By John A. Klea

IFCB 1983

In artificial incubation, we are attempting to reproduce an environment which is similar to that of the parent bird, in order to properly hatch a healthy chick. Failure to provide this proper environment will result in poor hatches.

A number of reasons can be cited for poor hatches and some of the more common ones are listed below:
1. Weak Parent stock, unhealthy or fed a nutritionally deficient diet.
2. Infertile eggs
3. Inbred stock
4. Egg shell contamination
5. Eggs too old when set
6. Improper care of eggs prior to incubation
7. Improper incubator ventilation, oxygen starvation
8. Eggs not turned often enough
9. Incubator temperatures too high, too low, or too variable during incubation.
10. Too little or too much humidity in the incubator

The above list is not all inclusive, but does point out a significant number of variables which must be controlled if high percentage hatches are desired. Neglect in any of these areas will result in failure. You will note that the last four reasons for poor hatches are directly related to the incubator or the operation of it.

In order to hatch an egg it must be a good fertile egg. This egg must come from well mated parent (unrelated preferred) stock and they must be fed a proper nutritional diet. Assuming this is the case and you wish to hatch the egg by artificial means, an incubator will be required.

The primary purpose of this paper will be to focus on the four latter reasons for poor hatches as they are associated with the incubator. Perhaps, at this point it would be wise to point out something about incubators that most people are not aware of. Most incubators, and this is particularly true if you have one which is more than 5-6 years old, were originally designed for the poultry industry. Many of them do a fine job of hatching chickens and turkeys but can be very disappointing when used to incubate gamebird or more exotic types of bird eggs. The reason for this is that poultry eggs have been bred for decades in incubators to such an extent that we have conditioned the eggs to withstand large environmental variations such as temperature without significantly reducing the percentage of hatch. Many incubators in use today do not have the capability to minimize these variables without modification to the incubator. Many incubator manufacturers sell their product (with an instruction booklet) with little if any first-hand knowledge of some of the types of rare and semi-rare bird eggs we are putting into them. To illustrate this point a little better, I would like to refer you to Figure 1.1 do not claim any accuracy to the graph shown as it is provided simply to illustrate the importance of providing the optimum temperature to insure a high percentage hatch. The graph depicts the optimum temperature of 99.5°F for pheasants as an example. A good deal of literature exists which states 99.75° to be the optimum temperature for pheasants, however. Note the difference between the curves for pheasants and poultry. The significance of this figure is to illustrate that proper temperature measurement and control (regardless of egg type) is vital to a large percentage hatch. Mother Nature also has different ranges of permissible temperature variation for various eggs, however. It is known that egg size, shell thickness, clutch size, temperature environment, bird characteristics and other parameters determine the degree of permissible environmental variation allowed before hatchability is significantly affected.

The numbers of bird breeders are increasing all of the time. The unfortunate part of many novice and experienced breeders is that many will purchase incubators, follow the instructions, and suffer the invariable learning curve we all must take before achieving a high percentage of successful hatch. I might add that many never achieve a high percentage hatch. Some get discouraged and give up, others will blame the incubator, incubator manufacturer, incubator instructions or themselves. In reality, it may be only one or a combination of all the above.

Let’s discuss the incubator and the problems associated with it in sufficient detail that it will improve your percentage of success. The incubator in reality is a simple device. Its function is to control the environmental conditions (temperature, humidity, oxygen supply and egg rotation if required) of a good fertile egg. Incubators vary in type (still air or forced air), construction, material, size, reliability and cost. The still air incubator remains in use today, but is not nearly as common as the forced air type. It is the device closest to duplicating the setting hen, however. The heat is transmitted to the egg from above and moisture from the bottom similar to a setting hen. This type of incubator permits a thermal gradient to exist across the egg. This can be difficult to control as drafts around the incubator can cause large variations in the egg temperature. Additionally, it is difficult to make an accurate humidity measurement in a still-air thermal gradient environment. Over 40 years ago, the forced air incubator was introduced. It needs less space, holds more eggs and generally will, in principle, outperform the still air incubator. The discussion which follows assumes the use of this type of incubator.

Before you set an egg in an incubator you must know the ideal or optimum conditions fora successful hatch. A good deal of printed information (temperature, humidity and turning frequency) is available for some types of semi-rare and rare eggs, but for others there is none. For these eggs, you may need to contact other successful breeders, avian department personnel of universities, avian department personnel of zoos or use your own trial and error method. For rare eggs, curves similar to that shown in Figure 1 do not exist because eggs would have to be sacrificed to develop such a curve. Because some eggs may be so precious, each breeder must guess at the ideal environmental conditions for the egg.


