What is Humidity Presentation PDF

What is Humidity?

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What is Humidity?

What Is Humidity?Objective: To define the various terms used when talking about humidity and relate them to humidification in the hospital environment. This presentation is important as the terms used will become relevant in subsequent presentations.

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Forms of WaterIce+ heat / energy

Water+ heat / energy

Water vapour

Forms of WaterObjective: To introduce concepts of water and energy.

There are three states of water:Ice (solid state) - consists of water molecules that are tightly bound each other,preventing any movement. When energy is added to ice in the form of heat, it turns into liquid water. Water (liquid state) - consists of clusters of water molecules loosely bound to each other. If energy is added to liquid water in the form of heat, it turns into water vapour. Water vapour (gaseous state) - consists of individual water molecules which are not bound to one another and move independently within a gas.

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• Upper airways add heat and moisture to inspired air

• Moisture is added as water vapour

• In hospitals moisture is added as

– Water vapour– Aerosols– Saline

Forms of Water in the Airways

LIQUID WATER

LIQUID WATER DROPLET

WATER VAPOUR

Forms of Water in the AirwaysObjective: To introduce the different ways water can be added to the airway, both physiologically and in the hospital.

In the upper airway, heat and moisture are added to inspired air. Moisture is added as water vapour.In the hospital environment, moisture or water can be added to the airways in different forms:Water vapour ʊ such as that seen with heated passover humidifiersLiquid water droplets, or aerosols ʊ such as those produced by nebulisers or bubble through humidifiersAnd liquid water ʊ such as that added when instilling saline

Does the form of water added to the airway make a difference?

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sLiquid Water and Water Vapour

•Passover humidifiers create water vapour which cannot transport bacteria or viruses

•Nebulisers create liquid water droplets which can transport bacteria or viruses

NEBULISED LIQUID WATER 1 - 40 MICRONS

BACTERIA 0.2 - 10 MICRONS

VIRUS 0.017 - 0.3 MICRONS

WATER VAPOUR 0.0001 MICRONS

1Hamill et al., 1995; 2Orec et al., 1997

Liquid Water and Water VapourObjective: To introduce bacteria and viruses, how they are created and their relative sizes. To tell the audience that different devices have different contamination aspects.

The size of the different forms of water does make a difference in the ability to carry pathogens. Aerosols (liquid water droplets) produced by nebulisers are 1 - 40 microns in size. Bacteria and viruses vary in size from 0.017 - 10 microns, which is less than the size of some aerosols. Therefore, it is possible for the aerosols to carry pathogens to the patient. Water vapour molecules are thousands of times smaller than bacteria and viruses, so they are incapable of transporting pathogens. The best way of adding moisture to the airways is through water vapour, as this cannot transport pathogens.

1Hamill et al., An outbreak of Burkholderia (formerly Pseudomonas) cepacia respiratory tract colonization and infection associated with nebulized albuterol therapy. Ann Intern Med 1995;122:762–6.2Orec et al., Mechanisms of bacterial movement in ventilator circuits. Anaesth and Intensive care 1997;25(5):568.

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Humidity

•Humidity is water vapour in a gas

• It is expressed in terms of:– Absolute humidity– Relative humidity– Dewpoint

HumidityObjective: To introduce the three terms used to define humidity. Tell the audience that we will use these terms through the day so they are very useful to remember.

Humidity is water vapour in a gas.We can express humidity in three ways: Absolute humidity - the actual amount of water vapour per litre of gas, measured in mg/L.

Relative humidity - the measure of how much water vapour is actually in the gas, compared to its capacity to hold water vapour. Relative humidity is measured as a percentage (%).

Dewpoint - the temperature where the gas is at 100% relative humidity (100% full) or saturated, measured in degrees Celsius. If a gas cools below this temperature, excess water vapour is lost as condensation.

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Absolute Humidity

(mg/L)

Is the actual amount ofwater vapour in a litre of gas

1L

Absolute HumidityObjective: To define the term absolute humidity

Absolute humidity is the actual amount of water vapour per litre of gas.It is measured in milligrams of water vapour per litre of gas. If we look at this bottle, which represents a litre of gas , it contains 22 mg of water vapour. Therefore, its absolute humidity is 22 mg/L.If more water vapour is added to the litre of gas, its absolute humidity will increase. There is however, a limit to the amount of water vapour a gas can hold and this is called its maximum capacity (to be covered later).

