1
Measuring Relative Humidity – Lab
Objectives
1. To measure relative humidity using a sling psychrometer (hygrometer).
2. To understand use of the saturation curve graph.
3. To determine humidity characteristics under varying atmospheric conditions.
Materials Supplied

Sling psychrometer (hygrometer) for measuring relative humidity

Instructions for use of sling psychrometer and tables for interpretation
Humidity
Air (such as the atmosphere) can hold up to a certain amount of water vapour (water in a
gaseous state) but the amount varies depending on the temperature. Humidity is a general
term that refers to the amount of moisture in air. Some other important terms to know
when dealing with moisture in the atmosphere are:
Specific Humidity (SH): the actual quantity of water vapour in the air, in grams per
kilogram (g/kg). It can be found by: SH = RH (%) 1324 100 x MSH
Maximum Specific Humidity (MSH): the maximum quantity of water vapour that could
be held in the air at a given temperature (g/kg). If the air is unsaturated, the SH is less
than the MSH.
Relative Humidity (RH): the ratio of SH to MSH, expressed as a percentage:
Specific Humidity
RH (%) = ----------------------------------Maximum Specific Humidity
x
100
Dew-Point Temperature (DT): the temperature at which air saturation and
condensation occur for a given value of specific humidity. Condensation is the change of
water from a gaseous state to a liquid state.
Saturation Curve: a graph (p. 4) showing the relationship between air saturation and
temperature. Once the saturation point is reached, the RH is 100% and no more water
vapour can be evaporated into the air.
For reference, an example using the Saturation Curve Graph is given. Follow the example of
how to correctly read the graph.
a.
b.
c.
d.
e.
f.
A sample of air is collected and determined to lie at Point A on the graph.
The air temperature is 30C.
The SH is 10.0g/kg (grams of water vapour per kilogram of air).
The MSH is 27.5g/kg.
The RH is 36% ([10.0 / 27.5] x 100, rounded to the nearest %).
The DT is 13C.
2
Measuring Relative Humidity
Background

A device for measuring relative humidity is called a sling psychrometer.

The paragraphs below describe the sling psychrometer and its operation – read carefully.
The Sling Psychrometer

The sling psychrometer contains two thermometers housed in a plastic casing attached to
a handle.

The casing is designed to be spun around the handle, vigourously.

The upper thermometer is a standard mercury thermometer with a Celcius temperature
scale – this is the dry-bulb thermometer.

The lower thermometer is similar, except it is covered by a wick at the far end- this is
the wet-bulb thermometer. If the wick is dry, unscrew the plastic cap and add water to
the reservoir.

The difference between the dry bulb and wet bulb temperatures is called the wet-bulb
depression (or depression of the wet bulb).
How the Sling Psychrometer Works

When the psychrometer is spun, evaporation causes the wet-bulb temperature to be
lowered.

The amount of evaporation from the wick is related to the relative humidity (RH). The
amount of evaporation is determined by the amount of water vapour already in the air
(the specific humidity) compared to the maximum amount of water vapour that can be
held at that temperature (maximum specific humidity).

If the RH is high, there will be relatively little evaporation and the dry and wet bulb
temperatures will be close to each other.

If the RH is low, there will be relatively more evaporation and the dry and wet bulb
temperatures will be further apart.
Using the Psychrometer

To use, spin the psychrometer vigourously for 30-40 seconds.

Read off the two temperatures to the nearest half degree and calculate the wet-bulb
depression.

On page 3 is a chart that will tell you the relative humidity for the air temperature you
measure (dry-bulb temperature) and the corresponding wet-bulb depression.

Read the chart down to the dry-bulb temperature you recorded, and across to the wetbulb depression to obtain the relative humidity value.

For example, if you measured a dry-bulb temperature of 34C and a wet-bulb
temperature of 24.5C, the wet-bulb depression is 9.5C and the relative humidity is 46%.

