UNDERSTANDING HUMIDITY IN INCUBATORS WITH AUTOMATIC HUMIDITY CONTROL.
Some confusion worldwide about these new digital control systems has come about because they are new and there is not a lot of people who have them and can give advice about them. I find 2 common responses to auto humidity control. One is to say I will set it on auto and just forget about it, the machine will do the rest. The other is to suddenly become fanatical about precision, now we can read the humidity, if it varies a couple of points something must be wrong. Both extremes may work fore a while, but both will let us down in the long run. It is better we understand what the humidity does and then see to understand how to make it work for us.
The most common method of reading humidity has been using a wet bulb thermometer. This is essentially a standard thermometer, which has a “wick” or cotton sock over the bulb at its bottom. The thermometer without the wick would read the same as the standard dry bulb thermometer. However, then the wick is placed over the bulb and wetted, the thermometer now reads differently. The water evaporating from the wick cools down the thermometer bulb so it now reads lower than the standard thermometer. The amount of cooling is dependent on the amount of water in the air of the incubator. If the air is very dry, a lot of water evaporates, so the thermometer reads a lot lower than the standard thermometer. If there is a lot of water in the air, not much evaporates off the wick, so there is not much cooling, so thee thermometer reads almost the same as the standard thermometer. This is called the WET BULB DEPRESSION. The difference between the standard thermometer and the wet thermometer directly relates to the humidity in a special chart called a psychometric chart. So we can use this to calculate the relative humidity which is what the digital controls read. In fact this is a common way of calibrating the digital control for humidity.
However this reading can be a problem. It’s the method used in most of the books on incubation, and tends to be regarded as an absolute.
” I read it in a book,” or “I saw it on the internet “, or “So and So told me this is what is correct”. The trouble is , unless the source is using the same incubator, with the same eggs, and the same calibration, the “recommended” humidity figure may be at best irrelevant, or just plain wrong..
So let me explain why.
There is only one thing that matters with humidity and only the egg can tell you. So learning how to read the egg is the single most important thing you can learn about humidity.
The egg needs to loose enough water by evaporation and respiration so that by 3 days before the egg is due to hatch the egg will be about 14% lighter in weight, or so it will be between 25% and 35% empty at the big end. So you can tell this by candling the egg with a torch, called a candler, or by using scales.
So what effects how much water evaporates out of the egg? ( we cannot effectively control the respiration, the chick is doing that)
Two things; The structure of the shell, and the humidity of the incubator. Because each species has a different shell structure, generally the humidity needed is related to the specific species being incubated. This is also a compromise when we want to do several species at the same time in the same incubator.
So now we are back to the actual humidity. But the actual humidity will depend on what sources of information I have.
But are the existing books and information sources correct? I usually say no, and this is why.
1/ It is a bit difficult to read the fancy charts so sometimes you can read it wrongly.
So the advice may be arrived at by wrong use of the charts.
2/ The advice is given by someone incubating eggs which may be very different to the shell structure of your eggs, due to genetics, diet, or age.
3/ The advice may well be from a different era. Lots of the methods from 30 years ago are now recognised as being based on a wrong understanding of the eggs.
4/ The advice may be based on incubators with different humidity systems. Most existing incubators use what I call STATIC humidity systems. That is you put water into a container, and then cover or uncover the container until you get a humidity reading which gives the correct weight loss or air cell size. This is ok, but we need to be reminded that the average humidity is all that matters. With a static system, the humidity in the incubator is the combination of the ambient humidity in the room, which may go up and down, plus the water evaporated from the eggs, plus the humidity added to the incubator from the evaporation of the water container. Now if I read the humidity in the afternoon, I will read a higher humidity than if I do it at 4am. So which reading is right? Calculating the average is somewhat difficult. If the operator is consistent, then he will adjust the humidity to achieve the right weight loss.
Also, if the machine is manual turn or semi automatic turn, then every time the incubator is opened, the humidity will escape. It will take an average machine 2-4 hours to get the humidity back to balance. If I do this 3 times a day then for a lot of the day the humidity will effectively be low, even though the temperature in the machine recovers quickly.
In the Automatic humidity machines, the control instrument is set for a particular humidity level. The control then actively adds water by evaporation to the incubator, or allows the humidity to fall as the fresh air comes into the incubator. If the outside humidity goes up, then the control doesn’t add any more moisture, but allows it to fall to the “Set Humidity”. If the outside humidity goes down the machine adds moisture, to bring it up to the set humidity. If I open the lid the machine actively adds extra moisture to quickly recover to the set figure. The result is the average humidity is the same as the reading. We can then change the setting if our eggs do not respond with the correct weight loss.
We have seen a lot of humidity figures quoted which are way above the real average figure, and these have caused a lot of confusion. A rule of thumb we have found is that the fully automatic incubators like the R-Com run 5-10% lower than the book recommendations based on the old technology incubators. So if your book says 60% start at 50%.
REMEMBER, it is easier to slow down the humidity loss by simply running high humidity, however it is very difficult if you get 2/3 of the way through and need to loose more weight, its much harder to correct.
I also noticed at the Grafton AVES conference several of the overseas speakers on incubation did not understand this relationship, yet the pictures they showed clearly told of significant humidity control problems, amongst others.