diff --git a/tests/lib/test_ts_operators.py b/tests/lib/test_ts_operators.py
index 369c8f35c44afe8fef4567ddb9e356546f2f82f9..4b6afaa81914648017c20ccc410d6e36e3ec2642 100644
--- a/tests/lib/test_ts_operators.py
+++ b/tests/lib/test_ts_operators.py
@@ -6,6 +6,7 @@ import pytest
 
 import saqc.lib.ts_operators as tsops
 import pandas as pd
+from pandas.testing import assert_series_equal
 
 
 def test_butterFilter():
@@ -45,3 +46,62 @@ F = False
 def test__exceedConsecutiveNanLimit(arr, maxc, expected):
     result = tsops._exceedConsecutiveNanLimit(arr, maxc)
     assert result is expected
+
+
+def dtSeries(data, freq="1d"):
+    index = pd.date_range(start="2020", periods=len(data), freq=freq)
+    return pd.Series(data=data, index=index, dtype=float)
+
+
+@pytest.mark.parametrize(
+    "func,data,expected",
+    [
+        ("identity", dtSeries([1, 2]), dtSeries([1, 2])),
+        ("count", dtSeries([0, 0]), dtSeries([2])),
+        pytest.param(
+            "first",
+            dtSeries([1, 2]),
+            dtSeries([1, 1]),
+            marks=pytest.mark.xfail(reason="BUG (the inner ts.first need an argument)"),
+        ),
+        pytest.param(
+            "last",
+            dtSeries([1, 2]),
+            dtSeries([1, 1]),
+            marks=pytest.mark.xfail(reason="BUG (the inner ts.last need an argument)"),
+        ),
+        (
+            "zeroLog",
+            dtSeries([1, 2, np.inf, np.nan]),
+            dtSeries([np.log(1), np.log(2), np.inf, np.nan]),
+        ),
+        pytest.param(
+            "zeroLog",
+            dtSeries(
+                [
+                    # 0,
+                    -2,
+                    -1,
+                    -np.inf,
+                ]
+            ),
+            dtSeries([np.nan, np.nan, np.nan]),
+            marks=pytest.mark.xfail(reason="zeroLog(0) did not return NaN"),
+        ),
+    ],
+)
+def test_tsop_functions(func, data, expected):
+    f = getattr(tsops, func)
+
+    resampler = data.resample("2d")
+
+    result = resampler.apply(f)
+    assert isinstance(result, pd.Series)
+
+    print()
+    print(result)
+    print()
+    print(expected)
+    assert_series_equal(
+        result, expected, check_names=False, check_freq=False, check_dtype=False
+    )