Publish dew point approximation to mqtt

config.schema.json

@@ -114,6 +114,10 @@
             "type": "string",
             "title": "Humidity Topic"
           },
+          "dewPointTopic": {
+            "type": "string",
+            "title": "Dew Point Topic"
+          },
           "batteryTopic": {
             "type": "string",
             "title": "Battery Topic"
@@ -169,6 +173,7 @@
         "mqtt.url",
         "mqtt.temperatureTopic",
         "mqtt.humidityTopic",
+        "mqtt.dewPointTopic",
         "mqtt.batteryTopic"
       ]
     },

lib/accessory.js

@@ -29,6 +29,7 @@ class HygrothermographAccessory {
 
     this.temperatureMQTTTopic = undefined;
     this.humidityMQTTTopic = undefined;
+    this.dewPointMQTTTopic = undefined;
     this.batteryMQTTTopic = undefined;
     this.mqttClient = this.setupMQTTClient();
 
@@ -37,6 +38,23 @@ class HygrothermographAccessory {
     this.log.debug("Initialized accessory");
   }
 
+  get dewPoint() {
+    if (
+      this.hasTimedOut() ||
+      this.latestTemperature == null ||
+      this.latestHumidity == null
+    ) {
+      return;
+    }
+    // Magnus dew point approximation
+    const t = this.temperature;
+    const h = this.humidity;
+    const a = 17.27;
+    const b = 237.7;
+    const alpha = (a * t) / (b + t) + Math.log(h / 100);
+    return (b * alpha) / (a - alpha);
+  }
+
   setTemperature(newValue, force = false) {
     if (newValue == null) {
       return;
@@ -51,6 +69,9 @@ class HygrothermographAccessory {
       .updateValue(newValue);
     this.addFakeGatoHistoryEntry();
     this.publishValueToMQTT(this.temperatureMQTTTopic, this.temperature);
+    if (this.dewPoint) {
+      this.publishValueToMQTT(this.dewPointMQTTTopic, this.dewPoint);
+    }
   }
 
   get temperature() {
@@ -74,6 +95,9 @@ class HygrothermographAccessory {
       .updateValue(newValue);
     this.addFakeGatoHistoryEntry();
     this.publishValueToMQTT(this.humidityMQTTTopic, this.humidity);
+    if (this.dewPoint) {
+      this.publishValueToMQTT(this.dewPointMQTTTopic, this.dewPoint);
+    }
   }
 
   get humidity() {
@@ -268,12 +292,14 @@ class HygrothermographAccessory {
       temperatureTopic,
       humidityTopic,
       batteryTopic,
+      dewPointTopic,
       url,
       ...mqttOptions
     } = config;
 
     this.temperatureMQTTTopic = temperatureTopic;
     this.humidityMQTTTopic = humidityTopic;
+    this.dewPointMQTTTopic = dewPointTopic;
     this.batteryMQTTTopic = batteryTopic;
 
     const client = mqtt.connect(url, mqttOptions);