Math Question?

> I need to form equation :

>

> Double vpsat = e(A/T + B + CT + DT2 + ET3 + F lnT)

>

> where:

> A = -1.044x10 (to the 4th)

> B = ?.129 465 0 x 10 (to the 1st)

> C = ?.702 235 5 x 10?

> D = ?.289 036 0 x 10?

> E = ?.478 068 1 x 10?

> F = ?.545 967 3

> T ?Temperature of the air in 癛,

> 癛 = 癋 + 459.67

>

> Questions are how do I form a number such as variable

> A?

use a float data type to store the value, also look at the math library !

> What does the e represent in the equation? I'm not a

> big math guy and I can't for the life of me figure

> out what the e represents.

e is a math constant, like pi. its value is 2.71828

I'm trying to calculate

> Vapor Pressure Deficit (VPD) from Temperature and

> Humidity based on the equations laid out on the 2nd

> to last page of this :

>

> http://ohioline.osu.edu/aex-fact/0804.html

>

> A shove in the right direction will be greatly

> appreciated.

--stallion--a at 2007-7-13 0:30:41 >

> Questions are how do I form a number such as variable A?double a = -1.044 * Math.pow(10, 4);

As was noted before e is just a number (2.71828). In your equation to take e to the power of the thing in parentheses.double vpstat = Math.exp(sum);

And for ln - natural logarithm - use Math.log().

(I'd never heard of 癛 before - had to look it up. 癛a==Rankine, not to be confused with 癛?=R閍umur)

[Edit] I've just seen the previous which you posted while I was figuring out why your constants were different from those on Wikipedia (who use 癒!). Post what you've got if you're stuck.

pbrockway2a at 2007-7-13 0:30:41 >

What do I do with the ln ? inF lnT from a google search I see it a natural logarith but what does this mean? and how do I apply it to the equation I so far have:

public static double getVPD(double temperature, double canopyTemp, double relativeHumidity){

double a = -1.044 * Math.pow(10, 4);

double b = -1.129 * Math.pow(10, 1);

double c = -2.702 * Math.pow(10, -2);

double d = 1.289 * Math.pow(10, -5);

double e = -2.478 * Math.pow(10, -9);

double f = 6.456;

double T = 0;

T = Conversions.CToF(temperature) + 459.67;

double sum = ( a/T + b + c*T + Math.pow(d*T,2) + Math.pow(e*T,3) + (f * (Math.log(T) )) );

double vpsat = Math.exp(sum);

double vpair = vpsat x relativeHumidty ?100;

double vpd = vpsat - vpair;

return vpd;

}

Does this process the equation correctly?

xavier33a at 2007-7-13 0:30:41 >

I assume temperature is in 癈, and that Conversions.CToF() does a 癈->癋 conversion. It looks OK providing you make a couple of changes to the d and e terms.double sum = ( a/T + b + c*T + d * Math.pow(T,2) + e * Math.pow(T,3) + (f * (Math.log(T) )) );

You can test it by calculating a few values and comparing them to what you get from the charts on the page you linked to.

pbrockway2a at 2007-7-13 0:30:41 >

You get a negative number? That's weird. Here's what I see (note: you still have x for * where you calculate vpair)public class Vpd {

public static void main(String args[]) {

// example A

double temp = (50.0 - 32) * 5 / 9;

double rh = 80;

System.out.println(getVPD(temp, temp, rh)); //0.0205psi

// example B

temp = (70.0 - 32) * 5 / 9;

rh = 80;

System.out.println(getVPD(temp, temp, rh)); //0.0416psi

// your example

rh = 38.7;

temp = 26;

System.out.println(getVPD(temp, temp, rh)); //0.1709psi

}

public static double getVPD(double temperature, double canopyTemp,

double relativeHumidity) {

double a = -1.044 * Math.pow(10, 4);

double b = -1.129 * Math.pow(10, 1);

double c = -2.702 * Math.pow(10, -2);

double d = 1.289 * Math.pow(10, -5);

double e = -2.478 * Math.pow(10, -9);

double f = 6.456;

double t = 0;

t = Conversions.CToF(temperature) + 459.67;

System.out.println("t=" + t);

double sum = a/t + b + c*t + d*Math.pow(t,2) + e*Math.pow(t,3) + f*(Math.log(t));

double vpsat = Math.exp(sum);

System.out.println("vpsat=" + vpsat);

double vpair = vpsat * relativeHumidity / 100;

System.out.println("vpair=" + vpair);

double vpd = vpsat - vpair;

return vpd;

}

}

class Conversions {

static double CToF(double celsius) {

return celsius * 9 / 5 + 32;

}

}

pbrockway2a at 2007-7-13 0:30:41 >