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

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.

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N_Cook
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A well-insulated toroidal inductor might be an interesting conductivity probe.

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John Larkin         Highland Technology, Inc 

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John Larkin

I'd probably do a capacitive conductivity gauge--look for the salt water shorting out the E field even more than fresh water. That way you don't have to expose the wiring to the water.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

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.

Reply to
George Herold

On a sunny day (Fri, 09 Aug 2019 16:07:57 +0100) it happened N_Cook wrote in :

Some of the ground water level meters here uses simply 2 electrodes, enough conductivity to to trigger a pumping station controller. I use the sampe principle for a water detector in a boat. I guess the salter the water the better the conductivity, so you could measure resistance perhaps, but not sure that will work right with polluted water, you need level too. For water level pressure sensors are used in the sewer systems here.

Level on the sensor rods + conductivity? You could use multiple sensor rods to give you height and then also measure conductivity.

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Maybe better use a commercial product? google 'salinity sensor' first hit:

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ebay has stuff too.

Reply to
Jan Panteltje

Sounds like a capacitive dipstick would do the job. Because salt water is conductive, stray capacitance due to fringe fields will go 'way up. A 74HC14 R-C oscillator (chip, battery, capacitor-probe, resistor) makes either an output frequency directly, or (with a load resistor) signals frequency on its power supply lines.

I've used a charge-pump with capacitors and diodes to convert frequency to current, comparator and set a trip point, but a multimeter that reads frequency would be sufficient.

Reply to
whit3rd

Is "fresh water" the appropriate term here? I say that only partly in jest because the various contaminants may make the sewer water as conductive as salt water.

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  Rick C. 

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Rick C

Length of graduated hose with a hydrometer at the end?

piglet

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piglet

I did something similar once

In principle applying an ac signal to the electrodes

Measured the capacitance with a dV/dt measurement

Then used energy consumption of the sensor to detect conductivity, that is salt concentration

Reply to
Klaus Kragelund

About the properties of salt water:

Put two isolated plates in water, use them as cap in an oscilator, and examine the frequency. (1.5 minute result from google).

Reply to
Sjouke Burry

"Remote Detection of Saline Intrusion in a Coastal Aquifer Using Borehole Measurements of Self-Potential" Note that this report was done in UK.

They used electrokinetic potential (two dissimilar electrodes to form a battery). See Table 1 for some differences in electrical characteristics of sea water and fresh water which you should be able to detect. For example:

Rock electrical conductivity seawater 5.2-5.75 (5.2measured) mS/cm Rock electrical conductivity groundwater 0.06-0.25 (0.25measured)mS/cm

I don't know what you're going to do about the sewage in the measurements. It could be anything from conductive (ionic) chemicals to corrosives that will eat or coat your electrodes. That's why this kind of testing is normally by collecting samples, and doing the analysis in a laboratory.

In food analysis, salt content is measured by titration. It can probably be adapted to testing concentration in your sewage plus saltwater mix.

If you're just measuring dissolved salt concentration, and ISE (ion selective electrode) probe and salt meter should be able to produce usable numbers. It's commonly used for salt water aquariums:

If you're cheap, use salinity test paper:

Again, all these tests assume that the salt will be diluted, not contaminated by sewage. Good luck.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

There are electrodes that are sodium-ion concentration specific, and food a nalysts are going to use them - rather than titration - for all but pickies t customers.

How long they might last if exposed to raw sewage is an open question

The contamination question is the one that suggests to me that an inductive conductivity measurement might be the way to go.

John Larkin mentioned usinga single toroid to induce a current in the surro unding (dirty) water. Two stacked non-progressively wound toriods do the jo b a whole lot more easily - you could pick out the current induced in the c onductive liquid threadig a single toroid, but it wouldn't be easy.

Toroids would stop working if lumps of - non-conducting - sludge block up t he hole in the toroid (or the stacked toroids) but nylon netting would prev ent that.

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Bill Sloman, Sydney
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Bill Sloman

My patent from many years ago:

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Reply to
klaus.kragelund

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

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Reply to
N_Cook

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.

NT

Reply to
tabbypurr

Both sodium and chlorine ion sensors can be contaminated by a wide variety of substances found in sewage. That's why the collection is done with a sampler and analysis is done in a lab.

I did some work for Manning Environmental when they were located in Santa Cruz CA: Helping design a refrigerator that fit in a manhole was not exactly the high point of my design career. The reason for the refrigeration was that many of the pollutants found in sewage are volatile. The only way to keep them in solution was to arrest evaporation by cooling the samples. The EPA wanted everything cooled to 0C - 4C. Preventing the collected samples from freezing was a major challenge. One ancient product, not found on the current web site, was a rotary sampler that resembled a miniature bottling plant, where sewage was sampled on a timed schedule, bottled, capped, tagged, and refrigerated for later analysis.

One thing nice about test strips is that they're not clogged by sewage. Just dip them in the flow, and take a digital photo to determine the color. However, I'm not sure what they'll do in the presence of the chemical cocktail found in industrial sewage.

Oops. That should read: Again, all these tests assume that the salt will be diluted by clean and neutral pH water and not contaminated by sewage.

I'm not sure how that would look, but in my very limited experience in sampling sewage, it will stick to anything that is not a smooth surface. Direct pumping is also problematic. Ask anyone who has a basement toilet pump: However, pumping raw sewage through one or two torroid orifices would be easy if the torroids were wrapped around a smooth teflon sampling pipe that does not present an obstruction. My guess(tm) is that pipe would need to be the same size as a typical toilet drain pipe (3in or

4in), which would require two rather huge torroidal cores.

Perforated silicone tubing is what's typically used for collector and vacuum collector hoses. Not much sticks to it or attacks it. For the nasty stuff, there's Teflon. If long life is not necessary, there's PVC. If there's a strainer screen, it's either disposable PVC, or stainless steel: "Manning Flex Strainer" For the various pump types, there is usually a "purge" cycle, which reverses the action, pressurizes the sampler strainer, and hopefully dislodges any obstructions.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

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

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John Larkin         Highland Technology, Inc 

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John Larkin

The toriodal core pair conductivity probe doesn't work like that. The current flows through the hole in both toriods, but also flows back around the outsides of both toroids - it's current loop thaqt threads both toroids.

The wholen thing has to be immersed in the liquid whose conductivity is to be measured.

The OP was looking for 4"x1" package to go on the end of a stick.

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Bill Sloman, Sydney
Reply to
Bill Sloman

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.

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.

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Bill Sloman, Sydney
Reply to
Bill Sloman

What about titanium instead?

Clifford Heath

Reply to
Clifford Heath

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