On 17 September 2017 at 20:12, <acb...@gmx.de> wrote: > The question is what accuracy you need. >
No a lot. I just want to find out if there's any voltage drops that are significantly higher than I would expect. The unit makes an RF transmission line, and the loss at RF is significantly higher than predicted by a computer model, which takes into account the skin depth of the materials. I'm wondering if there's something odd going on. I suspect the problem is the current in the aluminum is not being computed properly due to the oxide on the surface. But I just wanted to make sure there was no unexpected DC resistance. I don't think there will be, but I want to climate that possibility. > The classical way to do that (achieving high accuracy) is to apply a known > accurate current (say 10A) and measure the voltage drop accross the rod > with a nanovoltmeter. > As the piece of aluminum is isothermal you should not expect a big > thermovoltage. You could also compensate for this by reversing the current > and take the average, also by nulling the voltage reading prior to applying > any current. Generating precisely known AC currents (low uncertainty) is > difficult (i.e. measuring it precisely), therefore DC currents are ususaly > used also in metrology for this. > If you do some internet search you will find metrology reports about this. > If you do not have a nanovoltmeter you could build a measurement amplifier > with not that much of an effort (based on chopper amp or low drif precision > opamp) > The only nV meter I have is the lock-in amplifier, which has a full-scale sensitivity of 2 nV to 1 V in a 1-2-5 sequence. The only instrument I have able to measure > 3 A of current is a handheld multimeter. One of my power supplies can supply 35 A, and has an ammeter in it. I don't have any particularly accurate means of measuring DC current outside the limited of the 3457A. In terms of simplicity, getting a $10 audio amplifier from China and using the lock-in amplifier is the way to go, but I accept a metrologist would not like that idea! Dave _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
