Quick Routine to Convert deg C to deg F

John Couture · 2006-03-06 01:39

Ok, everyone is tired of hearing from me today but one last post.

For those of you that are working with temperature sensors that have deg Celsius and need to convert to deg Fahrenheit I worked up a quick routine that uses only integers.

The real equation:

·· deg F = (deg C * 1.8 ) + 32
68 deg F = (20 deg C * 1.8) + 32
68 deg F = (36 + 32)

Well, 1.8 is difficult to work with so what I did was multiply it by 2 then subtracted out the 0.2 after multiplying it also.

degC = 20
x1·· = degC * 2· ' this gives you 40
x2·· = x1········' make a copy of it
x1·· = x1 / 10···' I know it looks goofy, but it works
x2·· = x2 - x1·· ' 40 - 4 = 36
degF = x2 + 32·· '·go ahead, try it in a spreadsheet, it works for all temps > 0 deg C that a human can tolerate.
···

Finally, for those of you into trivia:

If zero degrees Celsius is the melting point of ice, what does zero degrees Fahrenheit represent?· (don't post the answer, make them look it up!)

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John J. Couture

San Diego Miramar College

Mike Cook · 2006-03-06 02:21

John,

Thanks for posting this. I was working on something similar to this today:

ºF = (ºC * 1.8) + 32

What I came up with is attached, prints a HEX value that is in ºF when divide by 10. Still not finished, too many projects!

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Mike
"OEM NMEA GPS Module" Now available on ebay for only $17.49
http://www.allsurplus.net/Axiom/

Jeff Dege · 2006-03-06 05:05

Zero farenheit was the lowest temperature that Farenheit could achieve in the lab - the freezing point of the salt water with the lowest freezing point of any concentration of salt that he tried.

100 farenheit was his body temperature.

Beau Schwabe · 2006-03-06 05:35

Why not?

degF = degC * 9
degF = degF / 5
degF = degF + 32

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Jon Williams · 2006-03-06 09:07

And if you're keeping everything as bytes and want to avoid roll-over...

degF = degC * 4
degF = degF / 5
degF = degF + degC
degF = degF + 32

... this assumes, of course, that everything is positive.

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Jon Williams
Applications Engineer, Parallax

LoopyByteloose · 2006-03-06 11:18

Okay, I know that you have to adjust by 5/9th [noparse][[/noparse]or its inverse of 1.8], then offset by 32

But is there a niffy way in Binary to deal with that awful 5/9ths?

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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Mike W · 2006-03-06 16:08

I have also been working with tempreature projects. Where I have been stuck is when the tempreature drops below 0. since where on the subject how do you convert negative temps.

Mike

Mike Cook · 2006-03-06 16:26

This is how I do it with the DS18s20, all readings are in ºC, in the process of writing a ºF routine.

tLSB = OW_READ          ' read LSB byte of temperature from DS18s20
tMSB = OW_READ          ' read MSB byte of temperature from DS18s20
 
temp1 = tLSB >> 1       ' remove fraction
 
IF tMSB.0 = 1 THEN      ' check for negative temperature ºC
  temp1 = temp1 | 128   ' OR 128 with the value for the 9th bit 
  temp1 = -temp1        ' two's compliment conversion
  TX_BYTE 45            ' set 'SIGN' of temperature to negative
ELSE 
  TX_BYTE 43            ' set 'SIGN' of temperature to positive
ENDIF
 
TX_DEC3 temp1           ' send temperature value to terminal
 
IF tLSB.0 = 1 THEN      ' Send fraction to terminal, 1 = .05 ºC
  TX_STRING ".5C"
ELSE
  TX_STRING ".0C"
ENDIF
 
RETURN

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Mike
"OEM NMEA GPS Module" Now available on ebay for only $17.49
http://www.allsurplus.net/Axiom/

John Couture · 2006-03-06 17:25

Beau,

Your routine of multiplying by 9 limits you to 28C when you use an 8 bit register (clever solution though)

Jon,

yours seems the easiest

Mike Cook

I actually do have a routine for bringing in the .5 but I wanted to keep it very simple (grin).

Mike W.

In the DS18S20 you download two bytes. The MSB (most significant byte) is the sign, the LSB is the degrees C (which includes the .5 deg bit). If the MSB = $FF then the reading is a neg number represented in two's complement. As for converting neg temps to deg F ... let met think about that for a couple of minutes.

