diff --git a/labsw/__init__.py b/labsw/__init__.py
index 75b60b9..57417ce 100644
--- a/labsw/__init__.py
+++ b/labsw/__init__.py
@@ -1 +1 @@
-__version__ = '0.1.5.6'
+__version__ = '0.1.5.7'
diff --git a/labsw/io.py b/labsw/io.py
index 5c6e8df..84cbb3b 100644
--- a/labsw/io.py
+++ b/labsw/io.py
@@ -33,7 +33,8 @@ def load_data_RH_logger(filepath, every=1):
 def process_data_RH_logger(filepath, every):
     df = load_data_RH_logger(filepath, every=1)
     # Derivative
-    df['dmdt'] = savgol_filter(df['weight'], window_length=10000, polyorder=1, deriv=1)
+    df['dmdt_SG'] = savgol_filter(df['weight'], window_length=10000, polyorder=1, deriv=1)
+    df['dmdt_diff'] = df['weight'].diff(periods=100) / df['time'].diff(periods=100)
 
     h5path = os.path.splitext(filepath)[0]
     h5path += '-processed.h5'