Laboratory setup -Capacitance vs. Temperature

[risingplague]

For my final project for second quarter physics for engineering, we are doing labs that we setup ourselves. The problem posed to my group was to measure the capacitance of a capacitor and it's dependence on external temperature, and then fit the data to what should be an exponential decay curve. My initial setup so far is this: I have an adjustable AC/DC power source, two multimeters, an adjustable resistor, a large 30v capacitor, and a switch. They are hooked in line as the following: Power source (+)->resistor->(+)multimeter(-)->(+)capacitor(-)->(+)multimeter(-)->switch->(-)power source. The capacitor is immersed in a hot water bath with DI water and a thermometer, on a hot plate. the multimeters have inputs to LoggerPro so that we can graph the data.

What can I do to make this setup more efficient, and collect the data that I need?
Any input would be greatly appreciated!

 

[marcusl]

I see several issues here. First, what is the time constant of your R-C? Your multimeter probably samples data every half second or so, so unless your tau is extremely long you won't get meaningful readings to fit to the exponential. Second, it seems from your description that you are measuring current. And are doing it twice, once with each meter. Generally this is a poor choice since the meter resistances will change your results. Why aren't you measuring voltage? Finally, the capacitance changes from 20-100C are likely to be small. You might want to rethink your approach. An AC measurement will have far better sensitivity.

 

[risingplague]

I see several issues here. First, what is the time constant of your R-C? Your multimeter probably samples data every half second or so, so unless your tau is extremely long you won't get meaningful readings to fit to the exponential. Second, it seems from your description that you are measuring current. And are doing it twice, once with each meter. Generally this is a poor choice since the meter resistances will change your results. Why aren't you measuring voltage? Finally, the capacitance changes from 20-100C are likely to be small. You might want to rethink your approach. An AC measurement will have far better sensitivity.

haha, we're actually looking for the time constant, and then we are to report the dependance. second, I had thought of that and was considering using just Loggerpro and some test leads going across the circuit, and using a test frequency of about 55-60Hz. We were using the multimeters to measure voltage. What kind of AC voltage should i be looking at to get some good readings? The capacitor has a max voltage of 30V, a max capacitance of 20 microfarads, and it's a TVA type.

 

[berkeman]

haha, we're actually looking for the time constant, and then we are to report the dependance. second, I had thought of that and was considering using just Loggerpro and some test leads going across the circuit, and using a test frequency of about 55-60Hz. We were using the multimeters to measure voltage. What kind of AC voltage should i be looking at to get some good readings? The capacitor has a max voltage of 30V, a max capacitance of 20 microfarads, and it's a TVA type.

You are not going to be making accurate measurements of capacitance with a DVM, unless that DVM has a "Capacitance" measurement setting. Why don't you use a digital oscilloscope to capture the RC transient and use the cursors on the 'scope to accurately measure the RC time constant?

 

[risingplague]

You are not going to be making accurate measurements of capacitance with a DVM, unless that DVM has a "Capacitance" measurement setting. Why don't you use a digital oscilloscope to capture the RC transient and use the cursors on the 'scope to accurately measure the RC time constant?

I'm not sure if we have an oscilloscope available at our lab, I'll have a look when I'm in on monday. If not, would my digital input into LoggerPro be able to give me access to the same information?