Reducing fringe effect, measuring values

In summary, the conversation discusses the measurement of low capacitance using parallel plates and the use of water as a dielectric. The speaker mentions the small change in capacitance they are interested in and the use of a control capacitor and 16bit A/D to measure it. They also mention a possible fringing effect surrounding the plates and the idea of using a guard material to isolate the test area. The conversation also touches on the possibility of using a different dielectric material, such as oil, and the importance of ensuring it is non-conductive. The speaker plans to redo their setup and enclose it, as well as considering using a different dielectric material.
  • #1
edmondng
159
0
Hi,

I'm trying to measure some low capacitance using parallel plates i built. We're probably looking into the pF, say maybe 20pF (with the pcb board etc). The change in capacitance that i am really interested is very small. For the moment, i have use water as my dielectric. The capacitance that i am measuring is measured in terms of voltage by using a control capacitor, then goes through my 16bit A/D. The differential voltage from 0ml to 250ml is approx 20mV. This is a really small value.

One thing i did notice is that there seems to be some sort of fringing effect field surrounding the plates. Put you hand within 1 feet of the plates and the voltage swings away. Stay 3 feet away and its more stable. The stability increases with distance, so the nearer you are the more the voltage swing. I take it because the capacitance and e-field is being disturbed, small capacitance change in the pF results in my voltage change.

I was thinking of using some kind of guard material to isolate my test area. What sort of material should i use? Plastic doesn't seem to have any effect. Also since i am looking at small values, is it possible to see a bigger change or amplify my signal? Its 16bit A/D, amplifying the voltage would not help. What i need is a bigger change like from 0ml to 250ml with maybe 1V change rather than 20mV. Any help, suggestion, books to look at would be appreciated.

Thanks
 
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  • #2
If the value changes when your hand is 1' away, that sounds more like AC mains interference into your test setup. And yes, grounded metal shielding would be good to have around your setup where you are measuring small values. Make a metal cylinder around the whole setup and ground it, keeping it spaced away from your capacitor by several multiples of the maximum plate spacing. The fringe field will not extend out more than a couple times the plate spacing, I wouldn't think.
 
  • #3
"The differential voltage from 0ml to 250ml is approx 20mV."
"What i need is a bigger change like from 0ml to 250ml with maybe 1V change rather than 20mV."

Try a different dielectric material. Maybe some sort of oil. You could check different things?
 
  • #4
i tried with factory capacitors and based on the values i use, i plotted a graph of voltage vs capacitance. seems that at a certain region the rate of voltage change is more and then it saturates. Since my plate resulted in only 20mV change the most and the voltage fell somewhere between the region where i could get a high rate of change (using factory caps), i think it has something to do with my whole setup measurement. I must be losing my field elsewhere. Not to mention calculated values were nowhere close.

I'm going to redo my whole setup, enclose everything, put a guard material to gnd around it as well. Any other things i should look at?

Thanks
 
  • #5
Make sure your dielectric is non-conductive. What kind of "water" did you use?
 
  • #6
i tried deionized and just regular tap water. would adding salt make it more conductive be better? i thought adding conductive would increase the capacitance so if i don't see a lot of 'action' could try it
 
  • #7
No, do not make it more conductive. I'm also not real comfortable with water as a dielectric anyway. Can you research oil-filled capacitors, and try it with an appropriate oil? At least I've heard of oil-filled caps.
 

1. How can the fringe effect be reduced in scientific measurements?

The fringe effect can be reduced by using specialized equipment and techniques, such as shielding and grounding, to minimize outside interference. Additionally, ensuring proper calibration and using multiple measurements can help reduce the impact of the fringe effect.

2. What is the best way to measure values in a laboratory setting?

The best way to measure values in a laboratory setting is by using precise and accurate equipment, following standardized procedures, and minimizing external factors that may affect the measurements. It is also important to consider the precision and accuracy of the measurements and use appropriate statistical analysis methods.

3. How does the fringe effect impact scientific research?

The fringe effect can impact scientific research by introducing errors and inconsistencies in measurements, leading to inaccurate results. This can hinder the progress of research and make it difficult to draw valid conclusions.

4. Are there any common mistakes to avoid when trying to reduce the fringe effect?

Some common mistakes to avoid when trying to reduce the fringe effect include using improper or outdated equipment, not following standardized procedures, and not considering the potential impact of external factors on the measurements. It is also important to regularly calibrate equipment and verify the accuracy of measurements.

5. How can researchers ensure the accuracy of their measurements when dealing with the fringe effect?

Researchers can ensure the accuracy of their measurements by using appropriate equipment, following standardized procedures, and considering the potential impact of external factors on the measurements. It is also helpful to compare measurements from multiple sources and perform statistical analysis to verify the results.

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