AQE Bug: Ridiculously High Readings

Short Story

I’ve been reading reports of really high (some would say ‘ridiculous’) values being reported by Air Quality Eggs in recent weeks as they’ve started to reach their homes. I think I’ve figured it out, and I’ve pushed an update to the Remote Egg software (more specifically, the Egg Bus Arduino Library). Basically there was a bug in the ‘extrapolation’ case of the getSensorValue function. If you want to skip reading the rest and go patch your Egg with the fix, just go download the latest zip from Git-Hub and reload the software as described in the tutorial videos we recently produced <LINK HERE>.

Details

In EggBus.c in the getSensorValue function, the way it works is it searches a table of (x, y) samples from the sensor hosting device (i.e. the Egg Shield) that relates the “raw” value from the sensor to a computed value that is something we are more used to thinking about.  For example, the table might take us from measured resistance in ohms (the raw value of the NO2 sensor) to concentration in ppb (parts per billion). The way the Nanode uses this table is it look for the two rows in this table that the raw value falls between and performs a linear interpolation – this is the most common case we should expect to see. There are of course a few other ‘edge cases’ that need to be handled: namely (1) exact matches to a table row, (2) values that are smaller than the smallest value represented in the table, and (3) values hat are larger than the largest value in the table. Exact matches are trivial, as we just return the mapped value with no further maths applied. The other two cases are slightly less trivial, but the principle is to calculate the slope of the sampled curve at either extreme and then perform a linear extrapolation in the corresponding direction off the sampled curve.

The problem was in the handling of the third case above – an extrapolation above the highest value in the table. Notably:

float EggBus::getSensorValue(uint8_t sensorIndex){

  // some setup code omitted here
  while(getTableRow(sensorIndex, row++, &xval, &yval)){
    real_table_value_x = x_scaler * xval;
    real_table_value_y = y_scaler * yval;

    // bunch of code here for the other cases omitted here...

    // store into the previous values for use in interpolation/extrapolation
    previous_real_table_value_x = real_table_value_x;
    previous_real_table_value_y = real_table_value_y;
  }

  // case 4: the independent variable is must be greater than the largest value in the table, so we need to extrapolate forward
  // if you got through the entire table without an early return that means the independent_variable_value
  // the last values stored should be used for the slope calculation
  slope = (real_table_value_y - previous_real_table_value_y) / (real_table_value_x - previous_real_table_value_x);
  return real_table_value_y + slope * (independent_variable_value - real_table_value_x);
}

I removed all the irrelevant stuff. Do you see the problem? Basically at the slope calculation right before the return is *guaranteed* to result in the calculation 0 / 0 which turns out to be Not-a-Number (nan) in floating-point-speak. So in the case where the “measured value” turns out to be on the high side, things would sort of fly off the handle. There is still the ‘problem’ that the values are coming out high to begin with, but that’s because we don’t have a calibration procedure at this point, and is a topic for a different post. Thanks for reading!