On the chart, if the exact dry-bulb temperature you recorded is missing, you must
interpolate between given values.
3
4
GEO 10
- Lab Assignment
5
Before Moving On

It is essential to understand the information provided up to this point, before moving
on. Three points that frequently require clarification are:
o
Dry-bulb temperature on the psychrometer is the air temperature.
o
Wet-bulb temperature is not the same as wet-bulb depression.
o
Wet bulb depression is the difference between dry-bulb and wet-bulb
temperatures. (How much lower, or “depressed”, is the temperature of the wetbulb compared to the dry-bulb?)
Practice Exercises Using Given Sling Psychrometer Values
Work out the relative humidity assuming the following temperatures were measured off a
sling psychrometer:
Dry-Bulb
Temperature (C)
Wet-Bulb
Temperature (C)
30
26
5
2
45
27.5
9.5
Wet-Bulb
Depression
Relative Humidity
(%)
4.5
Answers are given on page 8.
Using the Saturation Curve Graph
Saturation Curve Graph

On page 4 is a copy of the Saturation Curve graph. This graph can be used to
understand the relationship between temperature and humidity (including specific
humidity, maximum specific humidity, and relative humidity).

The curve on the graph represents the maximum specific humidity for a given
temperature. Therefore the curve shows the capacity of air at different
temperatures to hold moisture in the form of water vapour (gaseous form).

The vertical scale represents both specific humidity and maximum specific humidity
which are expressed in the same units.
Relative Humidity on the Saturation Curve Graph

Recall from the previous section that relative humidity is the ratio between specific
humidity and maximum specific humidity expressed as a percentage.

It is possible for the specific humidity to be lower than or equal to, but not higher
than, the maximum specific humidity. A point on the graph can only lie on or below
the line.
GEO 10
- Lab Assignment
6

A series of values will be used to demonstrate the relationships between temperature
and humidity of air.

In this example, the temperature of the air changes, forcing changes in the relative
humidity values. A starting point was selected with an air temperature of 20C and a
specific humidity of 10 g / kg.
Air
Temperature
(C)
Specific
Humidity
(g / kg)
Maximum
Spec. Humidity
(g / kg)
Relative
Humidity
(%)
Dew-Point
Temperature
(C)
20
10
15.5
65
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Relative Humidity Measurements

In a group of four people, use the sling psychrometer to measure the relative humidity in
three locations on the campus:
1. In classroom
2. In the cafeteria (or a another place in the school)
3. Outside, well away from any buildings or building entrances

Spaces are provided below to record your measurements and make calculations about the
humidity conditions (using the relative humidity results and the saturation curve graph).
Location 1: Classroom
Dry-Bulb Temperature
________________
Wet-Bulb Temperature
________________
Location 2: __________________
Dry-Bulb Temperature
________________
Wet-Bulb Temperature
________________
Location 3: Outside, away from buildings
Dry-Bulb Temperature
________________
Wet-Bulb Temperature
________________
GEO 10
- Lab Assignment
7
Location 1: Classroom
Wet-Bulb Depression
________________
Relative Humidity
________________
Maximum specific humidity for this air temperature
________________
Specific humidity (SH = RH / 100 x MSH)
________________
Dew-point temperature
________________
Location 2: ___________________
Wet-Bulb Depression
________________
Relative Humidity
________________
Maximum specific humidity for this air temperature
________________
Specific humidity (SH = RH / 100 x MSH)
________________
Dew-point temperature
________________
Location 3: Outside
Wet-Bulb Depression
________________
Relative Humidity
________________
Maximum specific humidity for this air temperature
________________
Specific humidity (SH = RH / 100 x MSH)
________________
Dew-point temperature
________________

At which location is the relative humidity highest?
______________

Which location has the greatest amount of water vapour in the air?
______________

If the answers to the preceding two questions are not the same, give an explanation of
why this is so. (If the answers are the same, assume for this question that they are not
the same.)

Plot your values on the Saturation Curve graph to see your values expressed visually.
GEO 10
- Lab Assignment
8
Answers to Practice Exercises

Relative humidity measurements using given sling psychrometer readings:
Wet-Bulb
Depression (C)
Relative Humidity
(%)
4
73
3
58
17.5
26
5

43
(average of
44 and 42)
Relative humidity readings in three locations:
o
It is not possible to give “answers” for your readings and calculations, since the
answers you get will be dependent on the actual conditions of temperature and
humidity in the air.
o
Rely on careful measurements, understanding the procedures, and careful
calculations to determine whether your answers are correct.
o
It is easy to tell which location has the highest relative humidity from a comparison of
your calculations.
o
“The amount of water vapour in the air” means the specific humidity; which location
has the highest specific humidity?
o
It is possible for the location with the highest relative humidity to not have the
highest specific humidity. Plot your values on the Saturation Curve graph to see your
values expressed visually. Before leaving this exercise, make sure you have a good
understanding and explanation of this possibility.