diff --git a/tests/util/test_fof_utils.py b/tests/util/test_fof_utils.py
index bc57bec..d7345fe 100644
--- a/tests/util/test_fof_utils.py
+++ b/tests/util/test_fof_utils.py
@@ -2,6 +2,8 @@
 This module contains unit tests for the `util/fof_utils.py` module.
 """
 
+import os
+
 import numpy as np
 import pytest
 
@@ -9,11 +11,11 @@
     clean_value,
     compare_arrays,
     compare_var_and_attr_ds,
-    fill_nans_for_float32,
     get_observation_variables,
     get_report_variables,
     primary_check,
     print_entire_line,
+    replace_nan_with_sentinel,
     split_feedback_dataset,
     write_lines,
 )
@@ -196,7 +198,7 @@ def test_fill_nans_for_float32_nan(arr_nan):
     Test that if an array containign nan is given, these values are replaced
     by -9.99999e05.
     """
-    array = fill_nans_for_float32(arr_nan)
+    array = replace_nan_with_sentinel(arr_nan)
     expected = np.array([1.0, -9.99999e05, 3.0, 4.0, -9.99999e05], dtype=np.float32)
     assert np.array_equal(array, expected)
 
@@ -206,7 +208,7 @@ def test_fill_nans_for_float32(arr1):
     Test that if an array without nan is given, the output of the function
     is the same as the input.
     """
-    array = fill_nans_for_float32(arr1)
+    array = replace_nan_with_sentinel(arr1)
     assert np.array_equal(array, arr1)
 
 
@@ -322,11 +324,18 @@ def test_compare_var_and_attr_ds(ds1, ds2, tmp_path):
     total1, equal1 = compare_var_and_attr_ds(
         ds1, ds2, nl=0, output=True, location=file_path
     )
-    total2, equal2 = compare_var_and_attr_ds(ds1, ds2, nl=4, output=True, location=None)
 
+    total2, equal2 = compare_var_and_attr_ds(ds1, ds2, nl=4, output=True, location=None)
     assert (total1, equal1) == (104, 103)
     assert (total2, equal2) == (104, 103)
 
+    script_dir = os.path.dirname(os.path.abspath(__file__))
+    grandparent_dir = os.path.dirname(os.path.dirname(script_dir))
+
+    path_name = os.path.join(grandparent_dir, "differences.csv")
+    if os.path.exists(path_name):
+        os.remove(path_name)
+
 
 @pytest.fixture(name="ds3")
 def fixture_sample_dataset_3(sample_dataset_fof):
diff --git a/util/fof_utils.py b/util/fof_utils.py
index 9400b00..9d35087 100644
--- a/util/fof_utils.py
+++ b/util/fof_utils.py
@@ -102,12 +102,18 @@ def compare_arrays(arr1, arr2, var_name):
     return total, equal, diff
 
 
-def fill_nans_for_float32(arr):
+def replace_nan_with_sentinel(arr):
     """
     To make sure nan values are recognised.
     """
-    if arr.dtype == np.float32 and np.isnan(arr).any():
+    if not np.issubdtype(arr.dtype, np.floating):
+        return arr
+
+    arr = arr.astype(np.float64, copy=False)
+
+    if np.isnan(arr).any():
         return np.where(np.isnan(arr), -999999, arr)
+
     return arr
 
 
@@ -217,9 +223,11 @@ def compare_var_and_attr_ds(ds1, ds2, nl, output, location):
     if output:
         if location:
             path_name = location
+
         else:
             script_dir = os.path.dirname(os.path.abspath(__file__))
-            path_name = os.path.join(script_dir, "differences.csv")
+            parent_dir = os.path.dirname(script_dir)
+            path_name = os.path.join(parent_dir, "differences.csv")
 
         with open(path_name, "w", encoding="utf-8") as f:
             f.write("Differences\n")
@@ -227,8 +235,8 @@ def compare_var_and_attr_ds(ds1, ds2, nl, output, location):
     for var in set(ds1.data_vars).union(ds2.data_vars):
         if var in ds1.data_vars and var in ds2.data_vars and var not in list_to_skip:
 
-            arr1 = fill_nans_for_float32(ds1[var].values)
-            arr2 = fill_nans_for_float32(ds2[var].values)
+            arr1 = replace_nan_with_sentinel(ds1[var].values)
+            arr2 = replace_nan_with_sentinel(ds2[var].values)
 
             if arr1.size == arr2.size:
                 t, e, diff = compare_arrays(arr1, arr2, var)