I am going to try to check and calibrate some electronics hobbyist humidity sensors using some salts including lithium chloride (gives RH 11.3% at STP), magnesium chloride hexahydrate (gives 33.1% RH at STP) and sodium chloride (gives 75.5% RH at STP). Instructions I find around the Internet seem to suggest that the salts should be prepared by adding water (one place said to the consistency of "wet sand"). My question is, if humidity where I live (United Kingdom) is not expected to go below 30% (it's usually 50-60% at the moment), then wouldn't it be wasteful of the lithium chloride to add water to it, since it's expected to be taking moisture out of the air and saturating some of it before use means I'll get much fewer uses out of it before its completely dissolved in water it took from the air?
I was also wondering how to determine how much air, say, 30 grams of anhydrous lithium chloride could dry if we assume the RH of the air is, say, 90%? How would I go about figuring that out? Also, am I right in thinking that heating it in an oven at, say, 80 °C (~180 °F) would drive off most of the moisture so I could keep reusing it forever. Thanks in advance for help in this.