diff --git a/tests/util/test_fof_utils.py b/tests/util/test_fof_utils.py
index 9564311..f0e61b1 100644
--- a/tests/util/test_fof_utils.py
+++ b/tests/util/test_fof_utils.py
@@ -2,6 +2,7 @@
 This module contains unit tests for the `util/fof_utils.py` module.
 """
 
+import os
 from unittest.mock import mock_open, patch
 
 import numpy as np
@@ -11,9 +12,9 @@
     clean_value,
     compare_arrays,
     compare_var_and_attr_ds,
-    fill_nans_for_float32,
     get_observation_variables,
     get_report_variables,
+    replace_nan_with_sentinel,
     split_feedback_dataset,
 )
 from util.log_handler import initialize_detailed_logger
@@ -196,8 +197,8 @@ def test_fill_nans_for_float32_nan(arr_nan):
     Test that if an array containing nan is given, these values are replaced
     by -9.99999e05.
     """
-    array = fill_nans_for_float32(arr_nan)
-    expected = np.array([1.0, -9.99999e05, 3.0, 4.0, -9.99999e05], dtype=np.float32)
+    array = replace_nan_with_sentinel(arr_nan)
+    expected = np.array([1.0, -9.99999e05, 3.0, 4.0, -9.99999e05], dtype=np.float64)
     assert np.array_equal(array, expected)
 
 
@@ -206,7 +207,7 @@ 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)
 
 
@@ -254,6 +255,13 @@ def test_compare_var_and_attr_ds(ds1, ds2):
         assert (total1, equal1) == (103, 102)
         assert (total2, equal2) == (103, 102)
 
+    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 f9e544d..466c3b7 100644
--- a/util/fof_utils.py
+++ b/util/fof_utils.py
@@ -104,13 +104,18 @@ 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 the input array has a floating dtype, it is cast to float64
+    and all NaN values are replaced with the sentinel value -999999.
+    If the array does not have a floating dtype, it is returned unchanged.
     """
-    if arr.dtype == np.float32 and np.isnan(arr).any():
-        return np.where(np.isnan(arr), -999999, arr)
-    return arr
+    if not np.issubdtype(arr.dtype, np.floating):
+        return arr
+
+    arr = arr.astype(np.float64, copy=False)
+
+    return np.where(np.isnan(arr), -999999.0, arr)
 
 
 def clean_value(x):
@@ -203,8 +208,8 @@ def process_var(ds1, ds2, var, detailed_logger):
     number of matching elements.
     """
 
-    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)
         if diff.size != 0: