Eggs need to lose sufficient amounts of moisture during incubation, in order to create a big enough air cell at the moment of internal pipping.

To be able to control the moisture loss, egg weight loss should be determined, as moisture loss and weight loss is identical. The usual procedure is to measure initial egg weight at moment of setting, and determine the egg weight again at moment of transfer (18 days). The weight loss should be expressed as percentage of the initial egg weight. The optimal percentage of weight loss at 18 days is between 11 and 12%. The minimum required weight loss is 10%. If the weight loss after 18 days of incubaiton is less than 10% of the initial egg weight, a reduction in hatchability can be expected. 

It is not needed to weigh individual eggs to determine weight loss. A number of incubator trays (usually 3 to 5 trays) per machine and per flock is sufficient to determine weight loss.

Mark the trays and weight them at start of incubation. Make sure that there are no eggs removed from the trays and weigh them again at transfer. Calculate the percentage of moisture loss as: initial egg weight divided by (initial egg weight minus egg weight at 18 days).

Make sure that the nett egg weight is recorded, so total tray + egg weight minus the tray weight. Although the influence of the tray weight is not that big, it can lead to the wrong assumption.

If the tray weighs 300 g and contains 100 eggs of 60 grams that each lose 6 gram over 18 days:

The nett egg weight will be 6000 g, and the egg weight at 18 days will be 6000 -(100 x 6) = 5400 g. This give a moisture loss of 6000 / (6000-5400) = 10%

If the tray weight is not substracted from the egg weight, the calculation wil be 6300 / (6300 - 5700) = 10,5%

Although the difference is not very substantial, it can lead to a false conclusion when moisture loss is on the edge of being sufficient.