Fix mA to PSI conversion

pod-operation/src/components/pressure_transducer.rs

@@ -20,7 +20,13 @@ const INA219_SCALING_VALUE: f32 = 160.0;
 // The pressure transducer outputs a current between 4 mA and 20 mA with 0 PSI
 // and 300 PSI respectively. Assuming a linear interpolation, a 1 mA increase
 // results in a 18.75 PSI increase.
-const MA_TO_PSI: f32 = 18.75;
+const REF_CURRENT_LOW: f32 = 4.0;
+const REF_CURRENT_HIGH: f32 = 20.0;
+const REF_PRESSURE_LOW: f32 = 0.0;
+const REF_PRESSURE_HIGH: f32 = 300.0;
+
+const REF_CURRENT_SPAN: f32 = REF_CURRENT_HIGH - REF_CURRENT_LOW;
+const REF_PRESSURE_SPAN: f32 = REF_PRESSURE_HIGH - REF_PRESSURE_LOW;
 
 impl PressureTransducer {
 	pub fn new(ina219_addr: u8) -> Self {
@@ -38,7 +44,10 @@ impl PressureTransducer {
 	// Read current from the INA219 and apply a scaling factor to translate
 	// the current reading to PSI.
 	pub fn read(&mut self) -> f32 {
-		self.read_current() * MA_TO_PSI
+		let current = self.read_current();
+
+		return REF_PRESSURE_LOW
+			+ REF_PRESSURE_SPAN * (current - REF_CURRENT_LOW) / REF_CURRENT_SPAN;
 	}
 
 	// Read from the INA219 and divide the reading by a scalar factor to