Why Is There a Huge Discrepancy in My Capacitor's Measured Capacitance?

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Discussion Overview

The discussion revolves around the significant discrepancy between the calculated and measured capacitance of a homemade capacitor constructed from aluminum foils and paper dielectric. Participants explore potential reasons for the measurement error, including issues related to measurement techniques and equipment accuracy.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant calculated the capacitance to be 3.3 nano Farads based on the dielectric constant, thickness, area, and permittivity, but measured it as 125 micro Farads, prompting questions about the source of error.
  • Another participant suggested that the large measured value indicates a serious error in the measurement procedure or instrument misuse.
  • Concerns were raised about the area calculation, specifically that the area should be converted to square meters before use, indicating a potential source of error in the capacitance calculation.
  • Questions were posed regarding the type of meter used for measuring capacitance, the resistance between the plates, and the method of holding the plates together, suggesting these factors could influence the measurement accuracy.
  • Some participants expressed uncertainty about how capacitance is measured, speculating on the accuracy of different measurement methods and the importance of selecting the correct range on the measuring device.
  • One participant mentioned their experience with an LCR meter and noted that it requires proper range settings for accurate readings, hinting at the possibility of measurement inaccuracies due to device limitations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the cause of the discrepancy. While some suggest measurement errors, others focus on calculation issues, and there is ongoing uncertainty about the measurement methods and equipment used.

Contextual Notes

Limitations include potential errors in unit conversion for area, the dependence of capacitance measurements on the characteristics of the measuring device, and unresolved questions about the construction and assembly of the capacitor.

eexmr7
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Hello,

I have made a capacitor with two aluminium foils separated with paper dielectric with,

dielectric constant (K) = 3.8 for paper material
paper thickness, D = 0.1mm = 1e-4 m
plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m
air permitivity, Eo = 8.85e-12 F/m

Calculated capacitance = (K Eo A)/d = 3.3 nano Farad

When I measured the capacitance i got,

Measured capacitance = 125 micro Farad

Can anybody explain why this huge error is existing? I have tried with different foil areas but could not find the reason of error.

I m looking forward, Cheers.
 
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eexmr7 said:
Hello,

I have made a capacitor with two aluminium foils separated with paper dielectric with,

dielectric constant (K) = 3.8 for paper material
paper thickness, D = 0.1mm = 1e-4 m
plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m
air permitivity, Eo = 8.85e-12 F/m

Calculated capacitance = (K Eo A)/d = 3.3 nano Farad

When I measured the capacitance i got,

Measured capacitance = 125 micro Farad

Can anybody explain why this huge error is existing? I have tried with different foil areas but could not find the reason of error.

I m looking forward, Cheers.
!
It's not just huge error, It's a huge value of capacity for described capacitor. My guess is you made a serious error in measurement procedure, readings or misuse of the instruments .
 
For one thing, this is incorrect: "plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m" Convert the two lengths to meters before multiplying, and your final area units need to be m^2.

Also, how did you measure the capacitance? What type of meter are you using? What do you measure for the resistance between the plates? How are you holding the plate stack together? Can you post a picture?
 
eexmr7 said:
Hello,

I have made a capacitor with two aluminium foils separated with paper dielectric with,

dielectric constant (K) = 3.8 for paper material
paper thickness, D = 0.1mm = 1e-4 m
plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m
air permitivity, Eo = 8.85e-12 F/m

Calculated capacitance = (K Eo A)/d = 3.3 nano Farad

When I measured the capacitance i got,

Measured capacitance = 125 micro Farad

Can anybody explain why this huge error is existing? I have tried with different foil areas but could not find the reason of error.

I m looking forward, Cheers.

Just by looking at your numbers, my feeling (based on 50 years of electronics experience) is that your calculated value is in the correct range. There is absolutely no chance at all that you can create a capacitor yourself with a value of 125μF.
 
I'm actually not exactly sure how devices measure capacitance (RC circuit? Or LC?), but I wouldn't be surprised if they don't show the same accuracy across all ranges (kinda how you have to dial in the right resistance range to get an accurate reading of a resistor).
 
rumborak said:
I'm actually not exactly sure how devices measure capacitance (RC circuit? Or LC?), but I wouldn't be surprised if they don't show the same accuracy across all ranges (kinda how you have to dial in the right resistance range to get an accurate reading of a resistor).

My personal inexpensive B&K LCR meter definitely needs to be put on the capacitance range that is closest to (but not less than) the capacitor to be measured. It also has a D (= 1/Q} measurement setting to let you know if the capacitor you are measuring is very lossy. If it is too lossy, that messes up the capacitance measurement. That's why I asked the resistance question in my earlier post... :-)
 
rumborak said:
I'm actually not exactly sure how devices measure capacitance (RC circuit? Or LC?), but I wouldn't be surprised if they don't show the same accuracy across all ranges (kinda how you have to dial in the right resistance range to get an accurate reading of a resistor).

I once built an LCR meter based on a Wien bridge. If anybody is interested, I could try to dig up the schematics.
 
berkeman said:
For one thing, this is incorrect: "plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m" Convert the two lengths to meters before multiplying, and your final area units need to be m^2.

Also, how did you measure the capacitance? What type of meter are you using? What do you measure for the resistance between the plates? How are you holding the plate stack together? Can you post a picture?
hi,

0.010043 is in meter-sq not in meters. the unit was wrong sorry for that. I measuring capacitance with LCR meter.
 

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