Temperature resolution - Githubissues

Conclusion

The function GetTypKTemp() in functions.cpp divides an int32_t over an int32_t. This results in only integer values for the thermocouple temperature difference. The ambient temperature sensor gives decimal values. Combining these two values gives what we have observed: large temperature jumps of approximately one degree with small variations at each jump. This also explains why the displayed thermocouple temperatures always carry the same decimal number as the ambient temperature.

Check out this smoothness. img_20150903_010626

Notes

I'm working downward from the highest level...

In t400.ino #53 double temperatures[SENSOR_COUNT]; creates a table containing the values to display. A double is floating point. I see a decimal number the display, so this appears to be good.

In t400.ino #169 temperatures[channel] = convertTemperature(temperature + ambient); writes a value to the specified table cell. When Celsius units are selected #86 double convertTemperature(double Celcius){} just returns Celcius. For simplicity, I'll just look at this case. #149 double temperature; declares a floating point, so that should be good. In t400.ino #54 double ambient = 0; declares a floating point variable. Should be no problem. #151 ambient = ambientSensor.readTempC16(AMBIENT) / 16.0; This is an integer value what gets divided by sixteen. Maybe something funky happens here, but probably not because I see a decimal value for ambient temperature on the display. AMBIENT is a keyword defined in the MCP980X library keywords.txt #26 AMBIENT LITERAL1. In MCP980X.cpp #45 int MCP980X::readTempC16(MCP980X_REGS_t reg){} returns the Celsius temperature as an integer which is °C times 16.0.

So far there appears to be no problem with combining the ambient temperature and the thermocouple temperature. The issue must be with determining what the thermocouple temperature actually is. Let's take a look at temperature.

In t400.ino #163 temperature = GetTypKTemp(measuredVoltageUv); converts measuredVoltageUv to temperature using the lookup table. In functions.cpp # 316 double GetTypKTemp(int32_t microVolts) verifies microVolts is within the expected range then returns LookedupValue, which is defined by #322 double LookedupValue;.

The problem could be in functions.cpp at #330 LookedupValue = ((double)-270 + (i)*10) + ((10 *(microVolts - valueLow)) / ((valueHigh - valueLow))); where all the numbers are integers except for (double-270). The problem may be from dividing integers rather than using floating point math.

Okay, I think that's the issue. I'm not really sure how all these variable types interact, so I wrote a quick c++ program.

// Use the lookup table in typek_constant.h to determine what the
// measured temperature should be. This does not include the
// junction temperature form the MCP9800.

#include <iostream>
using namespace std;

int main()
{
  int32_t microVolts;
  int32_t valueLow = -5354;
  int32_t valueHigh = -5141;
  double LookedupValue;
  uint16_t i = 0;

   cout << "Hypothetical uV: " << endl;
   cin >> microVolts;
   cout << "Corresponding i value: " << endl;
   cin >> i;
   LookedupValue = ((double)-270 + (i)*10) + ((10 *(microVolts - valueLow)) / ((valueHigh - valueLow)));
   cout << "Calculated: " << LookedupValue << endl;
   return 0;
}

I compile and execute to get the following output.

$ clear; g++ add.cpp; chmod +x a.out ;./a.out
Hypothetical uV: 
-5200
Corresponding i value: 
10
Calculated: -163

This code gives -163 whereas a hand calculate gives -162.769953....

Now I can change the code and cast all the integers at doubles.

...
   LookedupValue = ((double)-270 + (i)*10) + 
       ((10 *((double)microVolts - (double)valueLow)) / (((double)valueHigh - (double)valueLow)));
...

This gives the appropriate LookedupValue of -162.77.

$ clear; g++ add.cpp; chmod +x a.out ;./a.out
Hypothetical uV: 
-5200
Corresponding i value: 
10
Calculated: -162.77

Maybe there is a better way to do this, but it looks like it will work. I'll test on the hardware.

Actually, I think we can just denominator as a double after the subtraction. Casting -270 as a double does not appear to do anything.

   LookedupValue = (-270 + (i)*10) + 
       ((10 *(microVolts - valueLow)) / (double)((valueHigh - valueLow)));

This gives

$ clear; g++ add.cpp; chmod +x a.out ;./a.out
Hypothetical uV: 
-5200
Corresponding i value: 
10
Calculated: -162.77

On the hardware this change works, but causes another problem. As you can see in this image we are getting much getter resolution on the readings.

img_20150902_221453

However, there appears to be some kind of rollover when the displayed thermocouple temperature drops below the ambient temperature.

img_20150902_221517

Changing the denominator from (double)((valueHigh - valueLow)) to ((double)(valueHigh - valueLow)) does not change anything

Well, it turns out that (double)-273 was really important. The only change to any code is in function.cpp 'LookedupValue = ((double)-270 + (i)10) + ((10 (microVolts - valueLow)) / ((double)(valueHigh - valueLow)));' Just cast the denominator as a double and all is well.

Check this out. img_20150903_010626

PaxInstruments / t400-firmware

Temperature resolution #181

Conclusion

Notes