Humidity is one of four primary variables which must be controlled during egg incubation - the others being temperature, ventilation and movement (or turning). Humidity is the most difficult of the four to measure accurately and 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 most cases this gives excellent results so please check that you have followed these guide lines. However there are times when incorrect humidity levels do cause problems and further steps are needed to check that humidity levels are correct. This information sheet 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 – the greater the water loss through the shell, the larger the airspace. This air space plays a crucial part in incubation. Within 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 excessive incubation humidity the chicks will die having broken through the shell in one place (‘pipped’) either through weakness because of the lack of air to breathe in the shell or because of lack of space to turn 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 that in general that a slightly lower humidity level than optimum is likely to be less disastrous than a slightly higher than ideal level.

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 (called saturation). This maximum increases with temperature.

There are two commonly used ways to measure 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 vapour 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.

A good way of imagining this effect is to think of a bath sponge. When the sponge is squeezed to half it’s 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.

Wet Bulb temperature

This is the temperature (in degrees C or F) of a thermometer with a moist cotton wick around its bulb. Evaporation of water from the wick cools the bulb by an amount related to the relative humidity. This cooling effect is the same as the chill we feel when we step out of a shower. It is the difference between Wet Bulb temperature and air temperature that is important, so air or Dry Bulb temperature must also be known to define the RH. In incubators the Dry Bulb temperature is constant (we hope!) so WB is often quoted on its own.

Direct measurement of RH is not easy. Cheap hygrometers are available but you get what you pay
for; we have seen cheap instruments reading 30% different from out of the same new pack! More
expensive direct reading digital instruments are better but need re-calibrating regularly. When looking into digital hygrometers check both the accuracy quoted and the hysteresis percentage. Both figures should be better than +/- 5% - if either is not quoted, don’t buy it! For example: Brinsea’s Advance Humidity Pump uses a top quality sensor with accuracy of +/-3% and 0% hysteresis – see below for more details.

The most reliable, cheap method of measuring RH, is to measure wet and dry bulb temperatures and convert the information to %RH by using a simple chart.

A couple of points worth noting. The Wet and Dry bulb thermometers may be conventional (mercury) thermometers or they may be electronic sensors. There are two special cases where Wet and Dry bulb readings are identical; when the air is saturated (100%RH), and when the wet wick has dried out!
A further complication is that it is difficult to measure humidity in ‘still air’ incubators. Wet bulb
thermometers do not work well in near static air conditions. The other problem is that the temperature will vary by several degrees from the top of a still air incubator to the bottom and so RH readings will vary with height too. Fortunately the humidity level in still air incubators is probably less critical than fan assisted (or forced draught) machines (see information sheet So why fit a fan?).


Achieving correct humidity levels

There is a fairly easy and reliable way of measuring RH indirectly and directly measuring the effect
that RH level has on the egg. This is by weighing the eggs to monitor their water loss over the
incubation period. Most species of bird (with the exception of the ostrich family) need to lose between 13 and 15% of their weight from the time of setting the eggs in an incubator to hatching. By measuring the weights of the eggs at intervals during incubation, taking the average weights and comparing these to the expected weights needed to achieve the ideal weight loss by hatching, it is possible to see when the rate of water loss is too great due to humidity being too low and vice versa.
In practice this means drawing a graph (see below) with incubation time in days along the x-axis and weight up the y-axis. The average weight of eggs when set (day 0) can be entered and the ideal hatching weight (average day 0 weight less 14%) can be plotted on the day the hatch is due. These two points are then joined to give the ideal weight loss line. Average weights can then be taken every three or four days and plotted on the graph. If the actual average weights are lower than the ideal then humidity levels need to be increased and vice versa. Thus any deviation from the ideal weight loss line can be corrected as incubation progresses. The important point is to reach the ideal weight loss by hatching day; some deviation form the ideal weight loss line earlier in incubation will have little adverse effect.