diff --git a/ohmpi/hardware_system.py b/ohmpi/hardware_system.py
index 2ede95c137879ec953efb8c427e25daf070fe83e..0d6981147b16d19afa234c5731b50e9d670a30cb 100644
--- a/ohmpi/hardware_system.py
+++ b/ohmpi/hardware_system.py
@@ -317,7 +317,7 @@ class OhmPiHardware:
     def last_vmn_dev(self, delay=0.):  # TODO: should compute std per stack because this does not account for SP...
         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.last_vmn(delay=delay)
+            return 100. * np.std(self.readings[v, 2] * (self.readings[v, 4]) - self.sp) / self.last_vmn(delay=delay)
         else:
             return np.nan