co2_sensor: 2s measurements should not be used for forced calibration.

* Trying 10s instead.
* Adding warning if timestep < 10s

Forced calibration should hopefully be more reliable now.

ampel-firmware/co2_sensor.cpp

 #include "co2_sensor.h"
 
 namespace config {
-  // Values should be defined in config.h
-  uint16_t measurement_timestep = MEASUREMENT_TIMESTEP; // [s] Value between 2 and 1800 (range for SCD30 sensor)
-  const uint16_t measurement_timestep_bootup = 2; // [s] Measurement timestep during acclimatization
+  // UPPERCASE values should be defined in config.h
+  uint16_t measurement_timestep = MEASUREMENT_TIMESTEP; // [s] Value between 2 and 1800 (range for SCD30 sensor). 
   const uint16_t altitude_above_sea_level = ALTITUDE_ABOVE_SEA_LEVEL; // [m]
   uint16_t co2_calibration_level = ATMOSPHERIC_CO2_CONCENTRATION; // [ppm]
-  int8_t max_deviation_during_calibration = 30; // [ppm]
-  int8_t enough_stable_measurements = 60;
+  const uint16_t measurement_timestep_bootup = 5; // [s] Measurement timestep during acclimatization.
   const uint8_t max_deviation_during_bootup = 20; // [%]
+  const int8_t max_deviation_during_calibration = 30; // [ppm]
+  const int16_t timestep_during_calibration = 10; // [s] WARNING: Measurements can be unreliable for timesteps shorter than 10s.
+  const int8_t stable_measurements_before_calibration = 120 / timestep_during_calibration; // [-] Stable measurements during at least 2 minutes.
+
 #ifdef TEMPERATURE_OFFSET
   // Residual heat from CO2 sensor seems to be high enough to change the temperature reading. How much should it be offset?
   // NOTE: Sign isn't relevant. The returned temperature will always be shifted down.
@@ -131,14 +133,16 @@ namespace sensor {
         && co2 < (previous_co2 + config::max_deviation_during_calibration)) {
       stable_measurements++;
       Serial.print(F("Number of stable measurements : "));
-      Serial.println(stable_measurements);
+      Serial.print(stable_measurements);
+      Serial.print(F(" / "));
+      Serial.println(config::stable_measurements_before_calibration);
       switchState(PREPARE_CALIBRATION_STABLE);
     } else {
       stable_measurements = 0;
       switchState(PREPARE_CALIBRATION_UNSTABLE);
     }
     previous_co2 = co2;
-    return (stable_measurements == config::enough_stable_measurements);
+    return (stable_measurements == config::stable_measurements_before_calibration);
   }
 
   void startCalibrationProcess() {
@@ -146,8 +150,10 @@ namespace sensor {
      * For best results, the sensor has to be run in a stable environment in continuous mode at
      * a measurement rate of 2s for at least two minutes before applying the FRC command and sending the reference value.
      */
-    Serial.println(F("Setting SCD30 timestep to 2s, prior to calibration."));
-    scd30.setMeasurementInterval(2); // [s] The change will only take effect after next measurement.
+    Serial.print(F("Setting SCD30 timestep to "));
+    Serial.print(config::timestep_during_calibration);
+    Serial.println(F("s, prior to calibration."));
+    scd30.setMeasurementInterval(config::timestep_during_calibration); // [s] The change will only take effect after next measurement.
     Serial.println(F("Waiting until the measurements are stable for at least 2 minutes."));
     Serial.println(F("It could take a very long time."));
     switchState(PREPARE_CALIBRATION_UNSTABLE);
@@ -195,6 +201,9 @@ namespace sensor {
       Serial.print(F("Setting SCD30 timestep to "));
       Serial.print(config::measurement_timestep);
       Serial.println(F(" s."));
+      if (config::measurement_timestep < 10){
+        Serial.println(F("WARNING: Timesteps shorter than 10s can lead to unreliable measurements!"));
+      }
       scd30.setMeasurementInterval(config::measurement_timestep); // [s]
     }
     if ((current_state == PREPARE_CALIBRATION_UNSTABLE) || (current_state == PREPARE_CALIBRATION_STABLE)) {
@@ -313,7 +322,7 @@ namespace sensor {
   }
 
   void calibrateSensorRightNow(int32_t calibrationLevel) {
-    stable_measurements = config::enough_stable_measurements;
+    stable_measurements = config::stable_measurements_before_calibration;
     calibrateSensorToSpecificPPM(calibrationLevel);
   }
 }

ampel-firmware/config.public.h

@@ -27,7 +27,9 @@
  */
 
 // How often should measurement be performed, and displayed?
-//NOTE: SCD30 timer does not seem to be very precise. Variations may occur.
+//WARNING: On some sensors, measurements become very unreliable when timestep is set to 2s.
+//NOTE: 10s or longer should be fine in order to get reliable results.
+//NOTE: SCD30 timer does not seem to be very precise. Time variations may occur.
 #  define MEASUREMENT_TIMESTEP 60 // [s] Value between 2 and 1800 (range for SCD30 sensor)
 
 // How often should measurements be appended to CSV ?