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John J. Couture

San Diego Miramar College

Beau Schwabe · 2006-03-06 17:51

Jon Williams,

Nice! ....I didn't think of just dealing with .8 and then adding for the 1.0 relationship.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Jon Williams · 2006-03-06 18:25

I wish I could take credit -- I think Bean pointed that out to me when I was having trouble with the DS1620 in SX/B.

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Jon Williams
Applications Engineer, Parallax

Mike Cook · 2006-03-07 23:33

Ok, this looks like it should work for temperatures +/- 63ºC to +/-ºF. Check my math, I've run it several times and can't seem to find anything wrong with it. All done in bytes.

MAIN:
  sign = 0    
  for idx = 0 to 63
    degC = idx
    degF = degC * 4                     ' convert to Fahrenheit
    degF = degF / 5
    fraction = __REMAINDER
    degF = degF + degC
    fraction = fraction * 2             ' Double the fraction
    
    IF sign = 1 THEN
 
      IF degC = 0 THEN
        TX_BYTE Space
      ELSE
        TX_BYTE NEGATIVE 
      ENDIF   
      
      TX_DEC3 idx                      
      TX_STRING "  "
  
      IF degF >= 32 THEN
        degF = degF - 32
        TX_BYTE NEGATIVE
        TX_DEC3 degF
      ELSE
        INC degF
        degF = 32 - degF
 
        IF fraction = 0 THEN
          INC degF
        ELSE
          fraction = 10 - fraction 
        ENDIF

        TX_DEC3 degF
      ENDIF

    ELSE
      degF = degF + 32
      TX_DEC3 idx
      TX_STRING "  "
      TX_DEC3 degF
    ENDIF
 
    fraction = fraction + $30           ' add 30 HEX to it to make it ASCII
    TX_BYTE decimal                     ' print the decimal point
    TX_BYTE fraction                    ' print the fraction amount
    TX_STRING "F "                      ' print the units
    TX_STRING CrLf                      ' new line for the terminal

  NEXT

  
Finish: 
  END

Attached is the SX/B program I used to test it. To go negative set the sign variable to 1

<EDIT>

I had an error in IF degF > 32 THEN should be IF degF >= 32 THEN fixed listing and uploaded test program

ADDED: fraction amount from the degF = decF / 5 calculation

CORRECTED: error in fraction between -ºC to +ºF

</EDIT>

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Mike
"OEM NMEA GPS Module" Now available on ebay for only $17.49
http://www.allsurplus.net/Axiom/

Post Edited (Mike Cook) : 3/8/2006 11:22:37 PM GMT

Mike Cook · 2006-03-14 16:24

Attached is a test program written in SX/B to test the ºC to ºF conversion formula. Program works with byte values to output temperature in ºF in the range of: +146.3ºF to -82.3ºF, .9ºF resolution. This program tests a temperature conversion routines that I will be using for a future project. I'm posting it here to see if it might be helpful to others. If you find any problems with it or can think of a more efficient way of doing this then please post your comments.

The temperature sensor I will be using is a Dallas 1-Wire DS18s20, for the final project. A quick look at the DS1620 looks like this device outputs temperature in the same format as the DS18S20 so it might be useable for that part also. Please note that the DS18s20 is a 1-Wire device and the DS1620 looks to be a SPI device, never used a DS1620!

Data is output to a terminal program and then captured to a text file that was imported to Microsoft Excel. ºC is in column A and ºF is in column B. The temperature conversion formula is in column C (ºF = (ºC x 1.8) + 32). The check for Excel's PASS or FAIL formula =IF(B1=C1,"PASS","FAIL") is in column D.

One abnormally that I ran across is Microsoft Excel fails -18ºC. Not sure why, but the converted value (SX/B = -.4ºF) and (Excel = -.4ºF) looks to be the same. Yet the Excel formula for the 'check' prints FAIL, I calling this a 'PASS' because the data looks to be the same (not a Excel guru).

The following files are attached:

C_TO_F__V3.SXB -- SX/B program
sampledata1.txt -- capture of the ASCII output to terminal
sampledaat1.xls -- Excel worksheet with captured data and check formula

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Mike
"OEM NMEA GPS Module" Now available on ebay for only $17.49
http://www.allsurplus.net/Axiom/

Quick Routine to Convert deg C to deg F

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