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Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/12/2015 4:18 PM

We manufacture a conductivity sensor which has a cylindrical rod that is sandblasted to increase the surface area, subsequently increasing it's capacitance. The sandblasting operation currently involves a visual inspection "So the part starts out looking this way, and when it's done, it should look that way"

I'd like to implement a more quantifiable method of measuring the sandblasting results. I'm leaning toward trying a sleeve slipped over the sandblasted rod (0.100 dia x 0.718 long) and measuring capacitance in air. The test must be quick, easy, and I have little budget for expensive equipment, which steers me away from measuring the actual surface profile.

I'm open to ideas, web searches yield results that suggest measuring surface profile, visually, with depth micrometers or stylus. These methods work well, but are too time intensive and the equipment costs too much.

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#1

Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/12/2015 5:34 PM

Since the variable you are trying to test for is capacitance, why not use something like a wheatstone bridge?

https://en.wikipedia.org/wiki/Wheatstone_bridge

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#5
In reply to #1

Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 12:47 AM

Great idea! If this was 1895.

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#2

Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/12/2015 5:46 PM

Are you sandblasting by hand?

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#3

Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/12/2015 7:18 PM

Are the probes for measuring conductivity (as per the post title) or capacitance (as per the post content)?

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#4

Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/12/2015 9:50 PM

I'm confused. If the surface that is being sandblasted is one plate of the 'capacitor", what is the other plate used to determine the capacitance? Would your sleeve be covered in foil? Dipping the probe in a very mildly conductive liquid and measuring the resistance to a much, much larger plate (perhaps the metal shell of the 'bucket', might be a good way to quantify the surface area.

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#6
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Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 12:58 AM

This is probably the only way to do it. Except that a low conductivity solution would make "good" and "bad" electrodes look much the same. A high conductivity solution, say 10,000 uS, would give a lower resistance reading to "good" cells compared to "bad" cells. you would probably use about 1000 Hz.

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#11
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Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 1:06 PM

I was thinking mildly conductive to raise the overall resistance high enough so a 4-wire ohmmeter wouldn't be required. The drop in resistance will still be roughly linear to the surface area of the probe.

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#13
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Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 1:55 PM

But that's the idea. The non-linearity of the cell can be used to determine the condition of the surface. A simple two-wire circuit will work for that.

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#7

Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 1:01 AM

Who do you work for? I was in the water instrumentation business for about forever.

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#8

Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 9:44 AM

This is a "by hand" sandblasting operation, hence my desire to quantify the operations effect. The probes measure conductivity in fluid, so the conductivity solution sounds like the best answer, but contamination over time after checking multiple probes may be problematic. The rod being sandblasted is one "plate" of the capacitor.

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#9
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Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 10:17 AM

You might want to clean the probes before checking, in a strong solvent or a high alkaline cleaner.

However, if you extend the sand blasting period a little you might reach the point where you can't get it wrong. After that there might be no reason to test. Just a thought.

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#15
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Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/22/2015 1:51 PM

An ultrasonic cleaning bath would be safer and will give good results in cleaning surface debris.

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#10
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Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 11:39 AM

Testing AFTER the critical operation is performed is too late.

Find a way to control the sand blasting process to assure acceptable results.

"By hand" leaves too many variables uncontrolled.

A turntable, stepped/graduated movement and a timer comes to mind.

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#14
In reply to #8

Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 7:23 PM

You still appear to be confused about the difference between "capacitance" and "resistance".

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#16
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Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/22/2015 2:10 PM

If I remember right, we tumbled our conductivity parts. Might be more controllable than sand blasting by hand. The idea being to eliminate the need for testing.

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#12

Re: Measurable Parameter(s) for Sandblasting of Conductivity Probes

11/13/2015 1:41 PM

If I understood the right way your part is one of the electrodes in the final assembly.

- what Lyn suggested is correct since "quality must be in the process and not checked after". For this you will need a device to check the results after parameters variations. It would be good to use same order of magnitude for the capacity of the test fixture as in final use and if possible with same electronic module as it will be used.

- if you build up a capacitance for the check then you MUST guaranty a very good concentricity if not you will get errors due to variation of distance between internal and external surfaces.

I would suggest to use a sleeve of a non conductive material as ceramic with an electrode around it. It will present the advantage of low wear and, i think, it has same dielectric constant as air.

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Users who posted comments:

badbob (6); Brave Sir Robin (2); JohnDG (2); Kilowatt0 (1); lyn (2); MikeMack747 (1); nick name (1); TonyS (1)

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