Is anyone aware of a non-semiconductor voltage reference source with ~1% or better accuracy?
For humidity, a saturated NaCl paste is a decent 75% RH reference @25C.
For temperature an ice bath is a decent 0C reference.
One idea I consider is making a simple lead-acid cell. The output varies not only with temperature but with the sulfuric acid concentration. The temperature I can easily measure, but without a hygrometer I can't measure the sulfuric acid concentration. And even if I could, I doubt I could get 1% accuracy when temperature and acid concentration are factored.
Any ideas? I'm going to order some TL431A's (+- 0.8%), but while I wait for them to arrive I'm wondering if there's something I might already have around the house I could use.