Post by thread:[PIC]: Can you crack this formula with a PIC ?

It's a formula to calculate water vapour pressure given a temperature in Deg C float calculateWaterWVP(float Temp) { float a,tempInK,c,d,e,wvp; tempInK = Temp + 273.15; a = -6096.9385 * (1/tempInK); c = -2.711193 * 0.01 * tempInK; d = 1.673952 * 0.00001 * (tempInK * tempInK); e = 2.433502 * (log(tempInK)/log(10)/0.434294509); wvp = exp(a + 21.2409642 + c + d + e); return wvp; } */ If you can do this, you get a medal ! (Accuracy of result = 1% or better) Regards, Darren -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

At 01:08 PM 10/26/00 EDT, you wrote: >It's a formula to calculate water vapour pressure given a temperature in Deg C {Quote hidden}> > float calculateWaterWVP(float Temp) > { > float a,tempInK,c,d,e,wvp; > > tempInK = Temp + 273.15; > a = -6096.9385 * (1/tempInK); > c = -2.711193 * 0.01 * tempInK; > d = 1.673952 * 0.00001 * (tempInK * tempInK); > e = 2.433502 * (log(tempInK)/log(10)/0.434294509); > > > wvp = exp(a + 21.2409642 + c + d + e); > return wvp; > > } */ > >If you can do this, you get a medal ! > >(Accuracy of result = 1% or better) How nonlinear is this function OVER THE RANGE OF INTEREST? How much resolution do you need (for display, it may be unacceptable to have poor resolution, even if the accuracy is ok)? Since this is a SISO function of temperature, I'd be tempted to use either a lookup table with linear interpolation or a polynomial fit to the analytical function above. Using 16-32 bit fixed point integer math, depending. Best regards, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" spam_OUTspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> How nonlinear is this function OVER THE RANGE OF INTEREST? Very non-linear, exponential! Try this instead This one for temps above freezing. Note temps in Cel. VaporPressure := 6.11*Power(10,(T*7.5)/(T+237.3)) this one for temps below freezing IF you want the frost point! VaporPressure := 6.11*Power(10,(T*9.5)/(T+265.5)) These are pretty good over the meteorlogical range of temps (-40 to 50 Cel.) BTW, the vapor pressure is no better than the accuracy of the temperature measurement! Jim Korman Spehro Pefhany wrote: {Quote hidden}> > At 01:08 PM 10/26/00 EDT, you wrote: > >It's a formula to calculate water vapour pressure given a temperature in > Deg C > > > > float calculateWaterWVP(float Temp) > > { > > float a,tempInK,c,d,e,wvp; > > > > tempInK = Temp + 273.15; > > a = -6096.9385 * (1/tempInK); > > c = -2.711193 * 0.01 * tempInK; > > d = 1.673952 * 0.00001 * (tempInK * tempInK); > > e = 2.433502 * (log(tempInK)/log(10)/0.434294509); > > > > > > wvp = exp(a + 21.2409642 + c + d + e); > > return wvp; > > > > } */ > > > >If you can do this, you get a medal ! > > > >(Accuracy of result = 1% or better) > > How nonlinear is this function OVER THE RANGE OF INTEREST? > > How much resolution do you need (for display, it may be unacceptable > to have poor resolution, even if the accuracy is ok)? > > Since this is a SISO function of temperature, I'd be tempted to use > either a lookup table with linear interpolation or a polynomial fit to > the analytical function above. Using 16-32 bit fixed point integer > math, depending. > > Best regards, > > =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= > Spehro Pefhany --"it's the network..." "The Journey is the reward" > .....speffKILLspam@spam@interlog.com Info for manufacturers: http://www.trexon.com > Embedded software/hardware/analog Info for designers: http://www.speff.com > Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com > =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Hi, I was impressed by HiTech C's compilation of this. But. 1% is doable with 8 bits, i.e. table output can be on 8 bits. Input accuracy does not relate to this linearly however. In particular you lose precision in the log operation. So your input precision could be 10% to obtain output 1% under certain circumstances. On the other hand, I'd run the little program: for(temp = MIN_TEMP; temp <= MAX_TEMP; temp += TEMP_STEP) printf("%-10f\t%-10f\n",i,calculateWaterWVP(temp)); to obtain a pretty table that will speak volumes about what you need to do. You can modify the program to calculate the (output) table values themselves and to obtain the required input precision for each table domain (the precision will vary because of your nonlinear dependency - esp. the log operation). The easiest way to do this is to have a variable float temp0 = MIN_TEMP-1; and for each calculated value display only if the reduced (to 1%) value is different from temp0, then set temp0 so the calculated temp. This will remove all the intermediary results from the table, giving the domain boundaries for the table. In the end you will likely end up with a table that may be largish depending on your temperature domain or use several interpolation points, as Scott has said. This being said, maybe you need to purchase HITECH C ? ;-) Peter PS: I do not work for them. PS2: Of course you run this on a PC not on a PIC. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use listservKILLspammitvma.mit.edu?body=SET%20PICList%20DIGEST