How to add calibration to a humidity contoller using UNO and DHT22 sensor?

Question

I am using a setup that helps controlling humidity in an enclosure used to store a humidity sensitive product. My original posting is here.

I'm using a NANO and a DHT22 temperature/humidity sensor to acquire information on the actual condition in the enclosure; I am also using a 75% RH calibration kit to check the accuracy oh the humidity reading by the UNO and I found the value 9 percentage points higher than the kit's calibration value. Right now, until I find a solution to calibrate the unit, I have set my setpoint 9 points higher to compensate for the error. Any ideas on how to tackle this issue would be highly appreciated.

At the moment I can read the DHT sensor and show the humidity value on the LCD like this:

[...]
  int chk = DHT.read22(DHT22_PIN);
  lastcheck = millis();

  lcd.setCursor(0, 0);
  lcd.print("Humidity: ");
  lcd.setCursor(13, 0);
  lcd.print(DHT.humidity);
[...]

---

@Thomas_Weller, I'm running tests with your calibration routine and other code changes; I'm using your arbitrary values for testing purposes only; the compensation algorithm works, as sampled below, however the LCD output is the actual DHT22 reading, not compensated. I am testing with a different sensor, whose output is 9% below the actual RH value. Here's a sample of the serial output:

DHT value: 58
Compensated to: 55
DHT value: 59
Compensated to: 56
DHT value: 60
Compensated to: 57
DHT value: 61
Compensated to: 58
DHT value: 62
Compensated to: 59
DHT value: 63
Compensated to: 60
DHT value: 64
Compensated to: 60


setpoint = 72
DHT read = 66.80%
displ + backlights off
humidifier ON, fan OFF
setpoint = 72
displ + backlights off
humidifier ON, fan OFF
setpoint = 72
displ + backlights off
humidifier ON, fan OFF
setpoint = 72
DHT read = 66.80%
displ + backlights off
humidifier ON, fan OFF
setpoint = 72
displ + backlights off
humidifier ON, fan OFF
setpoint = 72
displ + backlights off
humidifier ON, fan OFF

Answer 1

To get better calibration, you need more calibration points. In the example below, I use 8 measurement points.

Given 8 humidity calibration bags for 20, 30, 40, 50, 60, 70, 75 and 80%.

int calvalues[] = {20, 30, 40, 50, 60, 70, 75, 80};

You would take several samples with the DHT sensor and build a table using the averages:

int dhtvalues[] = {15, 28, 39, 51, 63, 75, 84, 89};

As you can see, the DHT is below the real values in the lower range and above the real values in the upper range. But it could also be the opposite way.

You can then write this function:

#define COMPENSATION_POINTS 8

int compensate(int sensor_value)
{
  for(int index=1; index<COMPENSATION_POINTS; index++)
  {
    if (dhtvalues[index]>sensor_value)
    {
      int dhtLow = dhtvalues[index-1];
      int dhtHigh = dhtvalues[index];
      int calLow = calvalues[index-1];
      int calHigh = calvalues[index];

      return map(sensor_value, dhtLow, dhtHigh, calLow, calHigh);
    }
  }
}

which does linear interpolation for all the values that fall somewhere in between.

Here's some test code, which goes through the values in the expected range.

void setup() {
  Serial.begin(9600);
  for(int hum=15; hum<=89; hum++)
  {
    Serial.print("DHT value: ");
    Serial.println(hum);
    Serial.print("Compensated to: ");
    Serial.println(compensate(hum));
  }
}

void loop() {
  delay(1000);
}

Output:

DHT value: 15
Compensated to: 20
DHT value: 16
Compensated to: 20
DHT value: 17
Compensated to: 21
DHT value: 18
Compensated to: 22
...
DHT value: 87
Compensated to: 78
DHT value: 88
Compensated to: 79
DHT value: 89
Compensated to: 79

Applied to your case:

[...]
  int chk = DHT.read22(DHT22_PIN);
  lastcheck = millis();

  lcd.setCursor(0, 0);
  lcd.print("Humidity: ");
  lcd.setCursor(13, 0);
  int compensated = compensate(DHT.humidity);   // <- new line
  lcd.print(compensated);                       // <- changed line
[...]