diff --git a/ohmpi/hardware_system.py b/ohmpi/hardware_system.py
index e2316410f1968689b6a55a48b39f677042045132..45b25ea39d63589b2e4950e8b90a1cfd43ecea74 100644
--- a/ohmpi/hardware_system.py
+++ b/ohmpi/hardware_system.py
@@ -206,7 +206,8 @@ class OhmPiHardware:
     def last_rho(self):
         v = self.readings[:, 2] != 0
         if len(v) > 1:
-            return np.mean(np.abs(self.readings[v, 4] - self.sp) / self.readings[v, 3])
+            # return np.mean(np.abs(self.readings[v, 4] - self.sp) / self.readings[v, 3])
+            return np.mean((self.readings[v, 2] * self.readings[v, 4] - self.sp) / self.readings[v, 3])
         else:
             return np.nan
 
diff --git a/ohmpi/ohmpi.py b/ohmpi/ohmpi.py
index 9e9712aa5e9b8d9507ef456aefa5ff93932cac90..b12b8dc1c1d4624b079626356a32f798c5a0d453 100644
--- a/ohmpi/ohmpi.py
+++ b/ohmpi/ohmpi.py
@@ -459,6 +459,7 @@ class OhmPi(object):
             else:
                 delay = 0.
             x = np.where((self._hw.readings[:, 0] >= delay) & (self._hw.readings[:, 2] != 0))
+            print(f'length of series: {len(x)}')
             R = np.mean((self._hw.readings[x, 2] * self._hw.readings[x, 4] - self._hw.sp) / self._hw.readings[x, 3])
             R_std = 100. * np.std((self._hw.readings[x, 2] * self._hw.readings[x, 4] - self._hw.sp) / self._hw.readings[x, 3]) / R
             d = {