Ran some informal DC accuracy/drift tests with the UEi DL599, Brymen BM037, Amprobe AMP-25, and AMP-330 (made by Brymen):
Turn on, let sit for a few minutes, zero clamps, let sit another few minutes, DL599 and the Amprobes move around a tiny bit between -0.02A to 0.02A, BM037 slowly drifted up the longer I left it, ultimately 0.16A when I stopped the test.
Zero again, then send 5.000A DC through all of them, (AmpTip location and switch position on BM037 & AMP-330, centered in clamp on DL599 and AMP-25), BM037 and DL599 read 4.99A at start of test, AMP-25 was 5.07A, AMP-330 was 4.88A.
About 5 minutes later, DL599 is at 5.00A but I observed it move around a bit, up to 5.04A but then it goes back down and repeats. Couldn't determine any environmental reasons for this.
BM037 is acting the same as in the previous test and slowly drifts up, was at 5.20A when I stopped this test. AMP-25 is at 5.11A, AMP-330 is at 4.87A.
Turn off power, AMP-25 is 0.04A, AMP-330 and DL599 basically straight to 0.00A, DL599 then does the +/- 0.02A thing as before, BM037 also repeats and keeps drifting up, gave it a few more minutes and it reached 0.27A before I stopped the test.
Quick test @ 10A:
DL599 - 10.01A
BM037 - 9.99A
AMP-25 - 10.19A
AMP-330 - 9.85A