Next, I would like to discuss what I believe is the parameter which is responsible for most incubator related hatch failures. Temperature measurement and control leaves a great deal to be desired in many incubators in use today. Most commercially sold incubators today contain a thermometer which is satisfactory f or poultry perhaps, but not for more exotic types of eggs. To illustrate this point, Figure 2 has been provided to show a comparison between glass thermometers. The two thermometers on the left are typical of those found in incubators. You will observe that the smallest increments are 2.0°F apart, although some employ thermometers of 1.0°F increments. This severely reduces you ability to discriminate fractions of a degree, assuming the thermometer is truly accurate to begin with. Another important fact which most people are unaware of is that a general rule of thumb is used in the manufacture of thermometers. The general rule is that the accuracy of the thermometer is only as good as the smallest incremental division. In simple terms, your reading of 100°F on these types of thermometers may be 98 or 102°F in reality. These types of thermometers are mass produced at very low cost and are not inherently designed to be highly accurate devices. The thermometers identified as models 18F and 28F (Figure 2) are A.S.T.M. (American Society of Testing and Materials) thermometers. These thermometers are manufactured by a number of thermometer manufacturers (generally located in Eastern United States) to a rigid ASTM specification. They must meet or exceed the accuracy requirements of the specification. The 18F model must be accurate to within 0.2°F and the 28F model with 0.1 °F. This author has tested a number of these thermometers in a secondary standards laboratory which is traceable to the National Bureau of Standards Laboratory and found three different manufactured thermometers to meet or generally exceed the ASTM accuracy requirements. These thermometers are more expensive of course but not prohibitively so. These thermometers are readily available and can be purchased from chemical supply distributors or directly from thermometer manufacturers. Perhaps you would be reluctant to use this type of thermometer because it will not fit into your incubator. The solution would be simply to drill a hole into the side or top of your incubator in order to place the bulb portion of the thermometer in close proximity to the eggs. The bulb portion should not touch the egg or tray or an error in the temperature measurement will result.

For some incubators, even a more accurate thermometer will not help if it is placed in a position significantly removed from the eggs. Some incubators mount the thermometer behind a glass or clear plastic window in order to provide readability. The problem which may arise here is that without careful and proper incubator design the temperature of the air near the thermometer and the temperature around the eggs maybe significantly different. The air temperature around the egg is the desired measurement to be controlled.

Another advantage of the ASTM thermometer is that it lets you observe the temperature cycling or fluctuation which are a result of the temperature control systems. Other methods of temperature measurement exist and will be discussed briefly. Figure 3 has been provided to show another temperature measuring device. These dial thermostats can be found in 1°F increments but again lack the accuracy of the ASTM thermometers. The reason is that these devices employ a bimetallic coil or spring which effectively rotates a pointer directly related to temperature. The coil does not have the temperature sensitivity to accurately respond to temperature variations of less than 1 degree F.



Thermocouples are another form of temperature sensor. These devices are two dissimilar metals such as chromel and alumel which when welded together (in wire form) produce a small voltage which varies directly with temperature. However, thermocouple outputs drift with time and age; thermocouples should not be used to make temperature measurements which require better than 1 degree F accuracy.

Thermistors are similar to thermocouples in size and are relatively inexpensive. A thermistor is simply a resistor which varies resistance (generally orders of magnitude) with relatively small changes in temperature. However, thermistors too are unstable and will drift (in resistance) with time. Both of these types of sensors are sold in large quantities today. Usually they are connected to an electronic hand held box which has a digital display of the temperature reading. The display will often be capable of indicating temperature in tenths of one degree. However, you must read the accuracy specifications of these types of devices and they generally will not be accurate to better than 2.0°F.
Another device used to measure temperature is called an RTD (Resistive Temperature Device). These sensors are generally made from pure platinum wire. They can be very stable and very accurate temperature measuring sensors but can be fairly expensive when coupled with quality signal conditioning equipment. The biggest advantage with thermocouples, thermistors and RTD’s is that they can be remotely mounted from the readout device.

After temperature measurement in importance is temperature control. In order to control temperature, you must first be able to read it accurately. Temperature control systems have been improved in some types of incubators being manufactured today. Temperature controllers as a whole have been greatly improved in the past ten years largely due to new electronic components. Probably the most common temperature control system still used in incubators today is the thermal bellows and micro-switch. By today’s standards, this is an archaic method of fine temperature control. Please do not misunderstand this previous statement; with careful attention this type of control can work but it is not the most sensitive, or most reliable type of controller. It is one of the least expensive, however. Since it is a very common type of temperature controller, it will be discussed in great detail with answers as to why yours does not work very well.

