So it was sunny enough yesterday that I put my UT61E in a box in the conservatory for an hour. I also put the other UniT meter (Tenma 72-7755 otherwise known as UT70B) and Keysight U1232a in the box as well.
I did a quick check with each meter before putting them in the box.
Sadly, this was in the evening and the conservatory was cooling down.
The box only reached 30C, not the near 40C I was hoping for
So I did not bother moving the TTI1905a and AD594JN voltage standard .
I did a quick check by bringing the meters in and just quickly measuring the voltage standard.
Both the UniT meters showed a drift with temperature.
The Keysight read the same as before.
So, sadly the UniT meters do suffer from temperature drift
Both UniT slowly drifted back to the readings before I put them in the box, so they do recover.
Just looking at a tear down of the Keysight U1232a, I do not see the ceramic precision resistor divider array, so I wonder how they achieved the temperature stability.
I might try wrapping the UT61E precision resistor divider array with insulating polystyrene (my one came wrapped in the stuff) and see if that makes a difference, I doubt it since it is the substrate that need to remain stable.