diff --git a/labsw/__init__.py b/labsw/__init__.py
index 472a2ee..75b60b9 100644
--- a/labsw/__init__.py
+++ b/labsw/__init__.py
@@ -1 +1 @@
-__version__ = '0.1.5.5'
+__version__ = '0.1.5.6'
diff --git a/labsw/io.py b/labsw/io.py
index 77471b0..5c6e8df 100644
--- a/labsw/io.py
+++ b/labsw/io.py
@@ -26,16 +26,18 @@ def load_data_RH_logger(filepath, every=1):
     df = df.reset_index()
     del df['index']
 
-    # Derivative
-    df['dmdt'] = savgol_filter(df['weight'], window_length=10000, polyorder=1, deriv=1)
 
     return df.iloc[::every]
 
 
-def process_data_RH_logger(filepath):
+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)
+
     h5path = os.path.splitext(filepath)[0]
     h5path += '-processed.h5'
+    df = df.iloc[::every]
     df.to_hdf(h5path, key='data')