For improved reliability i recommend to look for bifurcated contacts, this gives very high reliability due to the fact that you are switching two contacts in parallel.
Panasonic relays that seem usable for metrology application are TQ (thermal EMF specified max. 2uV for the SMD version), AGN (bifurcated contacts with AgPd, Au clad) and DS (Au clad Ag contacts).
Greetings,
Rainer
i stumbled on something interesting
there are 2 version of TQ pdf
TQ2 states 1A life is 0.2million with therm emf ability
TQ2S states 1A life is 0.5million no therm emf ability stated
both look like current pdf can be downloaded from the usual octopart etc
but in the TQ2 pdf, it include TQ2S as part of the "total" lineup. it is a confusing story
this confusion almost infects TXS2 / TX2S. the TXS2 emf spec is 0.3uV. like the S and no S naming problem, the TX2S has no thermal ability stated. try read this line a few times, it helps in the confusion. now which is the model with low emf? 2S ? X2? S2? 2X ?
then there is TE, sometimes it is listed as part number, sometimes it is listed as model number.
(edit maybe all good DMM should have firmware to count relay ops to advise user of reliability issues)
updated butchered pic
bonus article : relay contact adhesion effect
maybe the contacts collided too hard !