Walmart sells agood digital unit..
use only one hygrometer and one thermometer that you know are accurate..
using two or three different ones only creates doubt.. and well it should..
suppose one reads wrong and it is reading 100 while the temp is actually 104.. and the other one is also wrong and it reads 100 while the temp is actually 110.. so now you take them and average the temp.. as many people will tell you to do..
the average is 107.. see what I mean ??
just use only one that you can trust..
it might take some time to get one that you trust, but it is worth the effort..
once you have one that is correct, you can then calibrate the rest to match it..
you put the trusted one and the untrue one together.. when your trusted one is reading 99.5, see what the other one is reading.. let's just say it is reading 96.. from now on, 96 on that thermometer is really 99.5.. make a note on it to that effect and you are good to go..
On bubble therms, you just have to make a mark at where the mercury level is and you no longer need the numbers..