diff --git a/mao_merge_45m/power_meter.py b/mao_merge_45m/power_meter.py
index ce26c02..88e8db8 100644
--- a/mao_merge_45m/power_meter.py
+++ b/mao_merge_45m/power_meter.py
@@ -108,7 +108,7 @@ def convert(
         df_["num_max"] = df_.groupby("time").max()["num"]
 
         df_.reset_index(inplace=True)
-        df_["s"] = (df_["num"]-1) / df_["num_max"] * 1000
+        df_["s"] = (df_["num"] - 1) / df_["num_max"] * 1000
         df_["time"] = df_["time"] + pd.to_timedelta(np.array(df_["s"]), unit="ms")
         df_.set_index(df_["time"], inplace=True)
 
diff --git a/tests/test_power_meter.py b/tests/test_power_meter.py
index 53924d5..33e81be 100644
--- a/tests/test_power_meter.py
+++ b/tests/test_power_meter.py
@@ -12,6 +12,7 @@
 # constants
 TEST_CSV = Path("data") / "power_meter_2022144230000Z.csv"
 
+
 # test function
 def test_convert() -> None:
     """Test whether the data of a CSV file is correctly parsed."""