Refer to Figure 4 and you will find an illustration of a typical incubator temperature control system. You should recognize the temperature sensor as a metal bellows generally made from rolled brass sheet. The sheet brass is pressed in a die to form convolutions into the disks. This strengthens and stiffens the bellow walls. The fours shaped disks and buttons are soft-soldered together. Ethyl Ether is added to the interior of the cavity and the injection hole is soldered closed. Now whenever this bellows is subjected to a warm temperature the wafer will expand linearly with temperature. It is designed so that the internal pressure will cause the greatest expansion to occur between the two center buttons. In Figure 4, the wafer is shown to actuate a microswitch. This switch allows the voltage to be impressed across the heater until the thermal wafer depresses the actuator pin of the switch in order to remove the power to the heater. The amount of expansion that the wafer must incur is directly related to the differential travel of the microswitch pin between the on and off positions. During the course of a breeding season the bellows will expand and contract thousands if not millions of times. This action, especially true if it must expand several thousandths of an inch, will cause eventual work hardening of the brass.


To illustrate this point Figure 5 shows a portion of the thermal wafer cross section. The section was taken from an area of greatest stress. It was polished, etched and photographed at 230X magnification. Review of Figure 5 shows a new or unused waferwall cross section. Here the grains of the brass are randomly distributed, but the wafer on the left which was used for one entire breeding season and accumulated approximately 100,000 cycles, shows a much different grain structure. Note the preferential alignment of the grains. This is indicative of twinning or work hardening. This study was conducted on additional wafers from other incubators as well. The results were similar. These wafers were removed during the startup of attempting to use them for a second year’s breeding season. The temperature control was erratic and unstable. The poor temperature control was caused by the failure of the wafer to expand and contract linearly with temperature. Replacement of the bellows rectified the temperature instability problem. Therefore, it is strongly recommended that new thermal wafers be placed into your incubator before starting the incubator each season.

Where you store your thermal wafers is very important as well. If you store them in an attic or garage where temperature is unusually warm the wafer will expand and be actually working without your knowledge. Another way to ruin a new thermal wafer is to screw it onto the adjusting bolt and leave it a significant distance from the microswitch pin before energizing the incubator. The incubator may reach an excessive temperature such as 110 degrees or above. At this point the internal pressure will nearly double, and most likely cause the wafer to expand beyond its yield limit. This will stretch the brass and render it useless as it will no longer expand and contract linearly with temperature changes. The solution is to slowly allow the temperature to rise in the incubator by slowly backing the wafer from the microswitch.

Another significant point about this type of temperature sensor is its susceptibility to error produced by barometric pressure changes. As an example, if the barometric pressure decreases the wafer will expand more readily because of its internal pressure. The additional expansion will cause a net result of lowering of the temperature set point of the incubator because the heater will be shut off due to a decrease in ambient pressure instead of a high temperature condition. Thus the thermal wafer becomes a barometer as well and will necessitate an adjustment to the adjustment bolt to rectify the problem.

Another bit of advice is to leave the incubator power on once you have started to use it for the breeding season. Intermittent significant periods of time which allow the thermal wafer to cool will allow it to take a set or again reduce its sensitivity to temperature changes. It will work best if you use it continuously throughout the hatching season.

Next, how can you tell if the thermal wafer is in satisfactory condition? There are several ways. One way is to shake the wafer at ambient temperature. You should be able to hear the liquid ether. If not, the liquid has escaped in the form of gas thru a small crack in one of the solder joints. This renders the wafer useless as it will no longer expand. Examine the edges where the solder is located. Cracks found here may cause a future failure at a most inopportune time. Another method is to examine the wafer for bulging. Refer to Figure 6. A cross section of a new unused thermal wafer is on the left. Note the flat characteristics. The wafer in the center was used for 3 months or one short season. The wafer to the right was used for several years for an unknown number of cycles but the temperature control using this thermal wafer was extremely poor(variations of ±5 to 6°F). The thermal wafer may be effectively worn out as a reliable temperature sensor if significant temperature variations occur. Work hardening or twinning of the material is not a visible characteristic without destructive testing.



