, I'll go with this simplest one first, I've got some s/s tube
and rod.

Don't be tempted to put one inside the other, makes keeping them usably clean a problem. A flat surface with both conductors on is best.
SS is generally coated with nonconductive oxide, not a good choice for conductivity measurement.

What about titanium instead?

Clifford Heath

 

[Bill Sloman]

On Saturday, August 31, 2019 at 9:42:46 AM UTC+10, Clifford Heath wrote:

On 10/8/19 8:29 pm, tabbypurr@gmail.com wrote:
On Saturday, 10 August 2019 09:07:17 UTC+1, N_Cook wrote:
On 09/08/2019 18:09, George Herold wrote:
On Friday, August 9, 2019 at 11:08:02 AM UTC-4, N_Cook wrote:

Any ideas. It will not be permanently immersed in salt-water, just the
odd sampling. Ancient unmaintained sewer system has breaches to tidal
sea-water at high tides from cracked and dislodged pipe-runs. Need a way
to monitor , firstly that it is partly at least seawater and then some
idea of concentration , to gauge how the situation worsens. At the
moment just the noise of the 1m fall at a manhole from the high-level
local system to the low-level district sewer , coincident with
high-tides, indicates it is mainly sea-water at thoe times. And
photographically to gauge the flow. Just needs a probe of some sort to
pass thru an air-balancing vent in a manhole, perhaps 1 inch x 4inches
and 6 to 10 feet below the surface.

--
Monthly public talks on science topics, Hampshire , England
http://diverse.4mg.com/scicaf.htm

Simplest might be some stainless steel probes and AC conductivity..
at ~1kHz or something. Maybe a SS tube with SS rod down the center,
or some other way to fix the geometry.

George h.


Thanks all, I'll go with this simplest one first, I've got some s/s tube
and rod.

Don't be tempted to put one inside the other, makes keeping them usably clean a problem. A flat surface with both conductors on is best.
SS is generally coated with nonconductive oxide, not a good choice for conductivity measurement.

What about titanium instead?

Same problem.

Stainless steel is protected by a coherent layer of chromium oxide, but it isn't non-conducting. I had to try to get a conductivity meter working with stainless steel electrodes, and it more or less worked, but the conductivity I measured was different if the solution was made conductive with NaOH or Na2CO3.

--
Bill Sloman, Sydney

 

[Jeff Layman]

On 31/08/19 03:55, Bill Sloman wrote:

On Saturday, August 31, 2019 at 9:42:46 AM UTC+10, Clifford Heath wrote:
On 10/8/19 8:29 pm, tabbypurr@gmail.com wrote:
On Saturday, 10 August 2019 09:07:17 UTC+1, N_Cook wrote:
On 09/08/2019 18:09, George Herold wrote:
On Friday, August 9, 2019 at 11:08:02 AM UTC-4, N_Cook wrote:

Any ideas. It will not be permanently immersed in salt-water, just the
odd sampling. Ancient unmaintained sewer system has breaches to tidal
sea-water at high tides from cracked and dislodged pipe-runs. Need a way
to monitor , firstly that it is partly at least seawater and then some
idea of concentration , to gauge how the situation worsens. At the
moment just the noise of the 1m fall at a manhole from the high-level
local system to the low-level district sewer , coincident with
high-tides, indicates it is mainly sea-water at thoe times. And
photographically to gauge the flow. Just needs a probe of some sort to
pass thru an air-balancing vent in a manhole, perhaps 1 inch x 4inches
and 6 to 10 feet below the surface.

--
Monthly public talks on science topics, Hampshire , England
http://diverse.4mg.com/scicaf.htm

Simplest might be some stainless steel probes and AC conductivity..
at ~1kHz or something. Maybe a SS tube with SS rod down the center,
or some other way to fix the geometry.

George h.


Thanks all, I'll go with this simplest one first, I've got some s/s tube
and rod.

Don't be tempted to put one inside the other, makes keeping them usably clean a problem. A flat surface with both conductors on is best.
SS is generally coated with nonconductive oxide, not a good choice for conductivity measurement.

