Dave,
Looking at your test setup, you might not be looking at all meter drift...Some other areas to look at when you're making measurements in the ppm area - just based on experience and some "gotchas" that we have discovered the hard way sometimes:
1. As others have pointed out, you've got un-shielded cables from meter to ref resistor. I would use a good quality shielded twisted wire paired assembly (as Dr Frank pointed out), and get a meter with a Guard or Shield connection - and make use of it. I use 4ea RG cables with guard amp driven from highest potential driver on your 4-wire setup, or just connect as basic grounded shields. If the DMM doesn't have a Guard point, get one that does - especially if you're going to be doing a lot of PPM-resolution measures.
2. You want another rag or iso thermal shield on those front meter connections. It's easy to get drift of some ppm just on those cheap banana plugs out there flapping in the breeze. Clean Tellurium copper spade connections where you can tighten a nut down to a known torque work well when you're chasing down the last half ppm drift, otherwise gold test clips are good. Good crimped / welded, gas-tight connections are much better than soldered joints at this point. Any crud or tarnish on the connections is a bad thing, and a clean silvered joint is better than a dirty gold or copper joint any day.
3. Turn off all switching power supplies in the vicinity. I've seen many a precision resistor - especially some non-bifilar wirewounds - easily pick up local switching noise. That includes unshielded flouro / LED lamp drivers. We use politically incorrect, electrically quiet incandescent lamps everywhere in the test lab here for illumination for that very reason. Plain old sunlight thru an IR-filtered glass window works too, if available. As long as it doesn't cause bad thermal effects.
4. If this were a '3458a meter, you'd want to run Auto Cal if your lab temp changes more than 1°C, or at least once every 24hrs when you're trying to play in the PPM accuracy pool.
5. You also want to flip your leads to the ref resistor periodically. It is unknown what thermal and diode effects are inside that box, especially when flying leads are present.. We even flip the ref resistor around physically occasionally to check for gravity-force stress effects at different orientations. You'd be surprised at what shows up sometimes - especially on cheap resistors.
6. Keep an eye on lab humidity and baro pressure also. Sometimes we've seen a drift pop up that is only seen during a local heavy storm or exceptionally sunny / rainy weather.
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