how do conductivity meters measure at winter conditions ? 1

[diapensiya]

Hello all,

I have been working on a system which requires (electrical) conductivity measurement of organic metarial in an aqueous stream. The aim is to prevent organic material to end up in drain lines. This is done by shutting off the valve on the pipeline when the conductivity meter shows very low values (organic material has very low conductivity with respect to water).
The conductivity is related with temperature. My question is how do condutivity meters work when the temperatures are below 0 degrees celcius ? Do they need insulation around ? Another thing is how do they correct the conductivity measurement when the temperature is changing during the day ?

 

[EmmanuelTop]

Low-temperature operation is a question for the instrument vendor. As for the measured values, I am sure you can implement a compensation block that will recalculate the result at any temperature within the operating range.

Dejan IVANOVIC
Process Engineer, MSChE

 

[Compositepro]

The conductivity of ice is much lower than water and, of course, water freezes at 0C. So conductivity would not be a good technique to use in freezing conditions.

 

[georgeverghese]

This is confusing to me - electrical conductivity in aqueous solutions is a function of the concentration of dissolved inorganic electrolytes - not heard of organic solutes in water contributing much (if at all) to total electrical conductivity. So you could have high organics and low conductivity readout if electrolyte concentration is low ?

 

[bimr]

Conductivity meters work from 0° to 50° C. If the water is frozen, the meter will not work.

Conductivity is the ability of a solution to pass an electric current. This depends on a number of factors
including concentration, mobility of the ions, valence of the ions and temperature. As the temperature of a
solution increases, the mobility of the ions in the solution also increases and consequently this will lead
to an increase in its conductivity. Therefore it is mandatory to always associate conductivity
measurements with a reference temperature, usually 20ºC or 25ºC.

Most conductivity cells and meters have automatic temperature compensation (ATC). During ATC, the instrument applies a temperature coefficient of variation to the measured conductivity and reports what the conductivity would be at the reference temperature, as illustrated in Figure 1. This is known as linear temperature compensation and can give a good approximation to the true conductivity when the temperature of the measured solution is close to the reference temperature.

Since your purpose is not to obtain a precise conductivity meter, the temperature effect is somewhat irrelevant. You are looking for a spike in the reading to indicate organics/

 

[georgeverghese]

I was referring to an aqueous stream at a fixed temp - agreed conductivity changes with temp.
Is this the right meter to measure biological oxygen demand and / or other organics in waste effluent streams ?

 

[diapensiya]

Thank you all for the answers.
I see now how the temperature is corrected.
It is true that I am interested in spikes in the conductivity measurement to detect organics.
The issue I am thinking of is what will happen if there is low measurement or spike due to lowered temperatures ?
Then there is a risk to close the valve although no organics present.
Maybe as you mentioned this a question to vendor.

 

[EmmanuelTop]

I doubt that decrease in temperature of water or aqueous solutions may cause conductivity to decrease to a point where it is in the same order of magnitude as of that of an organic material(s). But again, this depends on what is actually the aqueous solution, the organics, the level of contamination, and the range of expected temperature variations.

If the analysis shows that you are entering a zone in which reliability of the system is affected/reduced significantly, you will probably need to look for using different principle of contamination measurement. Discuss with the supplier.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Unless we are dealing with strong organic acids or bases in this effluent stream, electrical conductivity would be by far and away dominated by inorganic electrolytes. The only way you would have organic acids and bases controlling conductivity would be if this aqueous solution were to be fully de ionised water.

 

[bimr]

Would depend on what else is mixed in with the organics wouldn't it? Don't think the OP has provided enough to jump to conclusions. If the organics were mixed with a salt, it would work.

 

[EmmanuelTop]

Refer to Bela Liptak "Instrument Engineer Handbook, Vol.1 - Process Measurement and Analysis". I doubt there is any other comprehensive guide with such level of information out there.

Section 8.17 is dedicated to conductivity meters. The information extracted at ftp://ftp.unicauca.edu.co/Facultades/FIET/DEIC/Materias/Instrumentacion%20Industrial/Instrument_Engineers__Handbook_-_Process_Measurement_and_Analysis/Instrument%20Engineers%27%20Handbook%20-%20Process%20Measurement%20and%20Analysis/1083ch8_39.pdf shows overview of oil-in-water analyzer principles.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Dejan,

These conductivity probes in this attachment ( which have gained some notoriety in the business I come from) are for detecting an oil to water interface when there are two discrete phases to work with. These are still being used in phase separation operations ( by detecting interface level ) in the hydrocarbon industry.

Typical oil - in water analysers for effluent water stream monitoring are of the UV or IR type for measuring trace dispersed oil content in a continous water phase, which is what I believe is required in this application.

Even these UV and IR sensors dont give you the total organics content; vaguely recall some of these measure only dissolved oil in water, while others measure only free oil in water.

 

[georgeverghese]

It may well be that this water passes through an upstream open API separator or holding concrete tank. These contraptions have an arrangement of overweirs and underweirs to hold back a layer of oil in one of the compartments. This conductivity probe must be looking at this compartment to monitor that the oil phase does not carry under into the heavier water phase. In principle, that is how these devices are meant to be used. Ok

 

[cloa]

He may have significant freezing point depression so below 0 it may still work- depends on the organic material and salt content of the water.

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