Humidity starts out at one percentage but then needs to be raised at just the right point in order for a successful hatch. You *must* keep the humidity high in the incubator during hatch, and if the exterior air is too dry, you can cause problems by opening the incubator repeatedly. High humidity at the beginning of incubation can be corrected later on with lower humidity and vice versa. I use a sharpie to block the hole in mine if I need to. Too much of it or too little of it can ruin your hatch rate. Yes, it can affect the humidity inside the incubator some, so by lowering the humidity a bit in the incubator, you won't have to be concerned as much about incubator humidity when our ambient humidity is high. That said, I prefer to take chicks out once they’ve hatched so they One of the easiest ways to measure the water vapor is by figuring out the Relative Humidity percentage, also known as RH%.

I too have no idea where the red plug is. Humidity in the incubator has always been a tricky thing. Humidity should be increased on the last 2 days. The chicks may still be ok if it wasn't too long, and depends on where in the cycle they are. If the humidity gets too high, you can crack the lid a bit, and don't add anymore water. The membrane inside the shell dries out and is nearly impossible for them to get through. And, if you’re anything like me, you I know you can order more. On day 19 your humidity should be 65% and to the end. Humidity is calculated by measuring the water vapor in the air. I would keep it at 60% rather than too high and If the humidity in the incubator is too low, a chick can have a really hard time breaking out of their shell.