diff --git a/src/anemoi/datasets/compute/perturbations.py b/src/anemoi/datasets/compute/perturbations.py
index c09041f3..4fa964f9 100644
--- a/src/anemoi/datasets/compute/perturbations.py
+++ b/src/anemoi/datasets/compute/perturbations.py
@@ -7,59 +7,95 @@
 # nor does it submit to any jurisdiction.
 #
 
-import warnings
+import logging
 
 import numpy as np
 from climetlab.core.temporary import temp_file
 from climetlab.readers.grib.output import new_grib_output
 
-from anemoi.datasets.create.check import check_data_values
 from anemoi.datasets.create.functions import assert_is_fieldset
 
+LOG = logging.getLogger(__name__)
+
+CLIP_VARIABLES = (
+    "q",
+    "cp",
+    "lsp",
+    "tp",
+    "sf",
+    "swl4",
+    "swl3",
+    "swl2",
+    "swl1",
+)
+
+SKIP = ("class", "stream", "type", "number", "expver", "_leg_number")
+
+
+def check_compatible(f1, f2, center_field_as_mars, ensemble_field_as_mars):
+    assert f1.mars_grid == f2.mars_grid, (f1.mars_grid, f2.mars_grid)
+    assert f1.mars_area == f2.mars_area, (f1.mars_area, f2.mars_area)
+    assert f1.shape == f2.shape, (f1.shape, f2.shape)
+
+    # Not in *_as_mars
+    assert f1.metadata("valid_datetime") == f2.metadata("valid_datetime"), (
+        f1.metadata("valid_datetime"),
+        f2.metadata("valid_datetime"),
+    )
+
+    for k in set(center_field_as_mars.keys()) | set(ensemble_field_as_mars.keys()):
+        if k in SKIP:
+            continue
+        assert center_field_as_mars[k] == ensemble_field_as_mars[k], (
+            k,
+            center_field_as_mars[k],
+            ensemble_field_as_mars[k],
+        )
+
 
 def perturbations(
+    *,
     members,
     center,
-    positive_clipping_variables=[
-        "q",
-        "cp",
-        "lsp",
-        "tp",
-    ],  # add "swl4", "swl3", "swl2", "swl1", "swl0", and more ?
+    clip_variables=CLIP_VARIABLES,
+    output=None,
 ):
 
     keys = ["param", "level", "valid_datetime", "date", "time", "step", "number"]
 
-    def check_compatible(f1, f2, ignore=["number"]):
-        for k in keys + ["grid", "shape"]:
-            if k in ignore:
-                continue
-            assert f1.metadata(k) == f2.metadata(k), (k, f1.metadata(k), f2.metadata(k))
+    number_list = members.unique_values("number")["number"]
+    n_numbers = len(number_list)
 
-    print(f"Retrieving ensemble data with {members}")
-    print(f"Retrieving center data with {center}")
+    assert None not in number_list
 
+    LOG.info("Ordering fields")
     members = members.order_by(*keys)
     center = center.order_by(*keys)
-
-    number_list = members.unique_values("number")["number"]
-    n_numbers = len(number_list)
+    LOG.info("Done")
 
     if len(center) * n_numbers != len(members):
-        print(len(center), n_numbers, len(members))
+        LOG.error("%s %s %s", len(center), n_numbers, len(members))
         for f in members:
-            print("Member: ", f)
+            LOG.error("Member: %r", f)
         for f in center:
-            print("Center: ", f)
+            LOG.error("Center: %r", f)
         raise ValueError(f"Inconsistent number of fields: {len(center)} * {n_numbers} != {len(members)}")
 
-    # prepare output tmp file so we can read it back
-    tmp = temp_file()
-    path = tmp.path
+    if output is None:
+        # prepare output tmp file so we can read it back
+        tmp = temp_file()
+        path = tmp.path
+    else:
+        tmp = None
+        path = output
+
     out = new_grib_output(path)
 
+    seen = set()
+
     for i, center_field in enumerate(center):
         param = center_field.metadata("param")
+        center_field_as_mars = center_field.as_mars()
 
         # load the center field
         center_np = center_field.to_numpy()
@@ -69,9 +105,14 @@ def check_compatible(f1, f2, ignore=["number"]):
 
         for j in range(n_numbers):
             ensemble_field = members[i * n_numbers + j]
-            check_compatible(center_field, ensemble_field)
+            ensemble_field_as_mars = ensemble_field.as_mars()
+            check_compatible(center_field, ensemble_field, center_field_as_mars, ensemble_field_as_mars)
             members_np[j] = ensemble_field.to_numpy()
 
+            ensemble_field_as_mars = tuple(sorted(ensemble_field_as_mars.items()))
+            assert ensemble_field_as_mars not in seen, ensemble_field_as_mars
+            seen.add(ensemble_field_as_mars)
+
         mean_np = members_np.mean(axis=0)
 
         for j in range(n_numbers):
@@ -84,18 +125,22 @@ def check_compatible(f1, f2, ignore=["number"]):
 
             x = c - m + e
 
-            if param in positive_clipping_variables:
-                warnings.warn(f"Clipping {param} to be positive")
+            if param in clip_variables:
+                # LOG.warning(f"Clipping {param} to be positive")
                 x = np.maximum(x, 0)
 
             assert x.shape == e.shape, (x.shape, e.shape)
 
-            check_data_values(x, name=param)
             out.write(x, template=template)
             template = None
 
+    assert len(seen) == len(members), (len(seen), len(members))
+
     out.close()
 
+    if output is not None:
+        return path
+
     from climetlab import load_source
 
     ds = load_source("file", path)