How to calculate negative degrees Celcius from Fahrenheit?

[nerdegutta]

Hi.

I got this:

Temp = ADCval * 4
Temp = ADCval * 8 / 10 + Temp
Temp = ADCval * 8 / 100 + Temp

Temp_F = Temp  / 10

To take an ADC reading from a LM34, and make it Fahrenheit.

To convert it to Celsius I use this:

Temp_C = Temp_F - 32 
Temp_C = Temp_C * 56 / 100

But this only work above zero degrees Celsius. When it's getting below zero, it overflows or something. How to I calculate the minus degrees?

Temp, Temp_F and Temp_C are all word variables.


- nerdegutta

 

[matherp]

I've previously uploaded some generic signed arithmetic routines which can do this for you. You will only need the signed word multiply and signed word divide. Why do you use *56/100 rather than *100/180?

http://www.picaxeforum.co.uk/showthread.php?23588-Signed-word-and-byte-arithmetic

 

[nerdegutta]

Why do you use *56/100 rather than *100/180?

I was playing with my calculator, and *56/100 gave the right answer time after time.

Thanks for the link. Will start reading.

- nerdegutta

 

[westaust55]

@Nergegutta,

have a look at this thread: http://www.picaxeforum.co.uk/showthread.php?15464-DS18B20-Code-Examples/page3
where forum member marks has posted a number of permutations for calculation/conversion of termperatures from C ==> F and K
for use with the DS18B20 temperature sensor.

 

[nerdegutta]

Thank you for the link.

(Wonder why this doesn't show up when I'm searching the forum or google... )

 

[westaust55]

Not sure why a search would not shoiw that thread.

I had the advantage of knowing marks had done some work in that area (memory had not entirely failed yet - even if my fiendishly flailing fingers generate lots of typos )
Using Fahrenheit as the seach term it was about 15 down on a forum search.

 

[womai]

Or modify your code to treat positive and negative Celsius results separately:

if Temp_F >= 32 then
   Temp_C = Temp_F - 32
   Sign = " "
else
   Temp_C = 32 - Temp_F
    Sign = "-"
endif

Temp_C = Temp_C * 56 / 100

where Sign is a byte variable

 

[nerdegutta]

Thanks.

Was looking for something like this. I'll try when I get back home, on Friday.

- nerdegutta

 

[womai]

Of course all that breaks down when the temperature is negative even when expressed in Fahrenheit.. (the LM34 seems to be capable of operation down to -50 degC). You'd need to somehow encode this (sign flag, sign bit) and then treat the different cases separately, but you can do all this similarly to my code above.

 

[marks]

Hi nerdegutta,
I guess converting from Fahrenheit to Celsius would be handy for some hope you give this a try
C=F+76*5/9-60
should do the range you require ,using 5/9 is slightly more accurate then 56/100.
I guess others may want Celsius to Fahrenheit just reverse it.
F=C+60*9/5-76
heres some example code you can simulate

SYMBOL Temperature  = W0  SYMBOL TempMsb  = b1  SYMBOL TempLsb = b0
SYMBOL Sign         = b2
SYMBOL D0           = b3
SYMBOL D1           = b4 
SYMBOL D2           = b5 
SYMBOL D3           = b6 
SYMBOL D4           = b7

Main:
  W0 = -670    'example -67.0 F
' W0 = 3000    'example 300.0 F
 
       
  Convert:                                      
       Sign = " "                                                  '  Display + 
  Temperature = Temperature  +760 *5 /9 -600                       '  Fahrenheit to Celsius
     IF TempMsb > 127 THEN                       
       Sign = "-"                                                  '  Display - 
       Temperature = - Temperature                                     
     ENDIF 
    
     BINTOASCII Temperature,D4,D4,D3,D2,D1  
      IF  D4 = "0" THEN : D4 = " "                                 ' leading zero blanking 
      IF D3 = "0" THEN : D3 = " " : ENDIF                          ' leading zero blanking 
      ENDIF                          
                          
 SERTXD (CR, LF, "Temperature ",Sign,D4,D3,D2,".",D1," degrees Celsius", CR, LF) '  (0.1)resolutuion
   
GOTO Main

 

[nerdegutta]

Great! Thanks.

Too bad I'm stuck in a hotel this week.

 

[nerdegutta]

I got it working.

Now some cleanup with spacing.

Thanks for the help, guys.