This is the U1213A True RMS clamp meter I am using to measure current:
http://literature.cdn.keysight.com/l...1211-90011.pdf

I have been using this clamp meter for around 2 years. At most of the time i use it to measure DC and AC current on my product.
When I'm measuring current of a DC circuitry, I will press Shift to switch it to "DC", and i will switch it to "AC" when measuring on AC power supply.
And I know there is also "AC+DC" setting for voltage and current measurement but I seldom use it.

Till I found current value discrepancy when i'm measuring current of an unregulated DC power supply line with DC and "AC+DC" setting. The difference could be up to 2-3 Amps. I also did a quick comparison by hooking up my DC circuitry to a power meter to double confirm on the current reading.
The power meter (direct contact type, and with RMS setting) shows the equivalent current reading with "AC+DC" setting of this U1213A clamp meter.

My questions are:
1) Since U1213A is a True RMS clamp meter, regardless of the setting of DC, AC and AC+DC all the measurement readings should be given in RMS value, right?
2) How does this U1213A internally behave when switching from "DC, AC and AC+DC"? Does it switch internally to different kind of magnetic sensor for different settings above? Or it changes the method of sampling and calculation?

Thanks. Hope someone could help to clear my doubts.

 

1) Yes. Readings are true RMS.
2) DC measurements are made using Hall Effect sensors. True RMS is determined by taking multiple readings over a complete cycle and then computing the RMS.

Read this to find some answers:
http://www.fluke.com/fluke/caen/community/fluke-news-plus/articlecategories/clamps/ac-dc-clamp-meter

 

How does this U1213A internally behave when switching from "DC, AC and AC+DC"?

It changes the filtering.
It ignores or measures the AC and DC components by either analog and/or digital filtering of the signal.