ambient condition

Frosio course handout: Section 07 Ambient conditions

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Frosio 07 Ambient conditions slide # 1

AMBIENT

CONDITIONS

Humidity

Air Temperature

Substrate Temperature


Parameters influencing the corrosion speed. Atmospheric corrosion

x Humidityx Temperaturex Concentration of saltsx Amount of air pollution,

including acid rain, soot and dust particles


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Atmospheric corrosion rate depends on humidity

Relative Humidity, %

Corrosion rate

0 20 40 60 80 100


Surface temperature of the

structure must be minimum

3 C above the dew point

of the surrounding atmosphere

Atmospheric conditions.Atmospheric conditions.

Requirement during blasting and paintingRequirement during blasting and painting


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B-24/2-4

4847-693

ISO 8502 - 4Estimating probability of condensation

Cold liquid inside hull and warm air outside causes condensation

See how frames inside keeps the cold longer


Why Temperature & Humidity are important

Frozen overnight condensation, follows

the frames of the ship

Condensation follows the contours

of the double-bottom tank


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Purpose of climatic evaluation

Evaluate the likelihood of the moisture in the air to form

condensation on a substrate


Moisture / Absolute Humidity

When water evaporates, it forms water vapour or moisture in the air

We may measure this moisture as gram water per cubic meter air (g/m3)

The maximum amount of moisture that can be contained in the air depends on the temperature of the air

Warm air can contain larger amounts of moisture than cold air

The maximum moisture that can be contained in air at the various temperatures is called the Absolute Humidity


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Absolute humidity

DEFINITION:

The maximum content of water vapour in the air at a given temperature

The content will vary with temperature


At 8o C, the

maximum moisture

content in the air is

8.28 g/m3At 20o C, the

maximum moisture

content in the air is

17.31 g/m3


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Moisture / Relative Humidity

The Relative Humidity is a measure of how much of the Absolute Humidity (maximum moisture) that the air contains, expressed as percent.

If the air contains the maximum amount of moisture, it will have a Relative Humidity of 100%

If the air contain half of the maximum amount of moisture, it will have a Relative Humidity (RH) of 50%

If the air contain a quarter of the maximum amount of moisture, it will have a Relative Humidity of 25%

Question: Will air with RH=50% at 8o C have the same moisture content as air with RH=50% at 20o C?


Relative humidity

DEFINITION:

The ratio of The amount of water vapour present in a

given volume of air at a given temperature

and

The maximum amount of water vapour which this volume of air can hold at that temperature

RH is expressed as a percentage

RH outdoors will vary


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Relative humidity, RH

% R.H.:

Water vapour in the air as percentage of the total amount water vapour the air can contain at the same temperature.

Example

Air at 20o C can contain 17.31 g. water/m3, but the actual content is measured to 15 g/m3. What is the R.H. ?

15g x 100%

17.31g

R.H. = = 86.65%


Humidity / Dew Point

Air at 20o C and 73% RH contains 12.64 g/m3 water (12.64 as percentage of Absolute Humidity 17.31)

If the temperature drops to 16o C, it will still contain 12.64 g/m3, but the RH will increase to 92.69% (12.64 as percentage of Absolute Humidity 13.65)

If the temperature drops further to 12o C, what will happen?

At 12o C the maximum moisture content (Absolute Humidity) is 10.62 g/m3. This means the moisture content of 12.64 g/m3 is 2.02 g/m3 more than it can hold at this temperature

The consequence is that the excessive moisture will leave the air through condensation, ending up as dew on available surfaces or as rain or fog

The temperature at which a given moisture content will start to condensate is called Dew Point.


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Dew point

DEFINITION:

The temperature at which the air becomes saturated with water vapour

At this temperature the RH = 100% and water will condense

The chance of condensation is high when the weather is changeable


Dew Point / Substrate Temperature

On an early morning inspection we measure the air temperature to be 8oC and the RH to be 83%

Due to cold over-night temperature, the steel temperature of a structure to be painted was measure to only 4o C

Would this cause any problem?

At 8oC and RH 83%, the moisture content is 6.87 g/m3

Although the general air temperature is 8o C, the air next to the steel surface will be cooled by the steel and perhaps only be 4o C as well

At 4o C the max moisture content is 6.36 g/m3

This means that although we may not see it clearly, there will be condensation on the steel

This is why Steel Temp. must be min. 3o C above the Dew Point!


