My elevation is 210f for boiling. And no, there is no knowing for sure, but it's the best I can do or anyone can do without a calibration block - exactly why the reason I started this thread. Within a degree is the best I can hope for with these tests.
Anyway, apparently nobody can say there mash temps are dead on ( w/o calibration device) and if there thermometer is accurate after months or years of use.
"Its easy enough to add 150F to 180F water into an insulated jug or container and insert the reference and working thermometers." - Mabrungard, can you explain how you go about doing this... Thanks.