What about titanium instead?

Same problem.

Stainless steel is protected by a coherent layer of chromium oxide, but it isn't non-conducting. I had to try to get a conductivity meter working with stainless steel electrodes, and it more or less worked, but the conductivity I measured was different if the solution was made conductive with NaOH or Na2CO3.

Any idea why? NaOH shouldn't have any effect on a stainless steel oxide
coating at room temperature. Or is it simply that there are more sodium
ions in solution with NaOH than with Na2CO3, and that accounts for the
increase in conductivity (at least, I assume that's what you found. Or
was the NaOH solution /less/ conductive?).

i wondered if there was any possibility of using carbon rods from the
centre of old zinc chloride batteries as the electrodes? They will be
inert enough, but would they be physically strong enough for the
intended purpose?

--

Jeff

 

[Bill Sloman]

On Saturday, August 31, 2019 at 8:44:29 PM UTC+10, Jeff Layman wrote:

On 31/08/19 03:55, Bill Sloman wrote:
On Saturday, August 31, 2019 at 9:42:46 AM UTC+10, Clifford Heath wrote:
On 10/8/19 8:29 pm, tabbypurr@gmail.com wrote:
On Saturday, 10 August 2019 09:07:17 UTC+1, N_Cook wrote:
On 09/08/2019 18:09, George Herold wrote:
On Friday, August 9, 2019 at 11:08:02 AM UTC-4, N_Cook wrote:

Any ideas. It will not be permanently immersed in salt-water, just the
odd sampling. Ancient unmaintained sewer system has breaches to tidal
sea-water at high tides from cracked and dislodged pipe-runs. Need a
way to monitor , firstly that it is partly at least seawater and then
some idea of concentration , to gauge how the situation worsens. At the
moment just the noise of the 1m fall at a manhole from the high-level
local system to the low-level district sewer , coincident with
high-tides, indicates it is mainly sea-water at those times. And
photographically to gauge the flow. Just needs a probe of some sort to
pass thru an air-balancing vent in a manhole, perhaps 1 inch x 4inches
and 6 to 10 feet below the surface.

Simplest might be some stainless steel probes and AC conductivity..
at ~1kHz or something. Maybe a SS tube with SS rod down the center,
or some other way to fix the geometry.

Thanks all, I'll go with this simplest one first, I've got some s/s tube
and rod.

Don't be tempted to put one inside the other, makes keeping them usably clean a problem. A flat surface with both conductors on is best.

SS is generally coated with nonconductive oxide, not a good choice for conductivity measurement.

What about titanium instead?

Same problem.

Stainless steel is protected by a coherent layer of chromium oxide, but it isn't non-conducting. I had to try to get a conductivity meter working with stainless steel electrodes, and it more or less worked, but the conductivity I measured was different if the solution was made conductive with NaOH or Na2CO3.

Any idea why? NaOH shouldn't have any effect on a stainless steel oxide
coating at room temperature.

The chromium oxide layer isn't very thick, and hydroxyl ions getting into it presumably acted as charge carriers.

Or is it simply that there are more sodium
ions in solution with NaOH than with Na2CO3, and that accounts for the
increase in conductivity (at least, I assume that's what you found. Or
was the NaOH solution /less/ conductive?).

The hydroxly ion (-OH)is more mobile in water than the the carbonate ion so less NaOH than Na2CO3 was required to get the conductivity I was setting up.

The concentrations were chosen to give me series of solutions of predictable conductivity, and I'd checked them out with a regular conductivity meter with standard platinum electrodes.

For high conductivity solutions the stainless steel electrodes gave me less conductivity than I should have been seeing, due to the resistance of the passivating layer of chromium oxide on the electrodes, and the fact the the error was lower, but still obvious with NaOH solutions rather than with Na2CO3 solutions made what was going on even more obvious.

I wondered if there was any possibility of using carbon rods from the
centre of old zinc chloride batteries as the electrodes? They will be
inert enough, but would they be physically strong enough for the
intended purpose?

