The amount of water added to the incubator varies according to the weather, time of year, room the incubator is in and ventillation setting as well as the eggs you have etc so I could have lots of success with an incubator here adding no water, then give the unit to you and you don't have any luck at all using identical settings.
The humidity outside the incubator varies and the air that is drawn in could be high or low in humidity - so not adding water could mean it's just right, or completely wrong. Measuring humidity (or weight loss) is far more accurate.
Each egg needs to lose a certain amount of moisture for a successful hatch - the humidity / amount of moisture in the air determines the rate at which the moisture is lost, so high humidity = low moisture loss from the egg and low humidity = high moisture loss. Think of it like trying to dry your washing on a wet day versus a dry day.
So it's important to get an accurate humidity reading and to be able to maintain a given humidity in the incubator for the eggs to lose a certain amount of moisture (and weight).
The electronic meters are usually accurate - but it's worth cross checking it with a known good hygrometer.
You can make a wet bulb hygrometer. You need 2 thermometers, some wick (available from suppliers of Octagon 20 for the wet bulb thermometer for example) and a little plastic tub of water. Push the wick over the bulb of one thermometer and dip the other end into the water - wait for the reading to settle, this is the wet temperature, then, take the temperature of the other thermometer (dry bulb). There are lots of tables published of wet Vs. Dry temperatures (possibly in your manual) that give the humidity - you can then check your digital meter reading.
With a second thermometer, you can also check your existing thermometer's temperature reading. It's more likely a humidity problem though for dead in shell chicks.
It's worth trying some different eggs too if you've only had eggs from one place. I've had some strains that I can pop into the incubator and hardly look at the settings and I get 70-80% hatch rate with other pure breeds from show lines, I have had all die in the shell.
Poor health in the flock as well as posting eggs could also be the problem. Ideally, find someone local with fertile eggs that aren't from show strains and do a test run.
I'm going on but I hope this is of help
Tim
