diff --git a/ohmpi/hardware_system.py b/ohmpi/hardware_system.py
index 4509e0ceed4258962fb1de43ebec94e463cb3bad..f2526fb19c361a110d37d25236519b3f26eee8ed 100644
--- a/ohmpi/hardware_system.py
+++ b/ohmpi/hardware_system.py
@@ -270,7 +270,7 @@ class OhmPiHardware:
     def last_dev(self, delay=0.):
         v = np.where((self.readings[:, 0] >= delay) & (self.readings[:, 2] != 0))[0]
         if len(v) > 1:
-            return 100. * np.std(self.readings[v, 2] * (self.readings[v, 4] - self.sp) / self.readings[v, 3]) / self.last_resistance
+            return 100. * np.std(self.readings[v, 2] * (self.readings[v, 4] - self.sp) / self.readings[v, 3]) / self.last_resistance(delay=delay)
         else:
             return np.nan