Lab 5 Relative Humidity PDF

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Lab 5: Measuring Relative Humidity METR 1300 Intro to Meteorology The amount of water vapor present in the air is dependent upon temperature. Humidity is a general term that refers to the amount of water vapor in air. Relative humidity is a ratio of the amount of water vapor present to the amount possible based on temperature. Some other important terms to know when dealing with moisture in the atmosphere are: – Specific Humidity (SH) or “mixing ratio”- amount of water vapor present in a given mass of air (g/kg) – Maximum Specific Humidity (MSH) or “saturation point”- total amount of water vapor possible at a given temperature (g/kg) – Dew Point (degrees C or F)- The temperature at which saturation is reached, and condensation will begin (100% relative humidity) – Equation to calculate relative humidity; Specific Humidity – RH (%) = ----------------------------------- x 100 Maximum Specific Humidity

– Equation to calculate specific humidity; RELATIVE HUMIDITY – SH (G/KG) = --------------------------- x MSH (G/KG) 100 PART 1: Using the Saturation Curve

Example; Point A Specific Humidity 20 g/kg @ 30°C Maximum Specific Humidity 28 g/kg

20/28*100= 71% relative humidity

Using the saturation curve, at each point, find the following information;

Air Temperature (look at the x axis) Specific Humidity (look at the y axis) Maximum Specific Humidity (go up from your point until you hit the 100% curve, then look at the y axis) Relative Humidity (use the equation) Dew Point (go left from your point until you hit the 100% curve, then look at the x axis)

Point

TEMP (C)

SH (g/kg)

MSH (g/kg)

RH (%)

DP (C)

A

30 C

20

28

71%

25

B

15 C

10

10

100%

15

C

40 C

10

50

20%

15

D

45 C

35

70

50%

35

E

50 C

5

95

5%

5

F

50 C

70

95

74%

45

Part 2: Using the Sling Psychrometer

The sling psychrometer contains two thermometers housed in a plastic casing attached to a handle.

o One is a standard thermometer, this is the dry-bulb thermometer. o The other thermometer is covered by a wick at the far end- this is the wet-bulb thermometer. The difference between the dry bulb and wet bulb temperatures is called the wet-bulb depression (or depression of the wet bulb).

Using the Psychrometer - To use, dip the wet-bulb in distilled water (do not soak!). Spin the psychrometer vigorously for 30-40 seconds. -

Read off the two temperatures to the nearest half degree and calculate the wetbulb depression.

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On page 4-5 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 wet- bulb depression to obtain the relative humidity value.

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For example, if you measured a dry-bulb temperature of 34C and a wet-bulb temperature of 24.5C, the wet-bulb depression is 9.5C and the relative humidity is 46%.

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On the chart, if the exact dry-bulb temperature you recorded is missing, you must interpolate between given values.

How the Sling Psychrometer Works -

When the psychrometer is spun, evaporation causes the wet-bulb temperature to be lowered.

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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 vapor already in the air (the specific humidity) compared to the maximum amount of water vapor that can be held at that temperature (maximum specific humidity).

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If the RH is high, there will be relatively little evaporation and the dry and wet bulb temperatures will be close to each other.

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If the RH is low, there will be relatively more evaporation and the dry and wet bulb temperatures will be further apart.

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 wet- bulb compared to the dry-bulb?)

Practice Exercises Using Given Sling Psychrometer Value Work out the relative humidity assuming the following temperatures were measured off a sling psychrometer: Remember; Depression is the difference between the dry and wet bulb temperatures; Relative Humidity can be found using the dry bulb temperature and depression on the charts on page 4-5.

Dry-Bulb Temperature (C)

Wet-Bulb Temperature (C)

Wet-Bulb Depression

Relative Humidity (%)

30

26

14

72.8 %

5

2

3

57.8 %

45

29.5

17.5

24.6 %

9.5

4.5

5

42%

Relative Humidity Measurements -

In a groups of 2-4 people, use the sling psychrometer to measure the relative humidity in three locations on the campus: 1. In classroom 2. In another place in the school (hallway, bathroom, other building, etc.) 3. Outside, well away from any buildings or building entrances

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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 Find these using the Sling Psychrometer;

Dry Bubl Temp:

21 C

Wet-Bulb Temp: 18 C Then Find these: Wet Blub Depression:_____ 3 C _______ Relative Humidity _______74.8 % _________ (Use the relative humidity table on pages 4-5) Maximum Specific Humidity for this air temperature __ 18 g/kg (Use the Saturation Curve on page 8!) Specific humidity (SH = RH / 100 x MSH) : 748/100 x18= 13.4 g/kg Dew-point temperature 16 C (Use the Saturation Curve on page 8!)

Location 2: Hallway__ Dry-Bulb Temperature

21 C

Wet-Bulb Temperature

14 C

Wet-Bulb Depression

7C

Relative Humidity

48% Rh

Maximum specific humidity 18g/kg Specific humidity

8.1g/kg

Dew-point temperature

8.7

C

Location 3: Outside, away from buildings Dry-Bulb Temperature

12 C

Wet-Bulb Temperature

9C

Wet-Bulb Depression

3C

Relative Humidity

67%

Maximum specific humidity

9g/kg

Specific humidity

6g/kg

Dew-point temperature

6.1 C

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At which location is the relative humidity highest? Classroom 74.8%

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Which location has the greatest specific humidity? Classroom 13g/kg

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Plot the specific humidity and dry-bulb temperature for each location on the Saturation Curve below.

Classro Hallwa Outsid e...