Arduino的精度更高（浮动）

Question

I'm trying to do the Steinhart-Hart temperature calculation on an Arduino. The equation is

I solved a system of 3 equations to obtain the values of A, B and C, which are:

A = 0.0164872
B = -0.00158538
C = 3.3813e-6

When I plug these into WolframAlpha to solve for T I get a value in Kelvins that makes sense:

T=1/(0.0164872-0.00158538*log2(10000)+3.3813E-6*(log2(10000))^3) solve for T

T = 298.145 Kelvins = 77 Fahrenheit

However when I try to use this equation on my Arduino, I get a very wrong answer, I suspect because doubles do not have enough precision. Here's what I'm using:

double temp = (1 / (A + B*log(R_therm) + C*pow(log(R_therm),3)));

This returns 222 Kelvin instead, which is way off.

So, how can I do a calculation like this in Arduino?? Any advice is greatly appreciated, thanks.

Answer 1

Precision is not the main issue. Could even use float and powf() . A thermistor temperature calculation is not that accurate. After all the temperature is certainly not better than ±0.1°C accurate . Self heating of the thermistor is a larger factor.

OP's C code assumes log base 2, use log base e log() as the constants were derived using log base 2. @Martin R

// double temp = (1 / (A + B*log(R_therm) + C*pow(log(R_therm),3)));
double temp = (1 / (A + B*log(R_therm)/log(2) + C*pow(log(R_therm)/log(2),3)));`

Sample implementation, that avoids an unnecessary slow pow() call.

static const inv_ln2 = 1.4426950408889634073599246810019;
double ln2_R = log(R_therm)*inv_ln2;
double temp = 1.0 / (A + ln2_R*(B + C*ln2_R*ln2_R));

Answer 2

Yes, floating point arithmetic has limited precision on most arduinos.

Have you considered using fixed precision? If used correctly, this might give you better results. The requirement for this is to have rather narrow parameters, however, and be careful about unit conversions.

An unsigned long on arduino is 4 bytes too, so it can contain numbers up to 2^32-1 . If using fixed point, you might want to replace this 1/T by something like 100000/T , where the numerator constant and T have been scaled according to the desired precision.

You will also need to keep a (mental or paper) model of the number of decimals each variable contains, in order to optimize the operation order not to lose precision.

For the log2 function, I doubt it is available out of the box for integers. You could either cast the result or reimplement it . There is plenty of ressources for this problem, even here on SO.