Session 3 Humidity definitions & units

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1 Session 3 Humidity definitions & units 24 January 2018, Bordon Hill

2 Psychrometrics The properties of air & water vapour: Air can carry water as a vapour How much it can carry depends on its temperature The warmer it is the more it can carry The science of the subject is called psychrometrics It has some important definitions that we need to know

3 How much water can air hold? 1m 3 of air At 25 C,,?? 23g of water At 0 C,??g 5g of of water

4 How do we measure the water content of air? We refer to humidity - but there are different ways of expressing it 0 % 1/4 25 % 1/2 50 3/4 75 % % 100 Full % Pointer position is - Relative Humidity 100% level - Saturation

5 Measuring 5g g 00 % litres % litres 10g g litres % 20g litres % litres % Pointer position is - Absolute Humidity Full level - Saturated Moisture Content How much more can I get in the tank? - Humidity Deficit

6 and just to make things a little more confusing 0g 0kPa 5g 0.7kPa 10g 0.14kPa 0.21kPa 15g 0.28kPa 20g Pointer position is - Vapour Pressure Full level - Saturated Vapour Pressure How much more can I get in the tank? - Vapour Pressure Deficit

7 To extend the analogy With petrol we aim at keeping the tank full With humidity we aim at keeping the tank partially empty Saturation Vapour Pressure Deficit Humidity Deficit Saturated Moisture Content Saturated Vapour Pressure Dewpoint 0g 5g 10g 15g 20g Condensation

8 One more thing The size of the petrol tank changes with temperature 80% RH 91% 4.6g 2g HD 89% 3.6g 85% 2.6g 80% 2g 10g 13g 18g 23g 10 o C 15 o C 20 o C 25 o C

9 Important definitions Saturated Moisture Content (SMC) - Maximum amount of water vapour that air can hold (g/m 3 ) Absolute Humidity (AH) - Actual amount of water vapour in the air (g/m 3 ) Humidity Deficit (HD) - HD = SMC - AH (g/m 3 ) Relative Humidity (RH) - RH = AH/SMC x 100% Vapour Pressure Deficit (VPD) - Like Humidity Deficit but expressed as a water vapour pressure (kpa)

10 Growers use different control measurements Ornamentals growers tend to use - Relative Humidity - High Relative Humidity = BAD Edible growers tend to use - Humidity Deficit - Low Humidity Deficit = BAD Some use - Vapour Pressure Deficit (VPD) - Low VPD = BAD

11 Properties of air Psychometric chart

12 Mollier diagram Remove water Add water RH % Temp ( o C) Heat kpa Cool AH ( g / m 3 ) Absolute Humidity (g/m 3 )

13 Now we ll talk about measuring boxes

14 Mollier diagram Saturated moisture content RH % Temp ( o C) kpa FEC Services Ltd 2007 AH ( g / m 3 ) 14

15 Saturated moisture content The effect of temperature Temperature Saturated Moisture Content (g/m 3 ) ( o C) Changes are not linear

16 Mollier diagram Absolute moisture content RH % Temp ( o C) kpa FEC Services Ltd AH ( g / m 3 )

17 Mollier diagram Humidity Deficit RH % Temp ( o C) kpa Humidity deficit AH ( g / m 3 )

18 Humidity Vapour pressure: Air is a mixture of - Nitrogen - Oxygen - Water vapour - A few other bits & pieces Atmospheric pressure - 1 Bar = 101kPa - The total of the pressures exerted by each part Vapour pressure - The pressure exerted by the water vapour - Typically in the range of 1 to 3kPa

Humidity Vapour pressure deficit (vpd) - Is the pressure which drives the water vapour in the stoma out into the greenhouse Stoma - The gas / vapour in a stoma is at

19 Humidity Vapour pressure deficit (vpd) - Is the pressure which drives the water vapour in the stoma out into the greenhouse Stoma - The gas / vapour in a stoma is at saturation - The stoma valve has a resistance to flow = 0.14kPa Greenhouse - vpd must be > 0.14kPa to be on the edge of allowing transpiration vpd 0kPa HD 0 g/m 3 RH 100% vpd 0.14 kpa

