I never trust thermometers or hygrometers unless they have been calibrated. I’ve seen them off too much to really trust them. There are a lot of different types of thermometers and hygrometers so it’s not always easy to come up with a calibration method. With a thermometer I use a medical thermometer that is calibrated and a glass of warm water to see if they read the same. There is a method to calibrate a hygrometer using salt and a plastic bag but I’ve never done that myself. Through trial and error I’ve learned that when my hygrometer readings average around 40% during incubation I get good hatches, whatever the real humidity is.
I am not familiar with that specific make and model of incubator but it sounds a lot like my Hovabator. Sometimes with all the reservoirs empty I get a humidity of 15%. Sometimes I get a humidity of over 35% with all reservoirs dry. The temperature and moisture in the air going in can make that much of a difference. I really notice it at different times of the year. I can get that much swing with just one reservoir filled.
Instantaneous humidity isn’t that important, it’s average humidity over the incubation period. If mine runs at 50% a few days I’ll run it at 30% for a while to average it out. It’s about the total moisture loss over the total incubation period.
Ventilation will affect moisture levels in the incubator. Taking those plugs out as you did will help drop it a bit. Warm air holds more moisture and rises. If it can escape out of the top the humidity drops a bit. One time I was having trouble with humidity in mine, it was running several points lower than it should. It was driving me batty until I realized the cord to the turner was not in its slot correctly so the top was not sealed like it should have been. My heater was able to maintain proper temperature with the top cracked which made it harder to realize what was wrong. You might try cracking the lid a tiny bit but watch your temperatures very closely to make sure the heater can keep up. There is some risk involved in this approach. Watch the temperature.
When I refill my water reservoirs if I spill a bit of water the humidity will spike until that water dries up. Sometimes that can take hours. With the tiny amount of water you added and after you’ve dried it, that doesn’t seem to be the problem.
I don’t know how accurate your hygrometer is to start with, you’ve seen with that other one how far off they can be. You may not be in as bad a situation as you think. I think my first efforts would be toward getting a hygrometer I could trust to at least be fairly accurate. Do you have a drier room you could put the incubator in? You don’t want it where a window can let sunlight hit it and warm it up and you want to avoid AC/Heating vents. Cooler air going in can help drop the humidity too.
As Ron said there is a wide humidity range that works. The sweet spot for best humidity will vary for each of us, often you find that by trial and error. Commercial hatcheries have found that the same model incubators that you would think identical have to be tweaked for maximum hatch rate. Just moving one from one spot in the room to another can change that sweet spot. These are incubators that might hold 60,000 to 120,000 eggs. They adjust those by trial and error too but they use calibrated instruments so they know what they are working with.
I don’t know how much trouble you are actually in. I really don’t trust that hygrometer.