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Relative Humidity

•Expressed as a %• Is the actual amount

of water vapour in a gas compared to the maximum amount this gas can hold

44 mg/44 mg = 100% RH22 mg/44 mg = 50% RH

100% 50%

44 mg 22 mg

Relative HumidityObjective: To define the term relative humidity.

Relative humidity is a way of describing how saturated (full of water) a gas is.That is, how much water vapour is actually in the gas compared to its maximum capacity. We express this as a percentage. As we learnt in school, to work out a % we use the formulae% = Content/capacity x 100

For example, if we look at the bottle on the left which represents one litre of gas, it contains 44 mg of water vapour (content). At maximum capacity of a gas to hold water vapour at 37 °C is 44 mg per litre (capacity), therefore the gas is full or 100% relative humidity.The bottle on the right represents another litre of the same gas. It too could hold a maximum of 44 mg of water vapour (capacity), but has only 22 mg (content). % = Content/capacity x 100% = 22mg/44mg x 100 = 50%The gas is then only 50% full or is at 50% relative humidity.The maximum capacity, however, is not constant, it changes with temperature.

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•Warming the gas increases its maximum capacity to hold vapour

•Cooling the gas reduces its capacity to hold vapour

Maximum Capacity

Temperature (oC)

44

30

30 37

Absolu

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mg/L)

Maximum Capacity

Objective: To introduce the relationship between temperature and capacity to hold

water vapour - warming a gas increases its ability to hold water vapour

The maximum capacity of a gas to hold water vapour changes with temperature.

Warming a gas will increase its maximum capacity to hold water vapour while

cooling a gas will reduce its maximum capacity.

This graph shows how the maximum capacity (red line) of a gas varies with

temperature. For example;

At 30 °C, the gas can hold a maximum of 30 mg/L. At 37 °C the gas can hold a

maximum of 44 mg/L. If the gas is warmed from 30 °C to 37 °C, its capacity to hold

water vapour increases from 30 mg/L to 44 mg/L.

If the gas is then cooled from 37 °C back to 30 °C its capacity to hold water vapour

reduces from 44 mg/L to 30mg/L.

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Temperature and Capacity

30 mg/L32 mg/L

34 mg/L36 mg/L

38 mg/L40 mg/L

42 mg/L44 mg/L

30 °C 31 °C 32 °C 33 °C 34 °C 35 °C 36 °C 37 °C

Temperature & CapacityObjective: To reinforce the concept that the warmer a gas is, the more humidity it can hold.

The capacity of a gas to hold water vapour changes with temperature.This graph shows that as a gas is warmed it can hold more water vapour. At 30 °C the gas can hold a maximum of 30 mg/L. At 37 °C, the gas can hold a maximum of 44 mg/L.

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Dewpoint

•Expressed in °C

• Is a temperature where the gas is at 100% relative humidity (RH)

•Below this temperature water vapour is lost as condensation

Dewpoint

Objective: To introduce the concept of dewpoint. To help the audience

understand how and why condensate forms.

The third way of expressing humidity is dewpoint.

Dewpoint is a temperature where the gas is at 100% relative humidity. Below this

temperature water vapour is lost as condensation.

To illustrate dewpoint, think of a bottle that you take out of the refrigerator. This

bottle will be colder than the ambient temperature of the room. When the air in the

room touches the cold glass, it cools down. The room air can no longer hold the

same amount of water vapour at this colder temperature and the lost water

vapour appears as condensate on the glass.

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15 °C2% RH

0.3 mg/L

37 °C100% RH44 mg/L

40 °C86% RH44 mg/L

22 °C100% RH20 mg/L

AH, RH & Dewpoint

AH, RH & DewpointObjective: To use a practical example to bring the terms together. To use this example to explain how a heated humidifier and heated circuit works.

Let’s work through how heating & cooling a gas changes the absolute humidity, relative humidity and dewpoint of a gas. A container of dry medical gas is cold and virtually devoid of any moisture. If we heat the gas to 37 °C and add humidity to achieve 100% relative humidity, the gas can hold 44 mg of absolute humidity per litre of gas. At this temperature, the gas is at its dewpoint (37 °C). This is what happens in a chamber.If the gas is heated to 40 °C and no water vapour is added ʊ as would occur in a heated circuit ʊ the amount of water vapour stays the same. Its capacity to hold water vapour increases to 51 mg/L, therefore the relative humidity will reduce to 44/51 x 100 = 86%.Cooling the gas back to 37 °C will increase the relative humidity back to 100%. If the gas is cooled further the maximum capacity reduces and condensation will form.