Humidity in Incubation

Abstract

No aspect of incubation causes more confusion and concern than humidity. When other factors appear correct humidity is often blamed for poor hatches but whether too high or too low may still be in doubt. It doesn't have to be so; a few simple procedures can take the mystery out of humidity and put control back in the hands of the operator.

Humidity is one of four primary vari-

, ables which must be controlled during egg incubation - the others being temperature, ventilation, and movement (turning). Humidity is the most difficult of the four to monitor accurately and to control and therefore is commonly misunderstood. The operator instructions that accompany all incubators give guidelines to achieve correct humidity levels for most species under normal conditions and in the majority of cases this gives excellent results so first check that you have followed these guidelines.

However, there are times when incorrect humidity levels do cause problems :ind further steps are needed to check that humidity levels are correct. This article explains the effect of

different humidity levels, measurement of humidity, and the best techniques for achieving correct humidity levels.

Before spending time and effort checking incubation humidity levels, it essential to ensure that temperature and egg turning are correct - refer to the unit's operating instructions. Also check that the eggs are fertile and the parent stock healthy, properly fed, and free from in-breeding.

The Effect of Humidity Upon the Incubating Egg

Egg shells are porous - they allow water to pass through, and so all eggs, whether being incubated or not, dry out slowly. The amount of water that an egg loses during incubation is important and this is determined by the humidity levels within an incubator; if the humidity level is higher then the egg will "dry out" more slowly than if the humidity is lower.

All eggs have an air space at the round end and as water is lost through the shell it is replaced by air drawn through the shell into the air space which gradually increases in size. This air space plays a crucial part in hatching. It is the first air that the fully developed chick breathes and the space allows the developed chick some movement inside the shell to allow it to maneuver into hatching position.

If the incubation humidity has been too high the egg will have lost too little moisture and the chick will be rather large. In this case the air space will be too small, the chick's respiration will be affected and the young bird will have difficulty breaking out of the shell because of the lack of space.

Commonly, with excess incubation humidity, chicks will die just before or after having broken through the shell in one place C'pipped") either through weakness because of the lack of air to breathe in the shell or because of lack of space to tum and cut around the shell with their bill. Often, because of pressure within the egg, the bill protrudes too far out of the initial hole preventing the normal anti-clockwise progress of the bill chipping the shell from inside. The bill becomes gummed up with drying mucus.

Low incubation humidity levels lead

 

to small chicks with large air spaces by the time the hatch is due. These chicks will tend to be weak and may also die just before, during, or just after hatching. It should be noted in general that a slightly lower humidity level than optimum is likely to be less disastrous than a slightly higher than ideal level.

It is important also to understand that humidity does not directly affect embryo development unless the egg is seriously dehydrated. Only temperature and egg turning affect growth of the embryo directly. Humidity is important only to achieve the right balance between excessive dehydration and space within the egg as it reaches full term. Thus a temporary error in humidity can be corrected later provided the error is observed and the right action taken. Death of an embryo at early or mid-term stages of incubation is not usually attributable to incorrect humidity.

Measurement of Humidity

Many materials are capable of absorbing water or water vapor and air is one of them. Water vapor is a gas like any other gas, and air is a mixture of gases, one of which is usually water vapor. The difference is that the amount of water vapor varies widely whereas the other gases which make up our atmosphere remain fairly constant. The range of vapor may be from none to a certain maximum which the air can absorb. This maximum increases with temperature and is known as saturation level.

There are two commonly used ways to define humidity and the differences need to be clearly understood. These are:

Relative Humidity (RH) Expressed as a Percentage

This is a measure of the amount of vapor in air compared with the maximum that could be absorbed at that particular temperature. This is why relative humidity (RH) is quoted as a percentage. For example, an incubation RH level of 50% might be quoted. This means that at incubation temperature the air in the incubator contains half of its maximum possible water vapor capacity. Because maximum possible

 

water content increases at higher temperature, if the temperature was increased but no additional water added then the % RH level would drop. The air would become dryer.

A good way of imagining this effect is to think of a bath sponge. When the sponge is squeezed to half its normal size, clearly it can hold less water. Imagine a half-squeezed sponge soaked in water until no more can be absorbed (saturated). This is analogous to cold air at 100% RH - no more water can be absorbed. If the sponge is allowed to expand completely then, although the amount of water has not changed, the sponge is relatively dryer than before because it has greater capacity to absorb water. This is analogous to warmer air containing the same amount of water vapor which will now have a much lower RH level.

Conversely, when air cools, the capacity of the air to hold water vapor reduces and % RH levels will rise. If the air temperature drops below the saturation point (100%RH) the water vapor condenses. An example of this is dew forming on a cold night after a warmer day.

 

 

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