Extract unit conversion helper functions and document blind spot limitation

- Add getPressureConversionFactor() helper function for PSI/bar/kPa/mbar/inH2O conversions
- Add getDistanceConversionFactor() helper function for m/cm/ft/in conversions
- Refactor 3 places with duplicated pressure conversion logic to use helper
- Refactor ultrasonic distance conversion to use helper
- Document sensor blind spot limitation in README (sensors cannot detect liquid below mount height)

Co-authored-by: dorkmo <[email protected]>

Author: Copilot

TankAlarm-112025-Client-BluesOpta/README.md

@@ -102,6 +102,8 @@ Used for sensors like the Dwyer 626-06-CB-P1-E5-S1 (0-5 PSI) mounted near the bo
   - `sensorRangeUnit`: Pressure unit - "PSI", "bar", "kPa", "mbar", or "inH2O"
 - **Sensor Mount Height**: Height of sensor above tank bottom (usually 0-2 inches)
 
+**Known Limitation - Blind Spot:** Pressure sensors cannot detect liquid levels below their mount height. When the tank is empty (0 PSI), the reported level will be the sensor mount height (e.g., 2"), not 0". Mount the sensor as close to the tank bottom as possible to minimize this blind spot.
+
 **Pressure-to-Height Conversion:**
 The system automatically converts pressure to inches using these factors:
 - 1 PSI = 27.68 inches of water
@@ -122,7 +124,7 @@ The system automatically converts pressure to inches using these factors:
 
 **How It Works:**
 1. 4mA → 0 PSI → 0 inches of liquid above sensor
-2. Total height = 0 + 2" mount height = 2" (sensor position, not liquid)
+2. Total height = 0 + 2" mount height = 2" (minimum reported value due to blind spot)
 3. When tank fills: 12mA → 2.5 PSI → 69.2" + 2" = 71.2" total
 
 #### Ultrasonic Sensor (Top-Mounted)

TankAlarm-112025-Client-BluesOpta/TankAlarm-112025-Client-BluesOpta.ino

@@ -230,6 +230,23 @@ static size_t strlcpy(char *dst, const char *src, size_t size) {
 #define MBAR_TO_INCHES_WATER 0.4015f        // 1 mbar = 0.4015 inches of water
 #endif
 
+// Helper function: Get pressure-to-inches conversion factor based on unit
+static float getPressureConversionFactor(const char* unit) {
+  if (strcmp(unit, "bar") == 0) return BAR_TO_INCHES_WATER;
+  if (strcmp(unit, "kPa") == 0) return KPA_TO_INCHES_WATER;
+  if (strcmp(unit, "mbar") == 0) return MBAR_TO_INCHES_WATER;
+  if (strcmp(unit, "inH2O") == 0) return 1.0f;
+  return PSI_TO_INCHES_WATER; // Default: PSI
+}
+
+// Helper function: Get distance-to-inches conversion factor based on unit
+static float getDistanceConversionFactor(const char* unit) {
+  if (strcmp(unit, "m") == 0) return METERS_TO_INCHES;
+  if (strcmp(unit, "cm") == 0) return CENTIMETERS_TO_INCHES;
+  if (strcmp(unit, "ft") == 0) return FEET_TO_INCHES;
+  return 1.0f; // Default: assume inches
+}
+
 static const uint8_t NOTECARD_I2C_ADDRESS = 0x17;
 static const uint32_t NOTECARD_I2C_FREQUENCY = 400000UL;
 
@@ -1474,21 +1491,12 @@ static bool validateSensorReading(uint8_t idx, float reading) {
 
