diff --git a/measure.py b/measure.py
index ea0b50f827affeccd02a9e69d90f6640062efb01..53aab29d9e0ddc884a8e392421624e23afa47952 100644
--- a/measure.py
+++ b/measure.py
@@ -62,7 +62,9 @@ class OhmPiHardware:
     def _read_values(self, sampling_rate, append=False):  # noqa
         if not append:
             self._clear_values()
-        _readings = []
+            _readings = []
+        else:
+            _readings = self.readings.tolist()
         sample = 0
         self.tx_sync.wait()
         start_time = datetime.datetime.utcnow()
@@ -72,7 +74,7 @@ class OhmPiHardware:
             sample+=1
             sleep_time = start_time + datetime.timedelta(seconds = sample * sampling_rate / 1000) - lap
             time.sleep(np.min([0, np.abs(sleep_time.total_seconds())]))
-        self.readings = np.hstack([self.readings, np.array(_readings)])
+        self.readings = np.array(_readings)
 
     def _compute_tx_volt(self, best_tx_injtime=0.1, strategy='vmax', tx_volt=5,
                          vab_max=voltage_max, vmn_min=voltage_min):