flow through resistivity cell 2

[j.doe]

Hi, I'm designing a cell to measure the electrical resistivity of a dielectric liquid during flow. I performed simulations and found that if i use a design with two electrode rings within an insulating pipe, it would require relatively large dimensions and thus a large volume if i want the cell constant small enough to measure a high resistivity. So, i'm trying to design a cell with two perforated plates opposite each other to do the measurement as i would need less volume of liquid (i'm testing liquid R134a which is bad for the environment so can't use a lot of it) and i was wondering if you guys have any ideas about how to do this. I would like to minimise turbulence and have less holes for liquid to pass through so i can lower my cell constant by having a larger surface area of the plate touching the liquid. I'm trying to get a cell constant value of 0.1 m^-1 and i've calculated to find that i'd need a circular plate with a diameter of 15cm assuming no holes and a 2mm distance separating the two plates. Do i use many small holes or a few large ones? How can i find out the optimum hole to turbulence/cell-constant ratio? How would i attach these plates into position within a plastic pipe? Should i make the plate thick so that there is a larger area in contact with the liquid (i.e. the inside walls inside the hole), would this area factor into the cell constant equation, if so how? Is there a way i can create like a sluice gate mechanism within the pipe so that i can like store the liquid before releasing it to make the measurements with the plates to minimise required volume and ensure 134a is pressurised to liquid state during measurement? Thanks in advance.

 

[rothers]

Interesting, do you find the measured resistivity of R134a varries with flow rate ? What is your flow rate ?

 

[j.doe]

@rothers we haven't done any tests yet, i'm still trying to design the device to do the tests. We'll test how the flow rate affects the results once it's built. The reason for this experiment is because 134a is used as an inhaler propellant and we're trying to understand how the resistivity varies when the propellant flows down the inhaler valve stem using similar flow rates.

 

[EdStainless]

Are the plates parallel or perpendicular to the flow?
It is fairly easy these days to buy photo-etched SS parts. You can get various hole sises, shapes, thickness, and a so on.
IF you want max open space you might want to use hexagonal holes.

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[IRstuff]

Please refrain from double posting; you've posted 4 posts that are almost identical.

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[j.doe]

@IRstuff Hi, sorry i haven't read the rules about posting in these forums. This post is a continuation from the previous ones, the last one was mainly concerned about how to calculate the cell constant for a ring design cell and how to construct a ring design cell. Now that i've done the simulations and understood what the cell constant is for ring design cells in relation to their dimensions, i've confirmed that the perforated plate idea suggested in the previous forum is the way to go and am therefore asking about this plate idea in particular. Since my question this time mainly concerns fluid flow i thought i'd post in the mechanical engineering forum this time.

@EdStainless Hi Ed, i've decided to use your idea from the previous forum post about parallel plates placed edgewise to the flow. I'd like to maximise non-open space, i.e. the solid space of the electrode, to increase the surface area in contact with the liquid and reduce the cell constant while at the same time minimising fluid turbulence as a result of flowing through the plates, which i think is contradictory. Would hexagonal holes still be the preferred option in this case?

 

[IRstuff]

You do realize that multiple plates can be stacked? Actual capacitors have multiple plates that are interleaved to increase the effective surface area.

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[EdStainless]

I would just one or two long slots in each plate. Just enough to assure equal pressure on each side but no need for a lot of flow area.
I have had some small parts etched and the cost was reasonable. I also had business cards made, about $2.50 each for 100pcs.

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[j.doe]

@IRstuff Thanks a lot IRstuff, i think that is a really good idea. I'll simulate using several plates and calculate the cell constant. I think i can reduce the internal diameter of the pipe and the volume now with this.

@EdStainless I guess i'll simulate different designs including ones with slots and see how it works out.

 

[IRstuff]

Did you get it between those two posts?

The cell constant is gap/area, so if there are "n" gaps, then there is n*area(of a single gap) = total area, and go from there gap/(n*area). This is inverse of the basic capacitance calculation, minus the dielectric constant part.

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[j.doe]

@IRstuff Thank you. Yeah i sort of grasped what you were saying between those two posts. I really appreciate the help.

 

[EdStainless]

There are designs for three probe conductivity cells set up in a bridge arrangement.
You should be able to find reference designs on internet.


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[j.doe]

Hey,

Do you guys have any ideas about what kinda valve setup i should use for such a system where i use like 100ml of the refrigerant liquid (under pressure 700 kPa to keep it in liquid form) and make it flow across a short length of pipe (like 5cm max) with these plates inside. How would i control the flow rate the liquid travels at and what kinda valves would i use in such a setup? The liquid can exit into the atmosphere where it vaporizes, i don't need to collect it. Thanks in advance.

 

[EdStainless]

You are going to want to use a needle valve on the outlet. This will keep the pressure in the cell at the inlet pressure (almost) and allow you to adjust flow.

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[j.doe]

Thanks a lot Ed