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Temperature of substrate should be at a temperature of

min. 3oC

above dew point of the air in the vicinity

Main rule for pre-treatment and paint application:


Measuring ambient conditions

Electronic multi-function gauge

Electronic humidity gauge

Sling Psychrometer (Sling Hygrometer, Dry and Wet bulb)

(Hair hygrometer, not suitable!)

Dew point calculator

Steel thermometer


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Needs frequent calibration.

Dependent on batteries

Multi-function models will measure air temperature, surface temperature, relative humidity, and calculate dew point and 'T (*)

(*) difference between dew point and surface temperature)

Electronic humidity gauge


A dry and a wet thermometer put together in one unit.

The difference in the measured temperatures indicate the amount of humidity:

Large difference = low RH%

Small or none = high RH%

Sling Psycrometer


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ISO 8502-4Measuring the relative humidity

Photo: Sling Psychrometer for measuring:

- Dry temperature

- Wet temperature

Measure the temperature in the vicinity.

Calculate the relative humidity

Use together with dew point calculator

Cd-4932-88


ISO 8502-4Dew point calculator

For calculation of dew point

To be used together with surface temperature thermometer and sling psychrometer.

Use this frequently during pre-treatment, application and drying of the paint.

Recommendation: Every 6 hours and when weather conditions are changing

Cd-4932-89


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1. Air temperature

2. % Relative humidity

3. Steel temperature

4. Dew point

ISO 8502 ISO 8502 -- 44

HumidityHumidity

Guidance on the estimation of the probability

of condensation prior to paint application

Steel temperature min. 3 oC above the dew point


Contact thermometer

Electronic instrument for measuring the steel temperature

Other types of thermometers are also available. E.g. magnetic thermometers


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IX - diagram for determination of relative humidity (%RH)

Example:Measured valuesAir dry temp.: 14.0 CWet bulb temp.: 10.5 C

ReadRelative humidity: 65 %Dew point: 7 CAir-water content: 6.3 g/kg



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Using the IX diagram

Draw a horizontal line at the appropriate dry bulb temperature

Draw a 45o line at the appropriate wet bulb temperature

At the intersection you will determine the RH

Draw a vertical line from the intersection point all the way to the 100% line

This is the dew point


Ambient conditions, general

Painting should not be started when:

The relative humidity exceeds 85% (unless the paint manufacturer consider it safe)

The substrate temperature is less than 3o

C above the dew point


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Ways to control the climatic conditions

Dehumidifier, reduces the actual humidity in the surroundings

Heather, heat the objects (surfaces) prior to application in order to reduce the danger for condensation.

Forced ventilation is a must in order to secure a proper evaporation of the solvents from the paint film.


Ambient temperature will influence: shelf life (storage)

potlife (two-pack paints: time between mixing and no longer useable)

induction time (pre-reaction before application)

viscosity/sprayability

Steel Temperature

Ambient Temperature and Steel Temperature

Is the temperature important ? YES !

Steel temperature will affect: probability for

condensation

solvent evaporation

speed of cure (drying time)

degree of cure

recoating interval

service life of the coating


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Ventilation. Good practice

Solvent vapours

are heavier

than air

Suction from

lowest points

in enclosed areas


Ventilation. Bad practice

Solvent vapour Solvent vapour

Using an inlet fan Using an exhaust fan

Air flow Air flow

Air out

Solvent

vapoursAir in

Inlet fan, air through pipe


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Control of relative humidity and temperature

Check the atmospheric conditions before the work starts

If required: Install proper ventilation

Check the atmospheric conditions while the work is ongoing and during drying / curing

Paint technology: Humidity_control1


Temperature and humidity of air used for drying.

Supply of heated air immediately after application may lead to skin drying and entrapped solvents

Cold air will keep the film open longer and ensure proper evaporation

Avoid high air temperature (especially epoxy) High humidity will slow down the drying time Exhaust from heating equipment using propane or

paraffin oil contain water and Carbon dioxide and may create Amine sweating


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Blisters inside a tank due to poor ventilation. Entrapped solvents

Blisters sometime contain water

Rust will not be formed inside intact blisters, due to the high pH

As soon as the blisters break corrosion will start

Use cathodic protection to back up the paint system.

Cd-4934-68

ambient condition

Documents

air warm air

cold air

air pollution

maximum moisture content

moisture relative humidity

ambient conditions1frosio

ambient conditions2frosio

ambient conditions3frosio