20 Ketchup squeezy bottle analogy

21 Mollier diagram Vapour pressure RH % Temp ( o C) kpa AH ( g / m 3 )

22 Mollier diagram Vapour pressure deficit RH % Temp ( o C) kpa AH ( g / m 3 ) Vapour pressure deficit

23 Reason for humidity control drives the way it is measured Reasons for reducing humidity: - To aid / stimulate transpiration - To reduce the chance of condensation (disease) BUT Actively controlling humidity uses energy - Over-correction costs money It s important to choose a method of control that gives the required result at lowest energy cost

24 Transpiration Objective: to have a constant VPD What if we use Constant Relative humidity? Temperature o C Relative Humidity % Humidity deficit g/m 3 VPD kpa A constant RH gives a variable VPD From a transpiration point of the view plant sees varying conditions

25 Transpiration Using a constant humidify deficit Temperature Humidity Relative o VPD kpa C deficit g/m 3 Humidity % A constant HD gives a constant VPD - and a constant drive for transpiration As a guide, VPD = HD x 0.14 But observe how much RH varies

26 Humidity & plant temperature (another reason for humidity control) If plant temperature is higher than the air temperature - The microclimate around the plant slightly heats the air - RH falls and HD and VPD increase - No problems If plant temperature is lower than the air temperature - The microclimate around the plant slightly cools the air - RH increases and HD and VPD fall - Air approaches saturation /dew point - Higher disease risk

27 How condensation occurs RH % Air Plant Temp ( o C) kpa AH ( g / m 3 )

28 Humidity Dew point temperature Your glasses steam up when you walk into a greenhouse The temperature of your glasses is below the dew point temperature of the greenhouse air Dew point is the temperature at which this starts to occur in a particular set of circumstances

29 Mollier diagram Dew Point Temperature RH % Temp ( o C) kpa AH ( g / m 3 )

30 Humidity Dew point temperature Why is it important? - If the temperature of anything in the greenhouse (fruit, flower, stem, leaf, pot, slab, glass, frames) is at or below the dew point temperature, it will become WET - We might not see wetness but things feel clammy - There is no doubt that this is bad from a disease point of view Ensuring that the dew point temperature of the air is always below the temperature of the coldest part of the plant is a key part of disease control

31 Humidity & plant temperature Temperature RH HD Air RH seen by the plant HD seen by the plant Plant Plant

32 Humidity control for disease minimisation Controlling on HD? Temp HD Dewpoint dt Constant HD gives a variable temperature/ dew point difference Not easy to manage

33 Humidity control for disease minimisation Controlling on Relative Humidity? Temp RH Dewpoint dt Constant RH control gives a consistent and temperature/ dew point difference

34 Humidity Units & definitions Temperature ( 0 C ) Relative humidity 100% 95% 90% 85% 80% 75% 70% 65% 60% 55% 50% 45% 40% On disc - Relative Humidity vs Humidity Deficit table - Relative Humidity vs Absolute humidity kg/m3 g/m3 g/kg mmhg

35 Summary Vapour pressure deficit (VPD) - kpa - The driver that causes water vapour within the stomata to enter the air - What a plant really sees from a transpiration point of view Humidity deficit (HD) - g/m 3 - How much more water the air can hold - Direct relationship between HD & VPD, so HD is a good substitute for VPD 1 g/m 3 = 0.14 kpa Relative humidity (RH) - % - No easy relationship between RH & HD/VPD - RH tells you how close the air is to saturation and therefore the risk of a condensation event. - Controlling to a constant RH will deliver an almost constant difference between measured air temperature & it s dew point temperature therefore useful in disease control

36 Summary Plant temperature/air temp difference is critical - Plant temp higher than the air temperature - plant sees a higher HD & VPD, lower RH - Plant temp lower than the air temperature - plant sees a lower HD & VPD, higher RH Dew point temperature - If any part of the plant is at or close to the dew point temperature of the air you will get condensation - But you might not be able to see it

37 That s all for this session Any more questions at this stage?

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