Platinum is a lot more mechanically stable, and a lot easier to make connections to.

My eventual preference would have been to use electrodes plated onto a printed circuit board - a couple of patches of a thin layer of platinum (several microns thick) wouldn't have been particularly expensive - but I came late into the project and a lot of time and money had been spent on a set-up that the mechanical engineers had liked.

--
Bill Sloman, Sydney

 

[Winfield Hill]

Bill Sloman wrote...

Stainless steel is protected by a coherent layer of
chromium oxide, but it isn't non-conducting. ...

If the conductivity measurement is with an AC signal,
shouldn't a thin oxide layer not matter, even if it
was an insulator? What about the double layer, would
using a 10kHz measurement signal take care of that?
Platinized platinum is still preferred in such sensors
for high accuracy, but what about for modest-accuracy?


--
Thanks,
- Win

 

[Bill Sloman]

On Saturday, August 31, 2019 at 11:22:58 PM UTC+10, Winfield Hill wrote:

Bill Sloman wrote...

Stainless steel is protected by a coherent layer of
chromium oxide, but it isn't non-conducting. ...

If the conductivity measurement is with an AC signal,
shouldn't a thin oxide layer not matter, even if it
was an insulator? What about the double layer, would
using a 10kHz measurement signal take care of that?
Platinized platinum is still preferred in such sensors
for high accuracy, but what about for modest-accuracy?

With relatively conductive aqueous solutions the resistance of the oxide layer did matter, and it was lower with NaOH solutions than with Na2CO3 solutions made up to be of the same conductivity.

Our standard conductivity meter ran at 10kHz. I'd worked out a scheme which gave out a frequency that increased with increasing conductivity, from about 2kHz in tap water (300uS/m) to about 150kHz in 2% NaOH at 85C (about 300mS/m). There was a perfectly predictable built-in non-linearity to keep the maximum frequency low enough for the circuit to work.

--
Bill Sloman, Sydney

 

[N_Cook]

On 31/08/2019 11:44, Jeff Layman wrote:

i wondered if there was any possibility of using carbon rods from the
centre of old zinc chloride batteries as the electrodes? They will be
inert enough, but would they be physically strong enough for the
intended purpose?

I have , bought at a car boot sale out of interest, a couple of packs
of carbon-arc lamp graphite rods from a hundred years back maybe,
perhaps I'll give them a go.


--
Monthly public talks on science topics, Hampshire , England
<http://diverse.4mg.com/scicaf.htm>

 

[Winfield Hill]

Bill Sloman wrote...

On August 31, 2019, Winfield Hill wrote:
Bill Sloman wrote...

Stainless steel is protected by a coherent layer of
chromium oxide, but it isn't non-conducting. ...

If the conductivity measurement is with an AC signal,
shouldn't a thin oxide layer not matter, even if it
was an insulator? What about the double layer, would
using a 10kHz measurement signal take care of that?
Platinized platinum is still preferred in such sensors
for high accuracy, but what about for modest-accuracy?

With relatively conductive aqueous solutions the resistance
of the oxide layer did matter, and it was lower with NaOH ...

But my question was about accuracy. If the goal is simply
to determine if there's a significant salt-water intrusion,
wouldn't a lower-accuracy sensor scheme be OK?


--
Thanks,
- Win

 

[George Herold]

On Saturday, August 31, 2019 at 9:22:58 AM UTC-4, Winfield Hill wrote:

Bill Sloman wrote...

Stainless steel is protected by a coherent layer of
chromium oxide, but it isn't non-conducting. ...

If the conductivity measurement is with an AC signal,
shouldn't a thin oxide layer not matter, even if it
was an insulator? What about the double layer, would
using a 10kHz measurement signal take care of that?
Platinized platinum is still preferred in such sensors
for high accuracy, but what about for modest-accuracy?

I stuck two SS optical posts (1/2" diameter) into water
and measured AC conductivity just fine... ~a few kHz IIRC,
I sourced voltage and measured the current. The optical
posts were nice, tapped holes top and bottom, one to hold in
place and the other for electrical connection.

