I've just calibrated my hygrometers. I found this nice guide in case you need to calibrate yours. I just hate measuring equipment that can't be trusted.
http://www.cigarjournal.co/how-to-calibrate-a-hygrometer/ (http://www.cigarjournal.co/how-to-calibrate-a-hygrometer/)
I used the "Small Glass Method" described on the page. Actually I just used a bottle cap with a salt slush. Surprisingly my digital hygrometer actually showed exactly 75% as it should.
I placed a small bowl over the plastic bag and covered that with some towels to prevent temperature fluctuations.
I calibrated my thermometers as well by placing them in the small isolated space. I have a reference thermometer that is very precise (plus/minus 0.2 deg C while covering a wide temperature range). The other thermometers show a decimal but they are typically just within 1 degrees or so.
I normally use the precise reference thermometer for the vat. The other thermometers I use in the caves. If I wanted to use a different thermometer for the vat then I would calibrate it at around 35 degrees C. If you have an oral thermometer it should be reasonably precise at that temperature (I think) and can be used as a reference...
;-) Danbo