If the incubators have fans, leave one in and one out. I usually use the vent plugs to regulate small swings in temperature/humidity. As far as analog thermometers go, I would suggest buying 2 of these, then calibrating them http://www.walmart.com/ip/Aquaculture-Glass-Aquarium-Thermometer/45081765
They both have red vent plugs- one set of direction says nothing about them and to just leave them til hatch, the other set says to start out incubation with one out and only plug it in if you need more humidity desperately. Then it says to pull the second one 3 days prior to hatch to give them more fresh air?! Which directions do I listen to?