Index: firmware/App/Controllers/AirTrap.c
===================================================================
diff -u -r3698f3664dd6102ba378f85f7fa65fc9ee6dc5a1 -rb0e7750d1f612f98d21513eb18d0971fa1c7547a
--- firmware/App/Controllers/AirTrap.c	(.../AirTrap.c)	(revision 3698f3664dd6102ba378f85f7fa65fc9ee6dc5a1)
+++ firmware/App/Controllers/AirTrap.c	(.../AirTrap.c)	(revision b0e7750d1f612f98d21513eb18d0971fa1c7547a)
@@ -358,9 +358,9 @@
         }
     }
 
-    // Raise air trap level once at start of treatment (blood priming) or when air detected at upper level
-    else if ( ( TRUE == airTrapWasFilledAtStartOfTreatement ) ||
-              ( AIR_TRAP_LEVEL_AIR == getLevelSensorState( H16_LEVL ) ) )
+    // Raise air trap level once at start of treatment (blood priming) or when air detected at lower level
+    else if ( ( airTrapWasFilledAtStartOfTreatement != TRUE ) ||
+              ( AIR_TRAP_LEVEL_AIR == getLevelSensorState( H17_LEVL ) ) )
     {
         setAirPumpState( AIR_PUMP_STATE_ON, AIR_PUMP_MOTOR_FILL_PWM );
         set3WayValveState( H20_VALV, VALVE_3WAY_COMMON_TO_OPEN_STATE );  // open valve H20 to allow air to be pumped out of air trap to atmosphere
@@ -393,16 +393,17 @@
         set3WayValveState( H20_VALV, VALVE_3WAY_COMMON_TO_CLOSED_STATE );
         result = AIR_TRAP_MANUAL_CONTROL_STATE;
     }
-    // Transition back to closed state when air trap fill completed
+    // Transition back to lower level state when air trap fill completed
     else if ( AIR_TRAP_LEVEL_FLUID == getRawLevelSensorState( H16_LEVL ) )
     {
         airTrapWasFilledAtStartOfTreatement = TRUE;
-        setAirPumpState( AIR_PUMP_STATE_OFF, AIR_PUMP_MOTOR_OFF );
         set3WayValveState( H20_VALV, VALVE_3WAY_COMMON_TO_CLOSED_STATE );
-        airTrapLowerDelayStartTime = calcTimeBetween( AIR_PUMP_ON_DELAY_ADJUST_AFTER_FILL, getMSTimerCount() );
-        signalLowVenousPressureCheck();
-        SEND_EVENT_WITH_2_U32_DATA( TD_EVENT_AIR_TRAP_FILL, OFF, 0 );
-        result = AIR_TRAP_CLOSED_STATE;
+        set3WayValveState( H13_VALV, VALVE_3WAY_COMMON_TO_OPEN_STATE );   // open valve H13 to allow air to be pumped into air trap from atmosphere
+        setAirPumpState( AIR_PUMP_STATE_ON, AIR_PUMP_MOTOR_LOWER_PWM );
+        airTrapLowerStartTime = getMSTimerCount();
+        signalInitiatePressureStabilization( USE_SHORT_STABILIZATION_PERIOD );
+        SEND_EVENT_WITH_2_U32_DATA( TD_EVENT_AIR_TRAP_LOWER, ON, 0 );
+        result = AIR_TRAP_LOWER_LEVEL_STATE;
     }
 
     return result;
@@ -433,14 +434,15 @@
         set3WayValveState( H13_VALV, VALVE_3WAY_COMMON_TO_CLOSED_STATE );
         result = AIR_TRAP_MANUAL_CONTROL_STATE;
     }
-    // Transition back to closed state when air trap lower level operation completed
+    // Transition back to closed state when air trap lower level operation completed when air detected at upper level
     else if ( ( TRUE == didTimeout( airTrapLowerStartTime, airPumpStopTime ) ) ||
               ( AIR_TRAP_LEVEL_AIR   == getRawLevelSensorState( H16_LEVL ) ) )
 
     {
         setAirPumpState( AIR_PUMP_STATE_OFF, AIR_PUMP_MOTOR_OFF );
         set3WayValveState( H13_VALV, VALVE_3WAY_COMMON_TO_CLOSED_STATE );
         airTrapLowerDelayStartTime = getMSTimerCount();
+        signalLowVenousPressureCheck();
         SEND_EVENT_WITH_2_U32_DATA( TD_EVENT_AIR_TRAP_LOWER, OFF, 0 );
         // if lower level operation times out, alarm
         if ( TRUE == didTimeout( airTrapLowerStartTime, airPumpStopTime ) )