Hygrometer & Thermometer in Base of Forced Air Incubator

Understanding and Using Hygrometers to Monitor Relative Humidity Will Greatly Improve your Hatch Rate.

What is Relative Humidity?

Relative humidity (RH) is the moisture content (saturation) in the air in relation to a 100% saturation level based on temperature.

Higher temperatures can achieve higher saturation levels (RH).

Why is RH important to incubating and hatching eggs?

Eggs are porous. Moisture enters and leaves the shell through humidity.

Humidity levels that are too low can cause your eggs to dry out too much leading to unhatchable eggs or chicks that are too small and weak to survive.

Humidity levels that are too high can lead to the chick growing too large in a confined space leading to breathing issues as well as hatching issues.

For these reasons, having the correct relative humidity is one of the most important factors in achieving a good hatch rate.

Unfortunately RH is also one of the hardest factors to control. Luckily there’s a little wiggle room when it comes to RH.

What is a Hygrometer?

This is a great question; It is an instrument that is used to calculate relative humidity.

They're also known as relative humidity gauges. (Fun fact: Leonardo Da Vinci is credited with building the first one in the 1400’s.)

NOTE: In the picture above, they have the lid off of the incubator so you can see the digital hygrometer and thermometer better, when checking your RH, you'll want to have the lid on your incubator and look through the case if possible.


Managing Relative Humidity

The best way to manage RH is by using a hygrometer. There are a few types. There are electronic (digital) models that read both the temperature and RH.

These are nice because you don’t have to do the calculation and many of them give you a digital readout of both temperature and RH which is important for maintaining the proper environment in your incubator.

Another nice feature is that many of them have a probe attached to the gauge that you can insert in the incubator while the gauge is outside for easy access for readings.

The down side to these is that you often get what you pay for and inexpensive ones may not be too accurate and the expensive ones while much more accurate, are… well, expensive.

One of the most common types of RH gauges used in incubation is the wet & dry bulb psychrometer. These consist of two thermometers mounted side by side. One thermometer is dry and the other thermometer is considered wet.

These are inexpensive and fairly accurate so they’re frequently used for incubating eggs. You can also use the dry bulb thermometer to monitor the temperature within the incubator.

This type does require that you compare both the wet and dry numbers to come up with RH. This can be done with a chart or by calculation. Here is a nifty RH calculator.

Another type is the mechanical (analog) variety. These often use a blonde human hair held at slight tension by a spring to determine RH. With this type, you will need a temperature thermometer as well to monitor your incubator’s environment.

Regardless of which method you choose, you’ll want to make sure your gauge is accurate and correctly calibrated.

Directions for Calibrating your Hygrometer

Both analog and digital hygrometers can be calibrated using the salt test.

The principal behind the salt test is that a small amount of salt mixed with a little water in a sealed environment will always create 75% relative humidity.

There are 4 items that you’ll need to do the salt test:

  • Regular table salt
  • Clean water
  • A small container like a cap from a gallon of milk
  • A see through, seal-able container like Tupperware or a Zip-Loc bag

Once you've got all the necessary items, there are 5 steps to set up the salt test.

  • Put a small amount of salt in your cap until it’s about 2/3 of the way full.
  • Add drops of water until the salt is like wet sand. You don’t want it too wet or slushy.
  • Put the salt filled container along with your gauge in your sealed container.
  • Wait about 12 hours and check the reading while it’s still in the sealed container. It should read 75% RH.
  • Make any necessary adjustments. If it's reading 75%, you’re good to go. If not, than you’ll need to use the supplied adjusting screw to dial your hygrometer to 75%. On Digital models that are adjustable, these are usually marked. On analog models, the adjustment screw is normally on the back. You access it through a small hole using a small standard screwdriver. If your unit isn't adjustable, you’ll want to make a note of the variation and add or subtract accordingly. For example, if you’re reading 65%, you know its reading is 10% low and you’ll add 10% to the reading.

Hygrometers are really helpful in achieving good hatch rates and with a little practice you’ll soon be using yours like a pro.

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