George H.

--
Thanks,
- Win

 

[whit3rd]

On Saturday, August 31, 2019 at 7:05:26 AM UTC-7, N_Cook wrote:

On 31/08/2019 11:44, Jeff Layman wrote:
i wondered if there was any possibility of using carbon rods from the
centre of old zinc chloride batteries as the electrodes?

I have , bought at a car boot sale out of interest, a couple of packs
of carbon-arc lamp graphite rods.
...

If the intent is to probe a sump occasionally, zinc would do fine
(yeah, it'll rot away eventually, but a quarter pound of slug-stop
zinc strip will last a lifetime). The carbon would be easy to wash off
after use, but so would a lot of other options. And, the carbon
is useful if you want to arc-melt something.

I'm partial to the PVC pipework 'fork'; stick the fork into the pond and freshwater
makes a large gap - tine separation size- between electrodes,
while saltwater, conductive, makes a two-thicknesses-of-tubing
effective gap. A CMOS '555 oscillates it, and a multimeter reads out the frequency.

The important thing, is that the thing has to be at/near clean, and of
constant geometry, so that the liquid composition is the sensed variable.

 

[Bill Sloman]

On Sunday, September 1, 2019 at 12:50:27 AM UTC+10, Winfield Hill wrote:

Bill Sloman wrote...

On August 31, 2019, Winfield Hill wrote:
Bill Sloman wrote...

Stainless steel is protected by a coherent layer of
chromium oxide, but it isn't non-conducting. ...

If the conductivity measurement is with an AC signal,
shouldn't a thin oxide layer not matter, even if it
was an insulator? What about the double layer, would
using a 10kHz measurement signal take care of that?
Platinized platinum is still preferred in such sensors
for high accuracy, but what about for modest-accuracy?

With relatively conductive aqueous solutions the resistance
of the oxide layer did matter, and it was lower with NaOH ...

But my question was about accuracy. If the goal is simply
to determine if there's a significant salt-water intrusion,
wouldn't a lower-accuracy sensor scheme be OK?

Depends on exactly what conductivity the OP was looking for.

Sea-water and sewage are a lot more conductive than tap water (which is in turn about two orders of magnitude more conductive than distilled water).

The problem that I see is that the conductivity of the oxide layer depends on the nature of the contaminants in the water that it is exposed to, which does make interpreting the observations more complicated.

--
Bill Sloman, Sydney

 

[Bill Sloman]

On Sunday, September 1, 2019 at 5:41:15 AM UTC+10, whit3rd wrote:

On Saturday, August 31, 2019 at 7:05:26 AM UTC-7, N_Cook wrote:
On 31/08/2019 11:44, Jeff Layman wrote:
i wondered if there was any possibility of using carbon rods from the
centre of old zinc chloride batteries as the electrodes?

I have , bought at a car boot sale out of interest, a couple of packs
of carbon-arc lamp graphite rods.
..

If the intent is to probe a sump occasionally, zinc would do fine
(yeah, it'll rot away eventually, but a quarter pound of slug-stop
zinc strip will last a lifetime). The carbon would be easy to wash off
after use, but so would a lot of other options. And, the carbon
is useful if you want to arc-melt something.

I'm partial to the PVC pipework 'fork'; stick the fork into the pond and freshwater
makes a large gap - tine separation size- between electrodes,
while saltwater, conductive, makes a two-thicknesses-of-tubing
effective gap. A CMOS '555 oscillates it, and a multimeter reads out the frequency.

The important thing, is that the thing has to be at/near clean, and of
constant geometry, so that the liquid composition is the sensed variable.

The great virtue of the stacked-toroid conductivity sensor is that it measures the current induced in the solution that threads the stacked toroids without introducing any electrode surfaces to make life more complicated.

Getting two toroids wound with a non-progressive winding on each of them is going to be a performance, but once you've done that you've avoided a whole lot of other potential problems.

