Posted on by Jason G

 

To correctly calculate the VPD of your growing space you will need the following statistics:

  • Ambient environmental temperature
  • Relative humidity
  • Leaf surface temperature
  • Canopy level temperature

 

First, find your saturated vapour pressure (SVP) for a specific temperature.

TEMP DEG/C

SVP

0

611

1

657

2

706

3

758

4

813

5

872

6

935

7

1002

8

1073

9

1148

10

1228

11

1312

12

1402

13

1497

14

1598

15

1705

16

1818

17

1937

18

2064

19

2197

20

2338

21

2486

22

2643

23

2809

24

2983

25

3167

26

3361

27

3565

28

3779

29

4005

30

4242

31

4492

32

4754

33

5029

34

5318

35

5621

36

5940

37

6273

38

6623

39

6990

40

7374

41

7776

 

From the information above we can now compute saturation pressure.

How to Calculate Vapor Pressure Deficit (VPD) via room Temperature

Saturation pressure can be looked up using a Psychrometric Chart or calculating via the Arrhenius equation.

The reason growers would want this information, is because it is the measure of the drying power of the air. This directly impacts transpiration rates and water movement within the plant.

It also allows a grower to correctly calculate how much airflow is needed per environment and by maintaining stability with RH and temp an accurate SVP can be achieved.

As VPD is the saturated vapour pressure minus the actual vapour pressure (SVP - VPactual) and VPactual = (RH*SVP)/100

We may apply the formula:
VPD = (100 - RH / 100)) *SVP)

Alternatively, VPD = (1 - (RH/100)) *SVP)

(RH) is relative humidity and (SVP) is saturated vapour pressure

*For example, take 80% (RH) at 25 C. From the list above, SVP = 3167 Pa

100-RH = 20
 20/100 = 0.2
 0.2 * 3167 = 633.4 pascals (Pa)

Note: for convenience VPD may be given in kilopascals (kPa).
For this example, 0.6334 kPa.

Click here to View a extremely handy VPD Chart here - Source: http://www.just4growers.com

 

References:

http://physics.holsoft.nl/physics/ocmain.htm, cronklab,

http://www.just4growers.com