Can sombody please help me convert this formula to arduino code.

Projects Programming
1

Hi All. I an new here.

Can somebody please help me convert this formula so that the Arduino can use it.

It is to work out the wet bulb temperature if you have the dry bulb and relative humidity %

Tw = 20 atan[0.151 977 (50 + 8.313 659) to the power 1/2] + atan(20 + 50) - atan(50 - 1.676 331) + 0.003 918 38(50) to the power 3/2 atan(0.023 101 * 50) - 4.686 035

answer should be

13.7

2

What the *** is this?

0.003 918 38(50)
3

either it is .00391838 * 50 or .00391838 +/- 50 in the least significant digits.

tinkerboy, your notation is all screwed up mixing brackets and parens and multiplications. Also this:
atan(50 - 1.676 331) + 0.003 918 38(50) to the power 3/2 atan(0.023 101 * 50) - 4.686 035

is completely ambiguous.

Do us, and you a favor by doing the following:

  1. Does atan return degrees or radians?
  2. write constants without spaces as separators.
  3. write each term on one line:

atan(0.151777...)
atan(20+50)
...

4

It is a value. Sorry I see the copy paste made spaces in the value.

5

But I don't see any input, so why not use 13,7 then?

6 
void setup(void)
{
  Serial.begin(9600);
  while(!Serial);

// 13.6993418
float RH = 50;
float Tp = 20;
float Tw = Tp * atan(0.151977*sqrt(RH + 8.313659))
            + atan(Tp+RH) - atan(RH - 1.676331)
            + 0.00391838 * pow( RH,3./2.)*atan(0.023101 * RH)
            - 4.686035;
  Serial.println(Tw,7);
}

void loop(void)
{
}

Prints 13.6993418
It really helps to have a reference to the original equation (which I found before OP posted it)
[+edit] Changed a hardcoded "20" to Tp.

Pete

7

Found it

ARctan is in radians.

double TW = T * arctan(.151955 * sqrt(RH+8.313659))+atan(T+RH) - atan(RH-1.676331)+
pow(.00391838*RH,1.5)atan(.023101RH) -4.68035;

With T = temperature and RH = relative humidity

I tried it on the calculator and got 12.6, so check for typos.

8

But keep in mind, arctan (or other trigonometric functions) are not very easy for an Arduino Uno.

9

Thank you very much. I am controlling a PC with my phone via Teamviewer making the posts so things end up at wrong places.

Thank you very much for the help !!!!

10

septillion:
But keep in mind, arctan (or other trigonometric functions) are not very easy for an Arduino Uno.

I have never had one complain.

11

I did :slight_smile: Not the Arduino but the user who thought he could do 100 trigonometric calculations a second...

12

tinkerboy:
Hi All. I an new here.

Can somebody please help me convert this formula so that the Arduino can use it.

It is to work out the wet bulb temperature if you have the dry bulb and relative humidity %

What is it? I never heard the term "wet bulb temperature"

I know the terms saturation temperature" and "dew point"

Is it all the same?

13

I never heard the term "wet bulb temperature"

Well, it's similar to a "dim bulb" temperature... 8)

14

"Wet bulb" is the temperature that you measure with a wet bulb thermometer. A small sock fits over the bulb and dangles into water.

It's the way we used to measure humidity before electronic humidity sensors were created.

Another cool one: "Whirling Psychrometer" (I hope I've spelled that correctly.)

15

It's a type of hygrometer where you spin 2 thermometers around or otherwise provide airflow. One bulb is dry and the other is wet and from the temp difference you can get humidity.

16

MorganS:
It's the way we used to measure humidity before electronic humidity sensors were created.

There were the meters based on hair:

17

Thank so much. The code works 100%. I just had to add one more variable where there was a hard coded 20. I created a function and it works like a charm. Math formulas are not my strong point LOL.

18

Oh right. I've corrected my message #5.

Pete

19

septillion:
But keep in mind, arctan (or other trigonometric functions) are not very easy for an Arduino Uno.

This is why I used an approximation routine for atan()
Inverse kinematics use a lot of trig.
:slight_smile: