Measuring Relative Humidity

The Wet Bulb Method

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:

Relative Humidity

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.

Rel Hum1

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 humidity is



Rel Hum2

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 percentages.
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;
  1. Dry Bulb Temperature

  2. 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 Mollier diagram.
Yeah – That's what I thought!!! Ha! There's always an easier way so read on................ The hard part is done for you.

The following is a graph showing both dry bulb and wet bulb measurements including the Relative humidity for each reading.
The top row is the dry bulbtemperature (Green).
The Rednumbers are the wet bulbmeasurements.
The far left row (Blue) is the relative humidityin percentage.
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 incubator.

Wet bulb Reading for incubation temperatures

Rel. Humidity                     Temperature C
                               37.2    37.7    38.3   38.9
45%                    26.9    27.4    27.9   28.3
50%                    28.0    28.5    29.0   29.4
55%                    29.1    29.6    30.1   30.5
60%                    30.2    30.7    31.2   31.6
65%                    31.1    31.6    32.2   32.7
70%                    32.0    32.6    33.1   33.7

Rel. Humidity                     Temperature F

                              99       100     101     102

45%                    80.5    81.3    82.2   83.0

50%                    82.5    83.3    84.2   85.0

55%                    84.5    85.3    86.2   87.0

60%                    86.5    87.3    88.2   89.0

65%                    88.0    89.0    90.0   91.0

70%                    89.7    90.7    91.7   92.7

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 (84F)
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.
Have Fun!!