--
Bill Sloman, Sydney

 

[Martin Brown]

On 31/08/2019 00:42, Clifford Heath wrote:

On 10/8/19 8:29 pm, tabbypurr@gmail.com wrote:
On Saturday, 10 August 2019 09:07:17 UTC+1, N_Cook  wrote:
On 09/08/2019 18:09, George Herold wrote:
On Friday, August 9, 2019 at 11:08:02 AM UTC-4, N_Cook wrote:

Any ideas. It will not be permanently immersed in salt-water, just the
odd sampling. Ancient unmaintained sewer system has breaches to tidal
sea-water at high tides from cracked and dislodged pipe-runs. Need
a way
to monitor , firstly that it is partly at least seawater and then some
idea of concentration , to gauge how the situation worsens. At the
moment just the noise of the 1m fall at a manhole from the high-level
local system to the low-level district sewer , coincident with
high-tides, indicates it is mainly sea-water at thoe times. And
photographically to gauge the flow. Just needs a probe of some sort to
pass thru an air-balancing vent in a manhole, perhaps 1 inch x 4inches
and 6 to 10 feet below the surface.

Simplest might be some stainless steel probes and AC conductivity..
at ~1kHz or something.  Maybe a SS tube with SS rod down the center,
or some other way to fix the geometry.

Thanks all, I'll go with this simplest one first, I've got some s/s tube
and rod.

Don't be tempted to put one inside the other, makes keeping them
usably clean a problem. A flat surface with both conductors on is best.
SS is generally coated with nonconductive oxide, not a good choice for
conductivity measurement.

What about titanium instead?

Same only more so. Titanium like aluminium is a very reactive metal with
an incredibly thin self healing oxide barrier coat on the surface.

Silver foil might be OK in a wet chloride environment as a cheaper
alternative to platinum. Failed bursting disks from pharma plants is a
cheap source of such material if you know someone in the industry.

--
Regards,
Martin Brown

 

On Tuesday, 3 September 2019 09:01:19 UTC+1, Martin Brown wrote:

Same only more so. Titanium like aluminium is a very reactive metal with
an incredibly thin self healing oxide barrier coat on the surface.

Silver foil might be OK in a wet chloride environment as a cheaper
alternative to platinum. Failed bursting disks from pharma plants is a
cheap source of such material if you know someone in the industry.

Silver would certainly be close to ideal, but why not copper?
It will slowly corrode, but might still last a few decades and it
doesn't have the problem of an insulating oxide layer. It is also
readily available and fairly cheap.

John

 

[Martin Brown]

On 03/09/2019 09:27, jrwalliker@gmail.com wrote:

On Tuesday, 3 September 2019 09:01:19 UTC+1, Martin Brown wrote:

Same only more so. Titanium like aluminium is a very reactive metal with
an incredibly thin self healing oxide barrier coat on the surface.

Silver foil might be OK in a wet chloride environment as a cheaper
alternative to platinum. Failed bursting disks from pharma plants is a
cheap source of such material if you know someone in the industry.

Silver would certainly be close to ideal, but why not copper?
It will slowly corrode, but might still last a few decades and it
doesn't have the problem of an insulating oxide layer. It is also
readily available and fairly cheap.

Copper corrodes in moving water with both oxygen and chloride ions
readily available. Copper chloride is just about soluble enough to be a
problem (hence ferric chloride PCB etch baths).

In the open air on roofs it quickly develops a fairly thick greenish
blue carbonate surface coating that prevents any further corrosion.

You might get away with it and as you say copper pipe is cheap!

Silver chloride is not really soluble in brine at all and so survives
and makes quite a good biologically inert conductive sensor.

--
Regards,
Martin Brown

 

On Tuesday, 3 September 2019 10:18:25 UTC+1, Martin Brown wrote:

Silver chloride is not really soluble in brine at all and so survives
and makes quite a good biologically inert conductive sensor.

Silver/silver chloride electrodes are widely used for physiological
measurement as they are reversible and give low noise.

John