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Formula for Calculating Vapor Pressure Deficit

  1. 03-25-2014, 02:09 PM #1
    mooselodge
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    Formula for Calculating Vapor Pressure Deficit

    Hello,

    I am taking readings in the greenhouse with a temperature and Humidity Meter that is measuring in Celsius and Relative Humidity. However, in an attempt to better understand the effect of humidity on the plant I would like to add a column to the chart that illustrates the VPD (vapor pressure deficit).

    I have looked around for sources on this subject and found a thread on this site which brought me to the forum...

    http://www.excelforum.com/excel-gene...x-formula.html

    The formula is not giving me the results that I would expect as most greenhouse VPD readings to be between 2 and 8. see chart below.

    2014 03 25 Humidity VPD.PNG

    [code]A -10440.397
    B -11.29465
    CC -0.027022355
    D 0.00001289036
    E -0.0000000024780681
    F 6.5459673
    TC SEE TABLE
    TR [°R]*=*([°C]*+*273.15)*×*9⁄5
    VPsat 10 VPsat B10: =EXP(A/TR+B+CC*TR+D*TR^2+E*TR^3+F*LN(TR))
    [code\]

    when I use the following numbers I get VPD of 0.32093

    Temperature Humidity Rankine VPD
    19.109 °C 17.4 %RH 526.06 0.32093053

    this looks incomplete as I am looking for VPD and this formula says VPsat. Do I need another step here?
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  2. 03-25-2014, 02:56 PM #2
    ChemistB
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    Re: Formula for Calculating Vapor Pressure Deficit

    From this site
    http://cronklab.wikidot.com/calculat...essure-deficit
    there is a formula for calculating SVP (Saturated VP) from °C and
    calculating VPD from RH and SVP.
    The example it gives results in a 0.633 reading.
    If you upload some data in an excel sheet (Go advanced>Manage attachments) I can check to see if your formulas are set up correctly.
    ChemistB
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  3. 03-25-2014, 03:33 PM #3
    shg
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    Re: Formula for Calculating Vapor Pressure Deficit

    A
    B
    C
    D
    E
    F
    G
    H
    I
    J
    K
    L
    M
    N
    O
    P
    1
    		 A
    -1.0440397E+04
    		 C1: Input
    TEMP \ RH
    100%
    95%
    90%
    85%
    80%
    75%
    70%
    65%
    60%
    55%
    50%
    2
    		 B
    -1.1294650E+01
    		 C2: Input
    15
    0.0
    0.9
    1.7
    2.6
    3.4
    4.3
    5.1
    6.0
    6.8
    7.7
    8.5
    3
    		 CC
    -2.7022355E-02
    		 C3: Input
    16
    0.0
    0.9
    1.8
    2.7
    3.6
    4.5
    5.5
    6.4
    7.3
    8.2
    9.1
    4
    		 D
    1.2890360E-05
    		 C4: Input
    17
    0.0
    1.0
    1.9
    2.9
    3.9
    4.8
    5.8
    6.8
    7.8
    8.7
    9.7
    5
    		 E
    -2.4780681E-09
    		 C5: Input
    18
    0.0
    1.0
    2.1
    3.1
    4.1
    5.2
    6.2
    7.2
    8.3
    9.3
    10.3
    6
    		 F
    6.5459673E+00
    		 C6: Input
    19
    0.0
    1.1
    2.2
    3.3
    4.4
    5.5
    6.6
    7.7
    8.8
    9.9
    11.0
    7
    		 Temp [C] TC
    20.0
    		 C7: Input
    20
    0.0
    1.2
    2.3
    3.5
    4.7
    5.8
    7.0
    8.2
    9.4
    10.5
    11.7
    8
    		 Temp [R] TR
    527.7
    		 C8: =1.8 * TC + 491.67
    21
    0.0
    1.2
    2.5
    3.7
    5.0
    6.2
    7.5
    8.7
    10.0
    11.2
    12.4
    9
    		 VPsat
    0.3392
    		 C9: =EXP(A/TR+B+CC*TR+D*TR^2+E*TR^3+F*LN(TR))
    22
    0.0
    1.3
    2.6
    4.0
    5.3
    6.6
    7.9
    9.3
    10.6
    11.9
    13.2
    10
    		 RH
    70.0%
    		 C10: Input
    23
    0.0
    1.4
    2.8
    4.2
    5.6
    7.0
    8.4
    9.8
    11.2
    12.6
    14.1
    11
    		 VPair
    0.2375
    		 C11: =C9*C10
    24
    0.0
    1.5
    3.0
    4.5
    6.0
    7.5
    9.0
    10.4
    11.9
    13.4
    14.9
    12
    		 VPD [psi]
    0.1018
    		 C12: =C9-C11
    25
    0.0
    1.6
    3.2
    4.8
    6.3
    7.9
    9.5
    11.1
    12.7
    14.3
    15.8
    13
    		 VPD [mb]
    7.0
    		 C13: =C12*68.9475729
    26
    0.0
    1.7
    3.4
    5.0
    6.7
    8.4
    10.1
    11.8
    13.5
    15.1
    16.8
    14
    27
    0.0
    1.8
    3.6
    5.4
    7.1
    8.9
    10.7
    12.5
    14.3
    16.1
    17.8
    15
    28
    0.0
    1.9
    3.8
    5.7
    7.6
    9.5
    11.3
    13.2
    15.1
    17.0
    18.9
    16
    29
    0.0
    2.0
    4.0
    6.0
    8.0
    10.0
    12.0
    14.0
    16.0
    18.0
    20.0
    17
    30
    0.0
    2.1
    4.2
    6.4
    8.5
    10.6
    12.7
    14.9
    17.0
    19.1
    21.2


    That's a 2D data table on the right that duplicates the result in the picture you posted.
    Last edited by shg; 03-25-2014 at 03:45 PM.
    Entia non sunt multiplicanda sine necessitate
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