   if ((isCurrentLoop || isAnalogWithVoltageRange) && hasNativeRange) {
     // For sensors with native range, calculate max from sensor range
-    float conversionFactor = PSI_TO_INCHES_WATER;
     if (isCurrentLoop && cfg.currentLoopType == CURRENT_LOOP_ULTRASONIC) {
       // Ultrasonic: max level is sensorMountHeight (when tank is full)
       maxValid = cfg.sensorMountHeight * 1.1f;
     } else {
       // Pressure: calculate max from pressure range
-      if (strcmp(cfg.sensorRangeUnit, "bar") == 0) {
-        conversionFactor = BAR_TO_INCHES_WATER;
-      } else if (strcmp(cfg.sensorRangeUnit, "kPa") == 0) {
-        conversionFactor = KPA_TO_INCHES_WATER;
-      } else if (strcmp(cfg.sensorRangeUnit, "mbar") == 0) {
-        conversionFactor = MBAR_TO_INCHES_WATER;
-      } else if (strcmp(cfg.sensorRangeUnit, "inH2O") == 0) {
-        conversionFactor = 1.0f;
-      }
+      float conversionFactor = getPressureConversionFactor(cfg.sensorRangeUnit);
       maxValid = (cfg.sensorRangeMax * conversionFactor + cfg.sensorMountHeight) * 1.1f;
     }
     minValid = -maxValid * 0.1f;
@@ -1642,18 +1650,7 @@ static float readTankSensor(uint8_t idx) {
                                  cfg.sensorRangeMin, cfg.sensorRangeMax);
 
       // Convert pressure to liquid height in inches using appropriate conversion factor
-      float conversionFactor = PSI_TO_INCHES_WATER; // Default: PSI
-      if (strcmp(cfg.sensorRangeUnit, "bar") == 0) {
-        conversionFactor = BAR_TO_INCHES_WATER;
-      } else if (strcmp(cfg.sensorRangeUnit, "kPa") == 0) {
-        conversionFactor = KPA_TO_INCHES_WATER;
-      } else if (strcmp(cfg.sensorRangeUnit, "mbar") == 0) {
-        conversionFactor = MBAR_TO_INCHES_WATER;
-      } else if (strcmp(cfg.sensorRangeUnit, "inH2O") == 0) {
-        conversionFactor = 1.0f;  // Already in inches of water
-      }
-      // else assume PSI
-      
+      float conversionFactor = getPressureConversionFactor(cfg.sensorRangeUnit);
       float liquidAboveSensor = pressure * conversionFactor;
 
       // Total height from tank bottom = liquid above sensor + sensor mount height
@@ -1693,15 +1690,7 @@ static float readTankSensor(uint8_t idx) {
                                          cfg.sensorRangeMin, cfg.sensorRangeMax);
 
         // Convert distance to inches based on sensorRangeUnit
-        float distanceInches = distanceNative;
-        if (strcmp(cfg.sensorRangeUnit, "m") == 0) {
-          distanceInches = distanceNative * METERS_TO_INCHES;
-        } else if (strcmp(cfg.sensorRangeUnit, "cm") == 0) {
-          distanceInches = distanceNative * CENTIMETERS_TO_INCHES;
-        } else if (strcmp(cfg.sensorRangeUnit, "ft") == 0) {
-          distanceInches = distanceNative * FEET_TO_INCHES;
-        }
-        // else assume already in inches
+        float distanceInches = distanceNative * getDistanceConversionFactor(cfg.sensorRangeUnit);
 
         // Calculate liquid level: tank height - distance from sensor to surface
         levelInches = cfg.sensorMountHeight - distanceInches;
@@ -1719,18 +1708,7 @@ static float readTankSensor(uint8_t idx) {
                                    cfg.sensorRangeMin, cfg.sensorRangeMax);
 
         // Convert pressure to liquid height in inches using appropriate conversion factor
-        float conversionFactor = PSI_TO_INCHES_WATER; // Default: PSI
-        if (strcmp(cfg.sensorRangeUnit, "bar") == 0) {
-          conversionFactor = BAR_TO_INCHES_WATER;
-        } else if (strcmp(cfg.sensorRangeUnit, "kPa") == 0) {
-          conversionFactor = KPA_TO_INCHES_WATER;
-        } else if (strcmp(cfg.sensorRangeUnit, "mbar") == 0) {
-          conversionFactor = MBAR_TO_INCHES_WATER;
-        } else if (strcmp(cfg.sensorRangeUnit, "inH2O") == 0) {
-          conversionFactor = 1.0f;  // Already in inches of water
-        }
-        // else assume PSI
-        
+        float conversionFactor = getPressureConversionFactor(cfg.sensorRangeUnit);
         float liquidAboveSensor = pressure * conversionFactor;
 
         // Total height from tank bottom = liquid above sensor + sensor mount height