diff --git a/gui/main.py b/gui/main.py
index 94fc3cc..05ad131 100644
--- a/gui/main.py
+++ b/gui/main.py
@@ -47,8 +47,14 @@ class PHControllerGUI(QMainWindow):
         self.pump_status.setFont(QFont('Arial', 16))
         self.pump_status.setAlignment(Qt.AlignmentFlag.AlignCenter)
         
+        self.flow_rate_label = QLabel("ml/sec: --")
+        self.flow_rate_label.setFont(QFont('Arial', 14))
+        self.flow_rate_label.setAlignment(Qt.AlignmentFlag.AlignCenter)
+        self.flow_rate_label.setVisible(False)  # Initially hidden
+        
         status_layout.addWidget(self.ph_label)
         status_layout.addWidget(self.pump_status)
+        status_layout.addWidget(self.flow_rate_label)
         status_group.setLayout(status_layout)
         
         # Control Group
@@ -134,6 +140,13 @@ class PHControllerGUI(QMainWindow):
         self.tube_request_timer = QTimer()
         self.tube_request_timer.timeout.connect(self.request_tube_id)
         
+        # Timer for flow rate requests (when pump is running)
+        self.flow_rate_timer = QTimer()
+        self.flow_rate_timer.timeout.connect(self.request_flow_rate)
+        
+        # Track pump state
+        self.pump_is_running = False
+        
         # Enable/disable controls based on connection
         self.set_controls_enabled(False)
     
@@ -183,11 +196,16 @@ class PHControllerGUI(QMainWindow):
         # Stop tube ID polling
         self.tube_request_timer.stop()
         
+        # Stop flow rate polling
+        self.flow_rate_timer.stop()
+        
         self.connect_btn.setText("Verbinden")
         self.status_bar.showMessage("Getrennt")
         self.set_controls_enabled(False)
         self.ph_label.setText("pH: --")
         self.pump_status.setText("Pumpe: --")
+        self.flow_rate_label.setVisible(False)
+        self.pump_is_running = False
     
     def set_controls_enabled(self, enabled):
         self.pump_on_btn.setEnabled(enabled)
@@ -223,21 +241,31 @@ class PHControllerGUI(QMainWindow):
         if self.serial_connection and self.serial_connection.is_open:
             self.send_command("<13>")  # Request current tube size
 
+    def request_flow_rate(self):
+        """Request flow rate from Arduino when pump is running"""
+        if self.serial_connection and self.serial_connection.is_open and self.pump_is_running:
+            self.send_command("<18>")  # Request current flow rate
+
     def send_pump_command(self, state):
         if state == 1:
             self.send_command("<11>")  # Pump on
             self.pump_status.setText("Pumpe: Ein")
+            self.pump_is_running = True
+            self.flow_rate_label.setVisible(True)
+            self.flow_rate_timer.start(1000)  # Request flow rate every second
         else:
             self.send_command("<12>")  # Pump off
             self.pump_status.setText("Pumpe: Aus")
+            self.pump_is_running = False
+            self.flow_rate_label.setVisible(False)
+            self.flow_rate_timer.stop()
     
     def send_tube_command(self, index):
         # Only send command if not updating from Arduino response
         if not self.updating_tube_combo:
-            # Simple approach: send <1XX> where XX is 20+index
-            # This should trigger the default case in Arduino
-            # command_number = 120 + index  # 120-145 for tubes 0-25
-            self.send_command(f"<17{index}>")
+            # Send command in format <120> to <145> for tube indices 0-25
+            command_number = 120 + index  # 120-145 for tubes 0-25
+            self.send_command(f"<{command_number}>")
             self.status_bar.showMessage(f"Schlauchgröße {self.tube_combo.itemText(index)} ausgewählt")
     
     def send_calibration_command(self, ph):
@@ -288,6 +316,12 @@ class PHControllerGUI(QMainWindow):
                                 self.status_bar.showMessage(f"Schlauchgröße {self.tube_combo.currentText()} geladen")
                         except ValueError:
                             pass
+                    elif data.startswith('F'):  # Flow rate (ml/sec)
+                        try:
+                            flow_rate = float(data[1:])
+                            self.flow_rate_label.setText(f"ml/sec: {flow_rate:.4f}")
+                        except ValueError:
+                            pass
         except Exception as e:
             self.status_bar.showMessage(f"Lesefehler: {str(e)}")
     
@@ -296,6 +330,7 @@ class PHControllerGUI(QMainWindow):
         self.read_timer.stop()
         self.ph_request_timer.stop()
         self.tube_request_timer.stop()
+        self.flow_rate_timer.stop()
         self.close_connection()
         event.accept()
 
diff --git a/ph_controller/ph_controller.ino b/ph_controller/ph_controller.ino
index 65ade39..0aff296 100644
--- a/ph_controller/ph_controller.ino
+++ b/ph_controller/ph_controller.ino
@@ -293,18 +293,32 @@ if ( RxCmd[1] == '1' ){
           }
       break;
     }
-    case '8':{ // Schlauchdaten anfordern
-      Serial.print('<');
-      Serial.print('#');
-      double mlpersec =( ml[tubid]*korrf*2)/6;          // tube * 20 Um/min * korecturfaktor durch 60 sec
-      mlpersec = mlpersec*100000;
-      Serial.print(mlpersec);                          //      
-      Serial.println('>');      
+    case '8':{// Flow Rate anfordern (ml/sec)
+      Serial.print("<F");
+      double mlpersec = (ml[tubid] * korrf * 20) / 60;  // tube * korrektur * 20 rpm / 60 sec
+      Serial.print(mlpersec, 4);  // 4 decimal places
+      Serial.println(">");
       break;
     }
     
   }
-
+} else {
+  // Handle other commands like tube selection <120> to <145>
+  if (RxCmd[0] == '<' && RxCmd[1] == '1') {
+    // Parse 3-digit commands starting with 1
+    int cmd = (RxCmd[1] - '0') * 100 + (RxCmd[2] - '0') * 10 + (RxCmd[3] - '0');
+    if (cmd >= 120 && cmd <= 145) {
+      // Tube selection: 120-145 maps to tube indices 0-25
+      int new_tubid = cmd - 120;
+      if (new_tubid >= 0 && new_tubid <= MAXTUB) {
+        tubid = new_tubid;
+        EEPROM.put(ee_tubid, tubid);
+        Serial.print("<T");
+        Serial.print(tubid);
+        Serial.println(">"); // Bestätigung
+      }
+    }
+  }
 }