It looks like the coefficient table, or perhaps the polycomp()
function doesn't work correctly for some of the temperatures for a
Type T thermocouple. I'm working with negative temperatures, and it
totally freaks out.
I've taken the code out and put it in a standalone program that I
could test with. I've made some minor changes to the functions so it
will run independently, but nothing that would have caused these
problems (best I can tell).
The test program displays two tables -- one of ambient temperatures
to mV readings. All match up to the Type T table.
The other does a table with a bunch of mV readings from -270C
(-6.258mV) up to 0C (0.0mV). As you can see from the output,
something is very wrong. How was the type_t table generated? Could
it have been generated with the wrong source tables?
-Tom
Code:
#include <stdio.h>
struct thermocouple {
float v1, v2;
float rangeLow, rangeHigh;
float low[11];
float mid[11];
float high[11];
float mV[11];
};
const struct thermocouple type_t = {
-1.785, 2.556, -270.0, 400.0,
{-0.0856873, -3.35591, -58.1948, -587.947, -3829.53, -16790.4,
-50151.1, -100705, -130042, -97452.3, -32202.6,},
{0.000151427, 3.35406e-05, -0.00372056, 0.00326535, 0.022672,
-0.0222902, -0.0694538, 0.116881, -0.67161, 25.8257, -0.0024547,},
{-5.44871e-11, 6.49228e-09, -3.38465e-07, 1.02385e-05, -0.000201171,
0.00272364, -0.0263712, 0.192645, -1.30813, 27.0402, -0.900659,},
{-1.1498e-22, 7.18468e-21, 5.31691e-18, -2.88407e-16, -9.51445e-14,
4.02711e-12, 8.60657e-10, -5.74685e-08, 4.17544e-05, 0.0386747,
-0.000253129,},
};
/* Compute a 10th order polynomial with cooef being a10, a9, ... a0 */
float polycomp(float x, float * coef)
{
float r;
int i;
r = coef[0];
for (i = 1; i <= 10; ++i)
r = x * r + coef[i];
return r;
}
// T is temp in degrees C, mV is voltage in millivolts
float thermocouple(struct thermocouple *thermo, float T, float mV)
{
/* Correct voltage by adding reference temperature voltage */
mV = mV + polycomp(T, thermo->mV);
/* Find right range, them compute temparature from voltage */
if (mV < thermo->v1) {
return polycomp(mV, thermo->low);
} else if (mV < thermo->v2) {
return polycomp(mV, thermo->mid);
} else {
return polycomp(mV, thermo->high);
}
}
int main ()
{
int i;
float t;
float mV;
printf ("thermocouple table:\n");
for (i = 0; i < 30; i++) {
printf ("%3dC = %.5fmV\n", i, polycomp (i * 1.0, type_t.mV));
}
for (i = -6258; i < 0; i += 164) {
mV = i * 0.001;
t = thermocouple (&type_t, 0.0, mV);
printf ("%.3fmV = %.2fC\n", mV, t);
}
}
Output:
thermocouple table:
0C = -0.00025mV
1C = 0.03846mV
2C = 0.07726mV
3C = 0.11615mV
4C = 0.15511mV
5C = 0.19416mV
6C = 0.23329mV
7C = 0.27250mV
8C = 0.31179mV
9C = 0.35117mV
10C = 0.39062mV
11C = 0.43016mV
12C = 0.46978mV
13C = 0.50947mV
14C = 0.54925mV
15C = 0.58911mV
16C = 0.62905mV
17C = 0.66908mV
18C = 0.70918mV
19C = 0.74936mV
20C = 0.78963mV
21C = 0.82997mV
22C = 0.87040mV
23C = 0.91091mV
24C = 0.95149mV
25C = 0.99216mV
26C = 1.03292mV
27C = 1.07375mV
28C = 1.11466mV
29C = 1.15566mV
-6.258mV = 456.44C
-6.094mV = 379.28C
-5.930mV = 300.70C
-5.766mV = 237.98C
-5.602mV = 169.85C
-5.438mV = 123.53C
-5.274mV = 79.66C
-5.110mV = 48.65C
-4.946mV = 20.35C
-4.782mV = -2.68C
-4.618mV = -24.20C
-4.454mV = -37.53C
-4.290mV = -47.42C
-4.126mV = -55.99C
-3.962mV = -63.00C
-3.798mV = -66.14C
-3.634mV = -69.81C
-3.470mV = -71.88C
-3.306mV = -71.32C
-3.142mV = -70.61C
-2.978mV = -69.13C
-2.814mV = -66.84C
-2.650mV = -64.34C
-2.486mV = -61.48C
-2.322mV = -58.52C
-2.158mV = -54.95C
-1.994mV = -50.79C
-1.830mV = -47.00C
-1.666mV = -45.52C
-1.502mV = -40.78C
-1.338mV = -36.10C
-1.174mV = -31.49C
-1.010mV = -26.93C
-0.846mV = -22.43C
-0.682mV = -17.98C
-0.518mV = -13.59C
-0.354mV = -9.24C
-0.190mV = -4.94C
-0.026mV = -0.68C
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