The other important part of this type of temperature control system is the microswitch. These are mechanically actuated electrical switches (refer to Figure 2). As such, they have a useful life expectancy. Generally the mechanical life is 20 million cycles but the contact electrical life is 5 million cycles at rated current. Excessive force to the mechanical pin as well as excessive current through the contacts will severely reduce the useful life. In summary, if your microswitch is approaching to total of 5 million cycles it would be prudent to replace it. The number of cumulative cycles can be calculated readily by counting the number of actuations (heater power lamp on) the switch undergoes in a 5 min. period. Multiply by 288 to give you the number per day and then you can predict how long the switch should be used without significant danger of exceeding its normal life. They are generally reliable for years.

In addition to the reliability of the switch, the force and differential travel of the actuator pin are very important characteristics of the switch. Many microswitches found in incubators require a force of 9 to 13 ounces (2.5-3.61 newtons) and have a differential travel of.002 inches (0.05 MM) or more. This differential travel has a great bearing on the minimum and maximum temperature extremes during a typical on-off temperature cycle. The more sensitive the microswitch is, the less the thermal wafer has on expand and contract due to temperature variations. Microswitches are readily available which only require 4 ounces (1.11 newtons) of force to actuate them and have a differential travel of .0002 to .0005 inches (0.005-0.013 MM). These characteristics will significantly reduce the minimum and maximum temperature extremes during a typical thermal cycle.

The placement of the thermal sensor within the incubator is also very important. It must not be randomly located within the incubator. It is best to place the sensor between the heat source and eggs or in close proximity to the heater if thermal lag effects are to be minimized. If the temperature sensor is not near the direct heated air but placed near the eggs a thermal lag or poor response time will result. This will in turn cause significant temperature variations to occur. If your temperature control is poor, you may want to consider changing the components or installing a newer more reliable type temperature controller.

There are a number of better and more reliable temperature controllers on the market. One type uses a thermistor as the temperature sensor. It is a small bead which is very responsive to temperature change when coupled to a reliable electronic circuit. The power is turned on and off through a transistor which acts as an electronic switch. In principle, this type of temperature control is superior to the thermal wafer and microswitch which are mechanical devices with a shorter life. The thermistor temperature sensor is not affected by barometric pressure changes as well.

A similar type of temperature controller uses a small RTD (Resistive Temperature Device generally made from platinum wire) as the temperature sensor. These are generally a little more expensive but can be very reliable controllers when used with a well-designed circuit using quality electronic components.

Another temperature controller used in incubators utilizes a fixed contact thermometer. The thermometer is manufactured to place one of the contacts at a point where the mercury will touch the contact at a specific temperature. The thermometer is used in conjunction with a mechanical relay which is connected in a normally closed position. In this manner the power to the heater is on until the mercury column touches the fixed contact, allowing current to flow and open the relay contacts, shutting off the heat supply. As temperature starts to decrease and the mercury column touches the fixed contact, allowing current to flow and open the relay contacts, shutting off the heat supply. As temperature starts to decrease and the mercury falls away from the contact the heater power is reactivated. This operation continues in a on-off cycle fashion to maintain the proper temperature. No adjustments to the temperature setpoint can be made unless another fixed temperature contact thermometer or adjustable type contact thermometer is used. This type of temperature control system can work very well, but an accurate contact thermometer must be used, with a reliable relay. Its location within the incubator is important as well.

Proportional type temperature controllers are also available. This controller when properly matched to an incubator supplies only partial power to the heater in order to maintain a predetermined temperature. These devices can be excellent temperature controllers, but can be tricky to operate and are generally more expensive than the others.

In all of the temperature control systems discussed there remains room for error which most users are unaware of. If you want to maintain an ideal constant internal incubator temperature you must control a delicate heat balance of the system. The best temperature control system ever invented cannot take into account all of the variables that require control. As an example, suppose you have a small plastic type incubator and have reached excellent temperature equilibrium with six eggs inside and a constant ambient temperature of 70°F, Next let us suppose the ambient temperature decreases 10 to 15°F. This will undoubtedly increase the heat loss thru the plastic and cause a decreased internal temperature within the incubator. It will necessitate readjustment of the temperature control to rectify the problem. However, when the ambient temperature returns to 70°F the heat loss will decrease and the internal temperature will rise.

Another common method of upsetting the delicate heat balance is to add a significant amount of mass (several dozen cool eggs) to the incubator. This will cause the heat load to be significantly different. The average internal temperature will decrease immediately and probably will not achieve the original setpoint temperature after a substantial amount of time. an adjustment to the temperature controller will be required to acquire the original setpoint temperature. There are a significant number of variables which can affect the delicate heat balance of an incubator. Voltage dips caused by power companies can effectively reduce the power to your heater, resulting in a reduced heat output.

