diff --git a/ohmpi/hardware_system.py b/ohmpi/hardware_system.py
index 38f3782c2c8508304f1e4f57d691855783a61fb9..d7f51fc817860c9747c2ca137a649d460d48f360 100644
--- a/ohmpi/hardware_system.py
+++ b/ohmpi/hardware_system.py
@@ -326,8 +326,8 @@ class OhmPiHardware:
         plt.show()
 
     def calibrate_rx_bias(self):
-        self.rx._bias = (np.mean(self.readings[self.readings[:, 2] == 1, 4])
-                         + np.mean(self.readings[self.readings[:, 2] == -1, 4])) / 2.
+        self.rx._bias += (np.mean(self.readings[self.readings[:, 2] == 1, 4])
+                          + np.mean(self.readings[self.readings[:, 2] == -1, 4])) / 2.
 
     def vab_square_wave(self, vab, cycle_duration, sampling_rate=None, cycles=3, polarity=1, append=False):
         self.exec_logger.event(f'OhmPiHardware\tvab_square_wave\tbegin\t{datetime.datetime.utcnow()}')