The Wet Bulb
- Fortunately, you do not
have to go through what I went through to get this right! I am not a scientist so at first, this was a subject
on which the more I read, the more daunting the task became. When I first looked up Relative Humidity on the
web, this is what I came across:
The amount of water vapor in the air at any given time is usually less than
that required to saturate the air. The relative humidity is the percent of saturation humidity, generally
calculated in relation to saturated vapor density.
The most common units for vapor density are
gm/m3. For example, if the actual vapor density is 10 g/m3 at 20°C compared to
the saturation vapor density at that temperature of 17.3 g/m3 , then the relative
Like I said, I am not a
scientist – I just wanted to incubate a few chicks!!!
- So the search went on
until I found my answers. And they are provided here for you.
- Okay so what is relative
humidity? It is the amount of moisture in the air at any given temperature and measured in
- When incubating eggs, the
relative humidity that you need to maintain in the incubator insures that the eggs do not dry out and kill the
developing embryo inside; and at hatching time do not get too great as to make the shell too soft for the chick
to break out of.
- The easiest way to
measure RH is a good hygrometer, so here is the wet bulb method.
- To put it simply, air
humidity can be closely estimated by measuring;
Dry Bulb Temperature
Wet Bulb Temperature
- Dry bulb temperature can
be measured with an ordinary thermometer. The wet bulb thermometer is the same kind of thermometer with some
wet cloth, cotton or similar, around the base of the bulb.A white shoelace works perfectly! Cut off the end bit
then, with a piece of thread, tie the lace to the thermometer at the bulb or probe. This wick is placed in the
water of the incubator and read from there. There needs to be a continuous air flow over the bulb and wick of
the wet bulb thermometer. Easy in a fan forced unit.
- The difference in the
measurement of the two readings will give a relative humidity estimate when computed on a psychometric or
- Yeah – That's what I
thought!!! Ha! There's always an easier way so read on................ The hard part is done for
- The following is a graph
showing both dry bulb and wet bulb measurements including the Relative humidity for each
- The top row is
the dry bulbtemperature (Green).
Rednumbers are the wet
- The far left row
(Blue) is the relative humidityin
- Take a dry bulb reading
and a wet bulb reading inside your incubator. If using a digital thermometer, make sure the probe is waterproof
before attempting a wet bulb measurement. You will have to place the wet bulb probe inside the incubator and
leave it there for awhile to get a good reading. During incubation it is okay to leave it there the entire time
as it will not hurt anything.
- When you have the
measurements simply compare them to the table. There is no excuse now for you not knowing the RH in your
- Wet bulb Reading for incubation
37.2 37.7 38.3
26.9 27.4 27.9
28.0 28.5 29.0
29.1 29.6 30.1
30.2 30.7 31.2
31.1 31.6 32.2
32.0 32.6 33.1
99 100 101
80.5 81.3 82.2
82.5 83.3 84.2
84.5 85.3 86.2
86.5 87.3 88.2 89.0
88.0 89.0 90.0
89.7 90.7 91.7
- Let's say you take a dry
bulb reading. It should be, during incubation 37.8C (100.4F). Now, take a wet bulb reading ; 28.7C
- Your relative humidity is
between 50% and 55%.
- Consider the dry bulb
temperature first. The wet bulb measurement is placed vertically under this reading.
- So, there ya go! You not
only know what it is but how to get it.