Another troublesome area of concern for breeders who operate incubators is the measurement and control of humidity. There are a number of ways to measure humidity, but only the wet and dry bulb method will be discussed in detail because it is the most common method used in incubators today. Humidity measurements (wet-dry psychrometry) are indirect in nature – they must rely on the two temperature measurements and the use of a psychrometric chart. The basic principle is the depression in temperature caused by the thermal energy required to evaporate water from a moistened wick surrounding a thermometer bulb (wet bulb). The air to be measured must flow around the wick (minimum air flow velocity should be 2-3 meters/ second) and must be adiabatically saturated. The resulting temperature drop on the wet bulb thermometer depends on the degree of saturation of the air. A measurement of the ambient temperature (dry bulb) allows determination of the moisture condition of the air by use of Mollier’s diagram, a psychrometric chart, or special tables.

Although this method can theoretically be accurate (±2% R.H. is realistic) and is superficially simple, it does require attention to details. The following factors are all sources of possible error:
1. Temperature difference between the 2 thermometers when dry.
2. Accuracy and resolution of both thermometers.
3. Insufficient and irregular air flow velocity.
4. Loose fitting wick.
5. Radiant heat effects.
6. On the continuously wetted wick, heat transfer from wet-bulb reservoir.
7. Impurities in the water.
8. Presence of dust or chemicals in the air (diagrams are based on clean air).
9. Barometric pressure (diagrams are established at a standard pressure).
10. Accuracy of interpretation of the diagrams and charts.

Each type of egg requires different humidity conditions in order to optimize the weight loss or increase of air cell size within the egg. Periodic weighing or candling of the egg during the incubation period are the best methods of determining if the humidity is correct for a given egg. If you have various species of bird eggs which require the same temperature but different humidity requirements, you will be forced to use another incubator to obtain the proper environmental conditions.
If you know the ideal temperature and relative humidity requirements (specific values, not broad ranges) fora given type of egg, the correct wet bulb temperature can be determined from a psychrometric chart as shown in Figure 7.

Humidity control within an incubator is generally controlled by slowly adding water to containment areas in the bottom of the incubator. The amount of water surface area, incubator internal temperature, barometric pressure, the airflow (in and out) and the ambient air humidity will all have, 4 significant effect on the humidity within the incubator. Assuming all are relatively constant, most regulation is done by increasing or decreasing the water surface area and/or regulating the amount of air into and out of the incubator. Although precision humidity control is not required for most eggs, reasonable control of ±4% R.H. is not difficult to attain.


Two other minor incubator-related problem areas are adequate ventilation (oxygen starvation) and egg tipping or turning. Most incubators have been designed to ensure adequate ventilation provided the operator does not severely restrict the air vents. A power loss to the incubator will not only inhibit the proper temperature environment but oxygen starvation will result (no air flow when the fan is not operating) if the incubator is not opened adequately to increase oxygen supply.

Egg turning is very critical to most eggs to promote proper embryo development. Incubators with automatic turning mechanisms are very desirable. Most incubators rotate or tip the eggs on a frequency of once an hour or one half hour. If you are required to turn the eggs manually, your ability to keep a rigid frequent turning schedule will be severely taxed. Infrequent or irregular egg turning will generally produce deformed birds.

Another area of concern for breeders who desire high percentage hatches is the number of incubators that will be required. One incubator should be dedicated to hatching the eggs. When the chick has pipped it should be immediately transferred to another incubator which has a higher humidity and slightly decreased temperature. At this stage of the bird’s young life it is producing heat and can tolerate a slight lower temperature. The increased humidity will assist in softening of the egg in shell and permit easier hatching to occur. The hatching process leaves a messy residue which can provide breeding areas for bacteria. Once the hatching is completed the incubator should be thoroughly cleaned in preparation for the next batch of eggs. If hatching occurs in the primary incubator, these bacteria could ruin the remainder of the eggs within it.

In summary, I have tried to discuss incubator-related problem areas with practical solutions. I hope some of this advice will be of benefit to those who would like to be more successful breeders. The successful breeders have already learned most of the pitfalls discussed and have overcome them. Although many breeders have difficulties with incubators, prudent attention to quality instrumentation (thermometers), temperature control, and incubator care and operation will improve their hatching success. It is a most joyous reward to know you have played a vital role in the birth of one of god’s beautiful feathered treasures.

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