How to measure current in RMS value by using an AC/DC clamp meter

This is the U1213A True RMS clamp meter I am using to measure current:
https://literature.cdn.keysight.com/litweb/pdf/U1211-90011.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.

Hi,

I didn´t go through the documentation. But from experience with other RMS measurement devices:

I assume:
* DC: is the average DC current of the signal. AC is suppressed. No RMS value of the combined signal. But for DC speaking only: I_DC = I_RMS = I_AVG
* AC: gives the true RMS signal for a high pass filtered signal: DC is excluded. This is good to measure ripple current on a DC bisaed signal. The filter cutoff frequency usually is below 1Hz.
* AC+DC: gives the true RMS signal including DC. This is the true RMS signal for calculating power.

You may compare this:
I_AC+DC = sqrt ( I_DC^2 + I_AC^2)

Klaus