Analyzing the working principle of a conductivity sensor and obtained measurements

Thread Starter

Amr95

Joined Nov 13, 2020
1
Background to the conductivity sensor and the working principle: The conductivity sensor have two electrodes, through which the fluid passes through and acts as the electrical conductor between the sensor electrodes. The conductivity is measured by applying an alternating electrical current to the sensor electrodes and measuring the resulting voltage.

I am having a conductivity sensor to measure the concentration of fluids passing through it, the fluids passing through it are osmosis water and bicarbonate concentrate, the water is being pumped by a continuous flow pump while the bicarbonate concentrate is being pumped by a rotary vane piston pump, which produces a non-continuous flow. The two fluids pumped through two different tubes are then connected at the end by a T-connector, and then flow through the conductivity sensor. As the flow of the bicarbonate is not continuous, I'm expecting water only to be flowing through the sensor between each two full revolutions of the rotary vane pump, where the conductivity of osmosis water is almost 0 mS/cm while the conductivity of the bicarbonate concentrate is around 2,5. Following my assumption, I am expecting to get a sinusoidal signal with value alternating between 0 and 2,5 but in reality I am getting values between around 1,5 or 2 and 2,5 (mS/cm).

My question is what could be the reason that my sensor is not measuring the 0 values of the osmosis water and instead measures a lower value which doesn't represent the conductivity of water? is my sensor slow? are conductivity sensors in general not suitable for such measurements with such frequent change in its values? I would be thankful if someone could give me tipps how to interpret this behavior.
 

RKropp

Joined Jan 5, 2017
6
I think you should question your hydraulic assumptions instead of the sensor behavior. You never mentioned flow rates, but unless flow rates are very small and in very small diameter tubing there will be turbulence. The turbulent flow will cause mixing in the tee and cause the brine to mix into the RO flow even when the brine isn't flowing, which appears to be what you are seeing with the probe. Pulsating flow and direction changes like tees increase turbulence, and the more turbulent the flow the greater the amount of mixing. The only way to have no mixing is to incorporate a positive shut off valve to isolate the brine feed immediately adjacent to the mixing point or to have very small flow rates and pipes to ensure the flow is laminar (thermodynamically reversible flow, which implies no mixing). Even with laminar flow there would be diffusion of the bicarbonate ions that your sensor would pick up, but you would get closer to zero.

Fully modeling your system would require some CFD (computational fluid dynamics) work, but as an introduction look up something called Reynolds number which relates density, flow velocity, pipe diameter, and viscosity. Larger Reynolds numbers indicate increased turbulence. Water has a high density and low velocity, both of which result in high Reynolds numbers, so achieving laminar flow is pretty challenging. If you have ever seen a water feature with a fountain that looks perfectly transparent and "frozen in time" (Ballagio casino in Las Vegas, Nevada, USA is an excellent example to search for videos) you are seeing laminar flow. It takes surprisingly sophisticated engineering to make that happen.
 

RKropp

Joined Jan 5, 2017
6
I think you should question your hydraulic assumptions instead of the sensor behavior. You never mentioned flow rates, but unless flow rates are very small and in very small diameter tubing there will be turbulence. The turbulent flow will cause mixing in the tee and cause the brine to mix into the RO flow even when the brine isn't flowing, which appears to be what you are seeing with the probe. Pulsating flow and direction changes like tees increase turbulence, and the more turbulent the flow the greater the amount of mixing. The only way to have no mixing is to incorporate a positive shut off valve to isolate the brine feed immediately adjacent to the mixing point or to have very small flow rates and pipes to ensure the flow is laminar (thermodynamically reversible flow, which implies no mixing). Even with laminar flow there would be diffusion of the bicarbonate ions that your sensor would pick up, but you would get closer to zero.

Fully modeling your system would require some CFD (computational fluid dynamics) work, but as an introduction look up something called Reynolds number which relates density, flow velocity, pipe diameter, and viscosity. Larger Reynolds numbers indicate increased turbulence. Water has a high density and low velocity, both of which result in high Reynolds numbers, so achieving laminar flow is pretty challenging. If you have ever seen a water feature with a fountain that looks perfectly transparent and "frozen in time" (Ballagio casino in Las Vegas, Nevada, USA is an excellent example to search for videos) you are seeing laminar flow. It takes surprisingly sophisticated engineering to make that happen.
I noticed I said "water has a high density and low velocity". Substitute "viscosity" for "velocity". I don't use the forums much and I think the window for editing the other post has lapsed.
 

RobertPink

Joined Sep 25, 2020
7
Conductivity is notoriously difficult to get right. A large part of the problem is often the inter-reaction between the electrode material and the fluid being measured. It is not so much electrolytic effects, as you might imagine, but electrochemical effects at the surface of the electrode. The main problem is something called double layer capacitance. This is complicated but is basically a very thin ionic concentration caused by electrochemical interactions. It has a similar effect as a capacitor in circuit.

This may not be your problem but try different electrode materials. Copper is poor (too active), Platinum tends to be good but expensive. Lead is often used as the oxide formed at the surface is electrically conductive, stable and is not a good source of ions. Nickel is another candidate.
Conductivity can be the very devil to sort out and is one of the reasons non-contact conductivity probes are popular.

Take a look here;
https://en.wikipedia.org/wiki/Double-layer_capacitance
https://en.wikipedia.org/wiki/Electrical_conductivity_meter
 
Last edited:

Phil-S

Joined Dec 4, 2015
188
@RobertPink Non-contact conductivity probes?
Measuring the conductivity of a liquid is not difficult using the right equipment.
Certainly use AC for long-term use and most laboratory grade instruments will.
The probe itself will be fairly complex and often has two, three or even four annular electrodes and graphite is often used together with stainless steel in process instruments.
Results are reported to standard temperatures usually 20 or 25 degrees Celsius, so temperature compensation is needed or has to be part of the final calculation.
The problem seems to be in the flow arrangement.
The user is essentially describing a continuous flow analyser, used in industry and hospital laboratories, Technicon was one manufacturer.
A feature of continuous flow analysers was the introduction of bubbles to separate slugs of liquid, but the main feature was the mixing coil, usually glass and small bore, about 2-mm. After about 10 turns, the sample was de-bubbled and presented to the measuring device.
Another method uses discrete measurement where essentially all the reagents are mixed, either with a mixer or air bubbles
My question is what could be the reason that my sensor is not measuring the 0 values of the osmosis water and instead measures a lower value which doesn't represent the conductivity of water? is my sensor slow? are conductivity sensors in general not suitable for such measurements with such frequent change in its values? I would be thankful if someone could give me tipps how to interpret this behavior.
In answer to the questions
By osmosis water I guess you mean water that has been purified by reverse osmosis.
I would rather use de-ionised or distilled water that will have a conductivity close to zero, but in practice, with dissolved gases and other trace contaminants you will never get to zero.
If your value is lower than zero then your method is wrong.
Conductivity probes are not slow, but a period of mixing is always recommended.
The frequency of changes of conductivity don't matter providing you clean the probe between samples.
It would help if you said what you are trying to achieve by mixing water and bicarbonate solutions
 
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