The Hatch Scale, free in every order.

The Hatch Scale is a small digital scale that reads to a tenth of a gram, with a flip lid to keep the pan clean between batches. The weight-loss card that comes with it has a line per egg and a column for day 0, 7, 14 and 18. Both come in your parcel, with every order: an incubator, the brooder, a heat plate, a coop door or a spare tray.

It is not a thermometer and it does not replace the display on your incubator. It does something the display cannot: it tells you whether the eggs themselves are drying at the right speed.

An egg breathes through its shell. During incubation water leaves it as vapour, the air cell at the blunt end grows, and the chick needs that air cell to take its first breath and turn to pip. Texas A&M AgriLife puts the normal loss for chicken eggs at 12 to 14 % of the starting weight, and says racks of eggs can be weighed during incubation to detect a humidity problem before a hatch is destroyed.

Too humid, and the egg loses too little: the air cell stays small and the chick can drown in the shell at hatch. Too dry, and it loses too much: the chick is small, sticky and weak, and the membranes shrink-wrap it. A hygrometer tells you the air. The scale tells you the egg, and the egg is what hatches.

The Hatch Scale weighs an egg against itself, so it never argues with your incubator. A 60 g egg that reads 57 g on day 7 is on schedule whatever the display says.

If an egg wants to roll on the pan, stand it in a bottle cap: put the cap on first, press T to zero, then add the egg. Weigh at the same time of day, and weigh quickly: an egg out of a 100 °F incubator cools fast.

Other species run on their own calendars (duck 28 days, quail 17 to 18, goose 30) and the same 12 to 14 % loss is spread over their incubation. Take the lockdown day from the Hatch Guide and use the same rule: by lockdown, about 13 % lighter than day 0.