Temperature and humidity are the two settings that decide whether a batch succeeds. A few tenths of a degree or the wrong humidity can ruin a hatch. Here are the safe values and how to check that your incubator really holds them.
The right temperature
Aim for 37.5 °C in a forced-air incubator (with a fan), where heat is even. In a still-air incubator, with no fan, the air is layered: set it slightly higher, measured at the top of the eggs, because the bottom is cooler. This value applies to chicken (21 days), duck (28 days), quail (17 to 18 days), goose (28 to 30 days) and turkey (28 days).
Too hot or too cold
Too high a temperature speeds development: early hatches, deformed or undersized chicks. Too low a temperature delays hatching and spreads it over several days, with weak chicks. Stability matters as much as the exact value, so avoid sudden swings.
Dry or wet method
During incubation, keep humidity around 45 to 55%. The dry method aims for the low end of that range to let the air cell form properly, useful in an already humid climate. The wet method aims for the high end. The real judge is the air cell.
Reading the air cell
When candling, watch the air pocket at the large end: it should grow steadily as the egg loses water. Too small means the egg is too wet, so lower the humidity. Too large means the egg is drying out, so raise it. This is the best indicator, more reliable than a single number.
Humidity at lockdown
For the last 3 days, at lockdown (day 18 or 19 in chickens), stop turning the eggs and raise humidity to 65 to 75%. This higher humidity keeps the membrane from sticking to the chick while it pips and helps it get out.
Calibrate thermometer and hygrometer
Never trust the built-in display. Check the thermometer against a reference device and the hygrometer with the salt test: put damp salt in a sealed container with the hygrometer, and it should read 75% after several hours. Always turn the eggs at least 2 times a day, ideally 3, until lockdown.