From 583a26a6a4adb29336c7e5a6dbd8a7fdb582c668 Mon Sep 17 00:00:00 2001
From: Hendrik Langer <hendrik+dev@xd0.de>
Date: Wed, 7 Aug 2019 15:41:40 +0200
Subject: [PATCH] pressure compensation to sea level

---
 src/main.cpp | 37 +++++++++++++++++++++++++++++--------
 1 file changed, 29 insertions(+), 8 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index 5ee5ab7..2e0d7e9 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -59,7 +59,8 @@ XD0MQTT mqtt;
 struct __attribute__((packed)) sensor_readings_t {
   float temperature = NAN; // °C
   float humidity = NAN; // %H
-  float pressure = NAN; // Pa
+  float pressure = NAN; // hPa
+  float pressure_raw = NAN; // Pa
   uint32_t voc = 0; // Ohm
   float pm10 = NAN; // µg/m³
   float pm25 = NAN; // µg/m³
@@ -85,6 +86,8 @@ bool uv_active = false;
 bool light_active = false;
 bool sds_active = false;
 
+float station_height = 0;
+
 void helloWorld()
 {
   const char HelloWorld[] = "IchbinsBens!";
@@ -139,13 +142,13 @@ void getSensorMeasurements() {
     bme280.takeForcedMeasurement();
     sensor_readings.temperature = bme280.readTemperature();
     sensor_readings.humidity = bme280.readHumidity();
-    sensor_readings.pressure = bme280.readPressure();
+    sensor_readings.pressure_raw = bme280.readPressure();
   }
   if (bme680_active) {
     if (bme680.performReading()) {
       sensor_readings.temperature = bme680.temperature;
       sensor_readings.humidity = bme680.humidity;
-      sensor_readings.pressure = bme680.pressure;
+      sensor_readings.pressure_raw = bme680.pressure;
       sensor_readings.voc = bme680.gas_resistance;
     } else {
       Serial.println("Failed to perform reading :(");
@@ -161,7 +164,14 @@ void getSensorMeasurements() {
     sensor_readings.temperature_min = sensor_readings.temperature;
   }
 
-  history_pressure.addValue(sensor_readings.pressure / 100.0F);
+  // https://de.wikipedia.org/wiki/Barometrische_H%C3%B6henformel#Reduktion_auf_Meeresh%C3%B6he
+  // https://carnotcycle.wordpress.com/2012/08/04/how-to-convert-relative-humidity-to-absolute-humidity/
+  float absolute_humidity = (6.112*exp((17.67*sensor_readings.temperature)/(sensor_readings.temperature+243.5))*(sensor_readings.humidity/100)*18.02)/((273.15+sensor_readings.temperature)*1000*0.08314);
+  float pressure_compensation_factor = exp((9.80665/(287.05*(sensor_readings.temperature+273.15+0.12*((absolute_humidity*461.5*(sensor_readings.temperature+273.15)/100)/100)+0.0065*(station_height/2))))*station_height);
+
+  sensor_readings.pressure = (sensor_readings.pressure_raw / 100.0F) * pressure_compensation_factor;
+
+  history_pressure.addValue(sensor_readings.pressure);
 
   if (uv_active) {
     sensor_readings.uvi = uv.readUVI();
@@ -288,7 +298,7 @@ void displayValues() {
     display.print("Pressure");
     display.setFont(&FreeSansBold9pt7b);
     display.setCursor(135,y_offset+40);
-    display.printf("%.1f", sensor_readings.pressure / 100.0F);
+    display.printf("%.1f", sensor_readings.pressure);
     display.setFont(NULL);
     //display.print(" hPa");
     float pressure_diff = history_pressure.getElement(0) - history_pressure.getFirst();
@@ -430,8 +440,11 @@ void printValues() {
     Serial.println(" *C");
 
     Serial.print("Pressure = ");
+    Serial.print(sensor_readings.pressure);
+    Serial.println(" hPa");
 
-    Serial.print(sensor_readings.pressure / 100.0F);
+    Serial.print("Pressure [RAW] = ");
+    Serial.print(sensor_readings.pressure_raw / 100.0F);
     Serial.println(" hPa");
 
     Serial.print("Humidity = ");
@@ -442,7 +455,7 @@ void printValues() {
   if (bme680_active) {
     Serial.print("VOC = ");
     Serial.print(sensor_readings.voc / 1000.0F);
-    Serial.println(" hPa");
+    Serial.println(" kOhm");
   }
 
   Serial.println();
@@ -477,7 +490,7 @@ void sendValues() {
     delay(10);
     mqtt.publish(topic_humidity.c_str(), sensor_readings.humidity, "%.2f");
     delay(10);
-    mqtt.publish(topic_pressure.c_str(), sensor_readings.pressure / 100.0F, "%.2f");
+    mqtt.publish(topic_pressure.c_str(), sensor_readings.pressure, "%.2f");
     delay(10);
   }
   if (bme680_active) {
@@ -652,6 +665,14 @@ void setup()
   const char* fw_version_str = String(FW_VERSION).c_str();
   mqtt.publish(topic_version.c_str(), fw_version_str, strlen(fw_version_str));
 
+  if (WiFi.SSID() == "LNet") {
+    station_height = 135;
+  } else if (WiFi.SSID() == "Galaktisches Imperium") {
+    station_height = 30;
+  } else if (WiFi.SSID() == "nether.net") {
+    station_height = 111;
+  }
+
   ESP_LOGD(TAG, "setup done");
 }