logging the temperature to make sure your 5440 output is as stable as claimed around that magic 10 C band.
Thinking a little more about this, Randall - it's likely why I'm seeing these excursions on the low end of a reading on my meter for the 5440A output. It seems that typically, a warmer environment would trigger lower readings for a certain voltage output (at least with my units here), which may be counter-intuitive for most (including me). So if I, say, see 28C at my bench (hot for all lab typical conditions), I'll likely see a "lower than 10V" reading on the Prema (for instance, 9.9999995V, therefore lower than 10.0000000V). Importantly, I should probably suspect the Prema for this tempco far more than the 5440A, with its ovens for all critical stuff. The Prema has the famously uber-stable LTZ1000, but none of its circuitry is temperature stabilized. Almost everything of substance on the 5440A is.
Stepping back, there's all sorts of thermal inertias of things from heavy walls to slabs to... instruments to air convection and whatnot, so it's a bit hard to really grasp how this plays into a metrology reading, but my background in both physics and building engineering (HVAC etc.) helps. I mention it because that's what I think needs to be considered.
Well, I've got a basement lab which features ultra stable room temperature all over the year, i.e. basically +/- 1°C.
You can have constant temperature over days and weeks within +/- 0.2°C, w/o any HVAC, only heating in winter time.
Unstable room temperature, i.e. variations > 0.5°C over the measurement run (even with ACAL instruments) will ruin your day.
Variations of more than +/-2°C make precise/stable absolute measurements nearly impossible. Transfer / difference measurements are another story.
The 5440 might be more stable with his ovens, but my hp3458A has a (typical) T.C. w/o ACAL of +0.5ppm/°C, which you directly see in your measurements.
My better half once opened the small basement windows to get fresh air inside, when my 24h stability measurement was running.
That -0.5°C r.t. change created a -0.3ppm dip, so I was able to calculate the T.C., which comprises of the T.C. from the LTZ1000A circuit, plus the T.C. of U180, which can be separated by monitoring CAL? 72 and sketching that over TEMP?
The heating of the room by the instruments (200W in total, I guess), plus the 36W neon lamp, plus the 100W from the operator therefore pose a real problem for high stability and transfer measurements. You first need to let the room temperature stabilize to the new level, and then you have to make your transfers quickly and monitor the temperature to make sure it doesn't change more than 0.2 .. 0.5°C.
I append the temperature log from last month.
At the beginning, the data logging stick had to cool down from the temperature in my office upstairs, @ over 25°C, where I always read out the data.
Then you see weeks of mostly constant temperature. Outside it was quite warm, over 30°C, I think.
In the 2nd half of September, the outside temperature went down below 20°C. I have not yet switched on the gas heating, which stabilizes the r.t. of the lab even more.
On October 1st, I made my regular measurements, where you clearly see the warm-up by the instruments in the night before, starting @22:30.
Next morning ~ 8:15 ACAL ALL with fluorescent lamp on, 7x Ohm comparisons 8:42 .. 9:00, 3458A TEMP? increase by 0.1°C.
A break-fast later, there's the comparison of 13 volt references between 9:38 .. 10:06. Temperature increase inside the 3458A was +0.1°C only, but R.T. changed by +0.3°C already.
ACAL temperature had been 0.6°C lower, but the absolute reading of the 3458A does not interest here, only the transfer between the 13 sources.
I recently had sent two of my references to a fellow volt-nuts, together with another temperature logger.
I got very similar temperature profiles from his lab, and from his measurement session.
It would be helpful to make remotely controlled measurements, no lighting and no operator in the lab, but then you'd need a precision scanner.
Frank