Index: firmware/App/Modes/ModeTreatment.c
===================================================================
diff -u -rfba69244d94307e50fefaa1e88bcbb979584461e -r5046a41e436df0e43e5c9c83eac1bc3a3db20b3c
--- firmware/App/Modes/ModeTreatment.c	(.../ModeTreatment.c)	(revision fba69244d94307e50fefaa1e88bcbb979584461e)
+++ firmware/App/Modes/ModeTreatment.c	(.../ModeTreatment.c)	(revision 5046a41e436df0e43e5c9c83eac1bc3a3db20b3c)
@@ -1056,23 +1056,23 @@
          ( currentTreatmentState > TREATMENT_START_STATE ) && ( currentTreatmentState < TREATMENT_END_STATE ) &&
          ( CALC_ELAPSED_TREAT_TIME_IN_MIN() < treatmentTime ) && ( treatmentTime >= getMinTreatmentTimeInMinutes() ) )
     {
-        // Always adjust UF volume to accommodate treatment time change (not UF rate)
-        F32 uFVolume          = ( (F32)( treatmentTime - CALC_ELAPSED_TREAT_TIME_IN_MIN() ) * presUFRate ) + getUltrafiltrationReferenceVolume();
-        F32 uFVolumeL         = uFVolume / ML_PER_LITER;
+        // Always keep the UF volume the same and change the UF rate upon changing the treatment time
+        presMaxUFVolumeML = getTreatmentParameterF32( TREATMENT_PARAM_UF_VOLUME ) * (F32)ML_PER_LITER;
+        presUFRate        = presMaxUFVolumeML / (F32)treatmentTime;
+
         U32 dialVolume        = getTreatmentParameterU32( TREATMENT_PARAM_DIALYSATE_FLOW ) * treatmentTime;   // In mL
+
         // The minimum treatment time is either in the range of the institutional record or the 1-minute treatment config has been requested which is an exception
         BOOL isMinTxTimeValid = ( ( treatmentTime >= getU32TreatmentParamLowerRangeLimit( TREATMENT_PARAM_TREATMENT_DURATION ) ) ||
                                   ( TRUE == getTestConfigStatus( TEST_CONFIG_ENABLE_ONE_MINUTE_TREATMENT ) ) ? TRUE : FALSE );
         BOOL isTxTimeValid    = ( ( TRUE == isMinTxTimeValid ) &&
                                   ( treatmentTime <= getU32TreatmentParamUpperRangeLimit( TREATMENT_PARAM_TREATMENT_DURATION ) ) ? TRUE : FALSE );
-        BOOL isMinUFVolValid  = ( ( uFVolumeL >= getF32TreatmentParamLowerRangeLimit( TREATMENT_PARAM_UF_VOLUME ) ) || ( uFVolumeL <= 0.0F ) ? TRUE : FALSE );
-        BOOL isUFVolValid     = ( ( TRUE == isMinUFVolValid ) && ( uFVolumeL <= getF32TreatmentParamUpperRangeLimit( TREATMENT_PARAM_UF_VOLUME ) ) ? TRUE : FALSE );
+        BOOL isUFRateValid    = ( ( presUFRate >= MIN_UF_RATE_ML_MIN ) && ( presUFRate <= MAX_UF_RATE_ML_MIN ) ? TRUE : FALSE );
 
-        if ( ( TRUE == isTxTimeValid ) && ( dialVolume <= MAX_DIALYSATE_VOLUME_ML ) && ( TRUE == isUFVolValid ) )
+        if ( ( TRUE == isTxTimeValid ) && ( dialVolume <= MAX_DIALYSATE_VOLUME_ML ) && ( TRUE == isUFRateValid ) )
         {
             result = TRUE;
             sendTreatmentLogEventData( TREATMENT_DURATION_CHANGE_EVENT, ( presTreatmentTimeSecs / SEC_PER_MIN ), treatmentTime );
-            presMaxUFVolumeML = uFVolume;
             presTreatmentTimeSecs = treatmentTime * SEC_PER_MIN;
             setTreatmentParameterF32( TREATMENT_PARAM_UF_VOLUME, ( presMaxUFVolumeML / (F32)ML_PER_LITER ) );
             setTreatmentParameterU32( TREATMENT_PARAM_TREATMENT_DURATION, ( presTreatmentTimeSecs / SEC_PER_MIN ) );
@@ -1085,9 +1085,9 @@
             {
                 rejectReason = REQUEST_REJECT_REASON_TREATMENT_TIME_OUT_OF_RANGE;
             }
-            else if ( FALSE == isUFVolValid )
+            else if ( FALSE == isUFRateValid )
             {
-                rejectReason = REQUEST_REJECT_REASON_UF_VOLUME_OUT_OF_RANGE;
+                rejectReason = REQUEST_REJECT_REASON_UF_RATE_OUT_OF_RANGE;
             }
             else
             {