diff --git a/ohmpi/ohmpi.py b/ohmpi/ohmpi.py
index 682b9a2338ffa2f5dc7d7f3d2af1f71f0900bce9..42b7672ec556a491ffd7e471c046f2d3c7d74f28 100644
--- a/ohmpi/ohmpi.py
+++ b/ohmpi/ohmpi.py
@@ -470,7 +470,7 @@ class OhmPi(object):
             I = np.mean(self._hw.readings[x, 3])
             I_std = 100. * np.std(self._hw.readings[x, 3])
             R = Vmn / I
-            R_std = 100. * np.std((self._hw.readings[x, 4] - self._hw.sp) / self._hw.readings[x, 3]) / R
+            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 = {
                 "time": datetime.now().isoformat(),