Adds sp in rho and dev

ohmpi/hardware_system.py

@@ -207,14 +207,14 @@ class OhmPiHardware:
     @property
     def last_rho(self):
         if len(self.readings) > 1:
-            return np.mean(np.abs(self.readings[:, 4]) / self.readings[:, 3])
+            return np.mean(np.abs(self.readings[:, 4] - self.sp) / self.readings[:, 3])
         else:
             return np.nan
 
     @property
     def last_dev(self):
         if len(self.readings) > 1:
-            return 100. * np.std(self.readings[:, 2] * self.readings[:, 4] / self.readings[:, 3])/self.last_rho
+            return 100. * np.std(self.readings[:, 2] * (self.readings[:, 4] - self.sp) / self.readings[:, 3])/self.last_rho
         else:
             return np.nan