diff --git a/tests/concentrator/concentrator.py b/tests/concentrator/concentrator.py
index c4e8844..ab6655b 100755
--- a/tests/concentrator/concentrator.py
+++ b/tests/concentrator/concentrator.py
@@ -1,8 +1,8 @@
 #!/usr/bin/env python3
 from sensor import Sensor
-from sys import argv,exit
+from sys import argv, exit
 from flask import Flask
-from time import sleep
+from time import sleep, time
 import threading
 
 app = Flask('Sensor central unit')
@@ -43,7 +43,7 @@ flask_thread.start()
 while True:
     # logging: for now just writing to csv file (can be anything: write to db, mqtt...)
     for s in sensors:
-        log_string = ''
+        log_string = f'{time():.1f} '
         for reg_name, reg_number in s.input_registers.items():
             with modbus_mutex:
                 log_string += str(int(s.read_register(reg_number))) + ' '